Dr Radu Sporea SMIEEE MIET CEng

Associate Professor in Semiconductor Devices
+44 (0)1483 686086
28 ATI 02
By email appointment


Areas of specialism

Electronic devices and circuits; Flexible and printed electronics; Manufacturability and reliability of electronics; Modelling and simulation; Unconventional electronics

University roles and responsibilities

  • Admissions Tutor for the Department of Electrical and Electronic Engineering
  • Undergraduate final year project supervision
  • EEE Athena Swan committee
  • Local community engagement committee
  • Public engagement and outreach in Engineering
  • Nuffield/SATRO Research Placement host

    Affiliations and memberships

    Technical Program Committee member
    Technical Program Committee member
    Technical Program Committee member
    Publications Chair
    IEEE, IOP and RSC journals

    Business, industry and community links


    Research interests


    Postgraduate research supervision



    Eva Bestelink, Ute Zschieschang, Leslie Askew, Hagen Klauk, Radu A. Sporea (2023)Organic Source‐Gated Phototransistors with > 10 4 Photo‐To‐Dark Current Ratio in the Visible Range at Zero Gate‐Source Bias, In: Advanced optical materials

    Abstract With growing interest in organic phototransistors, as not only sensors but also neuromorphic computing elements, the vast majority of research investigates structures comprising Ohmic source/drain contacts. Here, it is shown how source‐gated transistors (SGTs), in which a source contact barrier dominates electrical characteristics, can be implemented as phototransistors. Organic photo‐SGTs (OPSGTs) based on vacuum‐processed small‐molecule dinaphtho[2,3‐b:2′,3′‐f]thieno[3,2‐]thiophene (DNTT) demonstrate low saturation voltage, exceptional tolerance to channel length variation, and photo‐to‐dark current ratio (PDCR) peaks over 10 6 for 819 µW broad spectrum incident light power. At zero gate‐source voltage, the PDCR reaches 10 4 , showing promise for simple sensor circuit implementation in medical and wellbeing applications.

    Massood Z Atashbar, Radu Sporea, Mitradip Bhattacharjee, Pedro Barquinha, Elonora Macchia (2023)Guest Editorial 2022 IEEE International Conference on Flexible, Printable Sensors and Systems (FLEPS), In: IEEE Journal on Flexible Electronics2(2)pp. 47-50
    K.D.G.I. Jayawardena, R. M. I. Bandara, M. Monti, E. Butler-Caddle, T. Pichler, H. Shiozawa, Z. Wang, S. Jenatsch, S. J. Hinder, M. G. Masteghin, M. Patel, H.M. Thirimanne, W. Zhang, R.A. Sporea, J. Lloyd-Hughes, S.R.P. Silva (2019)Approaching the Shockley-Queisser limit for fill factors in lead-tin mixed perovskite photovoltaics, In: Journal of Materials Chemistry A8(2)pp. 693-705 The Royal Society of Chemistry

    The performance of all solar cells is dictated by charge recombination. A closer to ideal recombination dynamics results in improved performances, with fill factors approaching the limits based on Shockley–Queisser analysis. It is well known that for emerging solar materials such as perovskites, there are several challenges that need to be overcome to achieve high fill factors, particularly for large area lead–tin mixed perovskite solar cells. Here we demonstrate a strategy towards achieving fill factors above 80% through post-treatment of a lead–tin mixed perovskite absorber with guanidinium bromide for devices with an active area of 0.43 cm2. This bromide post-treatment results in a more favorable band alignment at the anode and cathode interfaces, enabling better bipolar extraction. The resulting devices demonstrate an exceptional fill factor of 83%, approaching the Shockley–Queisser limit, resulting in a power conversion efficiency of 14.4% for large area devices.

    Rajapakshe Mudiyanselage Indrachapa Bandara, Shashini M. Silva, Cameron C. L. Underwood, K. D. G. Imalka Jayawardena, Radu A. Sporea, S. Ravi P. Silva (2022)Progress of Pb-Sn Mixed Perovskites for Photovoltaics: A Review, In: Energy & environmental materials (Hoboken, N.J.)5(2)pp. 370-400 Wiley

    Pb-Sn mixed perovskites are becoming increasingly popular as narrow-bandgap (1.2-1.3 eV) light absorbers in single-junction perovskite solar cells (PSCs) and as bottom cells for all-perovskite tandem solar cells, for high-efficiency, low-cost, lightweight, roll-to-roll printable photovoltaic (PV) applications. From the first report of planar Pb:Sn mixed PSCs in 2014, the power conversion efficiencies (PCE) have increased from 10% to 21% by the end of 2020 with an exponential growth in research conducted in this field. Despite much effort, the performance and stability of Pb-Sn mixed PSCs are still limited, which constrains their long-term use in all-perovskite tandem devices. This review highlights the avenues explored in improving different aspects of Pb-Sn mixed PSCs and provides a comprehensive discussion of the interdependent factors affecting the device performance. This includes compositional engineering of the perovskite crystal, absorber layer fabrication and crystallization methods, bandgap tuning, Sn4+ reduction, and surface passivation of the absorber layer, as well as the selection of interlayers and electrodes of the final PSC.

    Olivier de Sagazan, Andreï Uvarov, Eva Bestelink, Radu A. Sporea, Emmanuel Jacques (2020)Investigation on icp-cvd as a polyvalent low cost technology dedicated to low temperature μ-si tft prototyping., In: SID International Symposium Digest of technical papers51(1)pp. 1538-1541

    A Corial Inductively Coupled Plasma Chemical Vapor Deposition (ICP‐CVD) system has been investigated to produce un‐doped and doped μ‐Si layers, as well as insulators, leading to a general capability of performing N and P type TFTs. This enables to develop rapid prototyping of TFTs. Resistivity of layers and TFT issues from ICP‐CVD have been electrically characterized.

    Eva Bestelink, Kham M. Niang, George Bairaktaris, Luca Maiolo, Francesco Maita, Kalil Ali, Andrew J. Flewitt, Ravi Silva, Radu Sporea (2020)Compact Source-Gated Transistor Analog Circuits for Ubiquitous Sensors, In: IEEE Sensors Institute of Electrical and Electronics Engineers

    Silicon-based digital electronics have evolved over decades through an aggressive scaling process following Moore’s law with increasingly complex device structures. Simultaneously, large-area electronics have continued to rely on the same field-effect transistor structure with minimal evolution. This limitation has resulted in less than ideal circuit designs, with increased complexity to account for shortcomings in material properties and process control. At present, this situation is holding back the development of novel systems required for printed and flexible electronic applications beyond the Internet of Things. In this work we demonstrate the opportunity offered by the source-gated transistor’s unique properties for low-cost, highly functional large-area applications in two extremely compact circuit blocks. Polysilicon common-source amplifiers show 49 dB gain, the highest reported for a twotransistor unipolar circuit. Current mirrors fabricated in polysilicon and InGaZnO have, in addition to excellent current copying performance, the ability to control the temperature dependence (degrees of positive, neutral or negative) of output current solely by choice of relative transistor geometry, giving further flexibility to the design engineer. Application examples are proposed, including local amplification of sensor output for improved signal integrity, as well as temperature-regulated delay stages and timing circuits for homeostatic operation in future wearables. Numerous applications will benefit from these highly competitive compact circuit designs with robust performance, improved energy efficiency and tolerance to geometrical variations: sensor front-ends, temperature sensors, pixel drivers, bias analog blocks and high-gain amplifiers.

    Eva Bestelink, Olivier de Sagazan, Max Bateson, Radu A. Sporea (2020)The multimodal thin-film transistor (Mmt): A versatile low-power and high-gain device with inherent linear response, In: SID International Symposium Digest of technical papers51(1)pp. 444-447

    A new device, the Multimodal Transistor (MMT), separates charge injection from conduction. With design optimization, it can achieve a constant transconductance with independent on/off switching of output current. This functionality has ample applications in energy efficient analog computation and hardware learning.

    Pongsakorn Sihapitak, Juan Paolo Bermundo, Eva Bestelink, Radu A Sporea, Yukiharu Uraoka (2024)Optimizing a-IGZO Source-Gated Transistor Current by Structure Alteration via TCAD Simulation and Experiment, In: IEEE Transactions on Electron Devices Institute of Electrical and Electronics Engineers (IEEE)

    This study aims to increase the output current of the a-IGZO source-gated transistor (SGT) through TCAD simulation and experiment. As SGT proves to be useful in various low-power applications and wearable devices, the low output current characteristics limit the adaptability of this structure. It is estimated that a higher level output current is achievable by optimizing this architecture while retaining fast saturation characteristics. The SGT structure simulations have been performed with adjusted density- of-states (DoS) parameters from experiments; this made TCAD simulation more realistic and can be used to predict the results of fabricated SGTs. The results from this study show that longer source-channel overlap and thinner channel is preferable; the SGT with longer source-channel overlap will result in the Mode II injection dominating over the Mode I injection. The Mode I injection occurs at the edge of source contact near the channel region, while the Mode II injection occurs at the bulk of the source at the farthest region from the channel. The result from experiment shows that the fabricated SGT exhibits Mode II injection characteristics-that is the current will increase linearly with increasing gate voltage. From this study. it could be concluded that the source-channel overlap of 210 µm with a channel thickness of 20 nm and channel length of 5 µm is the optimized structure that could provide high output current with little temperature dependence. Further improvement of the output current could be achieved by utilizing lower work function source metal.

    S Georgakopoulos, RADU A SPOREA, MAXIM SHKUNOV (2022)Polymer source-gated transistors with low saturation voltage, In: Journal of materials chemistry. C, Materials for optical and electronic devices10(4)pp. 1282-1288

    A type of injection-limited transistor is demonstrated with a conjugated polymer semiconductor and fluoropolymer insulator. The source-gated transistor (SGT) is based on a source Schottky barrier, the effective height of which is controlled by the gate voltage, shifting the origin of current modulation from the channel to the source-semiconductor contact. SGTs fabricated in this work saturate at up to 30 times lower drain voltages than FETs at high gate voltages. Saturation in SGTs is retained for short channels without requiring downscaling of the insulator layer. As the transistor channel has reduced influence on current modulation, the SGT output current is only weakly dependent on the channel length contrary to traditional FETs. These features come at some current reduction due to the series resistance stemming from the source depletion region. The ability to function with thick insulators, low voltage operation and thus low power consumption, as well as the reduced sensitivity to channel length variations could be highly synergistic with printing techniques used to deposit materials in organic electronics.

    Radu A. Sporea (2022)50.2: Invited Paper: Contact‐controlled transistors:critical design parameters, In: SID International Symposium Digest of technical papers53(S1)pp. 488-488
    Eva Bestelink, Olivier Sagazan, Radu A. Sporea (2021)22.2: Invited Paper: Opportunities for Multimodal Thin‐Film Transistors in Displays and Beyond, In: SID International Symposium Digest of technical papers52(S2)pp. 294-297

    Multimodal transistors (MMTs) build on previous understanding of contact‐controlled transistors. Additionally, they allow precise control of the charge injected, distinct from controlling the state of conduction of the semiconductor channel. This approach leads to interesting functionality, of benefit to displays and other large area electronic circuits. Here, we summarize the construction and functionality of the MMT and outline potential applications.

    Eva Bestelink, Olivier de Sagazan, Radu A. Sporea (2020)P‐18: Ultra‐Compact Multi‐Level Digital‐to‐Analog Converter based on Linear Multimodal Thin‐Film Transistors, In: SID International Symposium Digest of technical papers51(1)pp. 1375-1378

    A new device, the Multimodal Transistor (MMT), separates charge injection from conduction and achieves a linear dependence of drain current on its control gate voltage. This functionality is used to implement a highly compact digital‐to‐analog‐converter, capable of performing 3‐level, 3‐bit conversion with minimal error (1.2% of LSB).

    Eva Bestelink, Kham M Niang, Indrachapa Rajapakshe Mudiyanselage, Georgios Bairaktaris, David M Frohlich, Radu A Sporea (2021)Oxide transistors: unconventional architectures and their applications, In: David J Rogers, David C Look, Ferechteh H Teherani (eds.), Proceedings of SPIE - The International Society for Optical Engineering116872585755pp. 116871G-116871G-8 SPIE

    Thin-film transistors deliberately comprising rectifying source contacts have attractive properties for sensor and driver circuits: high performance uniformity and geometrical tolerance; superior saturation; and high intrinsic gain. The paper reviews the source-gated and multimodal thin-film transistor configurations, and presents their proposed applications to ultra-compact sensing and data processing circuits. Source-gated transistors with nanoscale tunneling contacts offer an alternative to the Schottky-contact fabrication route, which presents processing challenges. Emerging multimodal transistors overcome limitations of traditional contact-controlled devices and add to the list of useful properties: high gain or constant transconductance by design; immunity to drain voltage variations in floating gate configuration; and a significantly faster response time than source-gated transistors. These devices form the foundation for the design of compact, yet extremely versatile, thin-film circuits for sensing, signal conditioning and signal conversion. Finally, a vision is presented in which the properties of these circuits will be essential to convey seamless user interactivity to physical objects, transforming them into intuitive user interfaces beyond traditional displays screens.

    Georgios Bairaktaris, Brice Le Borgne, Sirpa Nordman, Samuli Yrjänä, Vikram Turkani, Rudresh Gosh, Vahid Akhavan, Peter Bagge, Timo Turpela, David M Frohlich, Radu A Sporea (2020)78‐2: Using Physical Books as Interfaces to Digital Displays, In: SID International Symposium Digest of technical papers51(1)pp. 1159-1162

    A new form of interaction with digital displays is described, using the pages and binding of a physical book as the interface. This leads to a form of augmented book, or a‐book, which can seamlessly trigger multimedia content on a nearby device using embedded light, pressure or touch sensors.

    Radu Alexandru Sporea, K. D. G. Imalka Jayawardena, Marios Constantinou, Munroe Ritchie, Alistair Brewin, William Wright, S. Ravi P. Silva (2016)Heterostructure Source-Gated Transistors: Challenges in Design and Fabrication, In: Meeting abstracts (Electrochemical Society)MA2016-02(33)pp. 2160-2160

    Source-gated transistors (SGTs) [1, 2] use a potential barrier at the source to produce low-voltage saturation and flat saturated characteristics. This, in turn, results in excellent power efficiency and amplification performance [3-5]. Usually, SGTs are made with a Schottky source contact which acts as the current-control mechanism. These devices are easy to fabricate in a multitude of semiconductor systems, but may suffer, depending on design, from reduced dynamic range of the current above the threshold, and from significant temperature dependence of the on-current. We propose an alternative method of making SGTs, using an ohmic source contact and a bulk barrier. We call devices realised following this principle bulk unipolar barrier source-gated transistors (BUSGTs). Several strategies for realising the bulk barrier exist, which depend on the material system and fabrication process considered [6]. Here, we realise the bulk barrier at the source with a tri-layer semiconductor heterostructure (Figure 1, top). The advantages of this architecture include improved dynamic range (Figure 1, bottom) and reduced temperature dependence of drain current, as well as the usual energy and gain characteristics of SGTs. Measurement results on C70/C60/C70 n-type devices show: low hysteresis, indicating good quality interfaces; a large threshold shift due to the bulk barrier and residual charge in the heterostructure; and FET-like saturation, due to the interplay between semiconductor mobility and barrier height. A strong correlation is found between measurements and simulations using Silvaco Atlas. We discuss the benefits of such structures, and also the fabrication and design challenges associated with this type of transistor (chemistry and materials, barrier realisation, interface properties, requirements on charge carrier mobility). [1] S. D. Brotherton, Introduction to Thin Film Transistors: Physics and Technology of TFTs, Springer, 453–480, Cham (2013); [2] A. Valletta et al., J. Appl. Phys . 114 064501 (2013); [3] R. A. Sporea et al., Scientific Reports 4 4295 (2014); [4] R. A. Sporea, ULSI vs TFT V (2015); [5] R. A. Sporea et al., AA3.07, MRS Fall , Boston (2015); [6] J. M. Shannon, APL 35 (1) 63–65 (1979). Figure 1 – Top: fabricated tri-layer fullerene heterostructure, showing the position of the bulk barrier at the source; bottom: transfer characteristic of the fabricated devices, illustrating high dynamic range above threshold, low hysteresis, and significant threshold shift as due to the bulk barrier. Figure 1

    Radu Alexandru Sporea (2021)(Invited) Multi-Gate Contact-Controlled Transistors, In: Meeting abstracts (Electrochemical Society)MA2021-01(32)pp. 1058-1058 The Electrochemical Society, Inc
    Eva Bestelink, Olivier de Sagazan, Radu Alexandru Sporea (2020)Total Gain Recovery in Floating Gate Thin-Film Transistors for Neuromorphic and Edge Computing, In: Meeting abstracts (Electrochemical Society)MA2020-02(28)pp. 1947-1947

    We propose a floating-gate (FG) thin-film transistor architecture which alleviates a significant limitation present since the inception of FG field-effect transistors, namely the loss of gain due to parasitic capacitive coupling on the FG [1]. The interest in large area electronics has grown beyond traditional applications, such as display and sensing arrays, with recent trends including neuromorphic and edge computing. However, the challenges of fabricating robust thin film transistor (TFT) circuits have remained despite the many significant achievements in material systems and process development. In order to realise the full benefit of low-cost, high-throughput manufacturing methods, device shortcomings need to be addressed [2]. In FG devices, specifically, one major limitation is the degradation of gain compared to conventional devices, as a result of capacitive coupling of the channel and drain potentials to the floating gate. The problem is far more severe in contact-controlled TFTs, such as the staggered-electrode, high-gain source-gated transistor (SGT) [3, 4], and crippling in conventional TFTs manufactured in materials such as InGaZnO [5]. SGT operation differs from conventional field-effect TFTs, whereby the semiconductor is fully depleted at the source edge by an energy barrier (e.g. Schottky), which is reverse biased by the application of drain voltage. Charge injection is modulated via the gate/source overlap, producing very low saturation voltage and ultra-low output conductance g d [3]. This latter feature yields the distinctive and extremely high intrinsic gain observed in such devices. A large voltage drop occurs across the source depletion region, producing the notable early saturation and, consequently, the channel potential is maintained at a value close to the drain potential for operating conditions in which a conventional TFT would still be working in the linear region. As such, when a floating gate is used in a SGT, the FG couples strongly to both the drain and the channel. FG potential varies considerably with drain voltage, thus increasing charge injection along the source, ultimately raising g d . While g d is still orders of magnitude lower than that of a TFT or FG TFT (see Figure 1), many analog applications would benefit from a solution to this problem [1]. Here, we present a staggered-electrode contact-controlled device, the multimodal transistor (MMT), which shares SGT charge injection principles, however the switching mechanism of its channel is controlled by a separate gate [6]. In a FG MMT, the source gate responsible for charge injection and channel switching gate are designed as FGs (FG1 and FG2, respectively) and a main control gate (CG) may overlap both FGs. FG MMTs have been fabricated in low T°C technology [6] with Ni source and drain contacts. ICP-CVD (Inductively Coupled Plasma Chemical Vapour Deposition) via the Corial 210D reactor was used to deposit µ-Si as active layer, as well as the insulators of the gate stack (Figure 1a). Temperature never exceeded 250°C in any of the processing steps. The MMT exhibits the same high-gain properties of the SGT and preserves it even in FG configuration. With the channel and drain potentials coupling to FG2, its potential is raised significantly higher than FG1 (see Figures 1b and 1c for TCAD simulation with Silvaco Atlas), which is effectively shielded. As charge injection is exclusively modulated by FG1, the low saturation and extremely low g d are maintained, in contrast to the FG SGT (see Figure 1c). Thus, extremely high gain can be obtained from the FG MMT structure, with a wide range of applicability to traditional analog applications such as displays and sensors, as well as emerging neuromorphic circuits. Figure. 1 a) Photomicrograph of a µ-Si floating gate MMT (FG MMT); b) FG2 potential is significantly raised, while FG1 is shielded from parasitic capacitive coupling from the channel and drain potential, keeping charge injection at the source constant with drain voltage. c) Output conductance g d comparison confirms FG MMT g d is significantly lower than that of the equivalent FG SGT. References: [1] E. Rodriguez–Villegas, Low Power and Low Voltage Circuit Design with the FGMOS transistor . London: The IET, 2006. [2] Y. Kuo, Electrochem. Soc. Interface , vol. 22, no. 1, pp. 55–61, 2013. [3] R. A. Sporea et al ., Sci. Rep. , vol. 4, pp. 1–7, 2014. [4] J. Zhang et al. , Proc. Natl. Acad. Sci. U. S. A. , vol. 116, no. 11, pp. 4843–4848, 2019. [5] T. Qin et al. , Acta Phys. Sin. , vol. 67, no. 047302, pp. 1–7, 2018. [6] E. Bestelink et al. , SID , vol. 31.1, 2020. Figure 1

    Georgios Bairaktaris, Fasihullah Khan , Imalka Jayawardena, David Mark Frohlich, Radu A Sporea (2022)Printable and flexible photodetectors via scalable fabrication for reading applications, In: Communications Engineering140 Nature Portfolio

    Printing techniques have been widely adopted in the fabrication of flexible electronic components. However, its application is still limited in complex control and communication circuitry due to the low performance and low fabrication uniformity amongst printed devices, compared to conventional electronics. Thus, the electronic systems in real-world applications are hybrid integrations of printed and conventional electronics. Here we demonstrate a low-cost, low-complexity, fully-printable flexible photodetector that can withstand over 100 1 mm-radius bending cycles using a simple and scalable two-step fabrication process. The prototypes are implemented in an augmented book system to automatically detect the ambient light through optical apertures on paper of a printed book, and then transmit the information to an adjunct device. This technique demonstrates the utility of low-cost materials and processes for robust large area sensing applications and could act as a gateway to pertinent multimedia information.

    Radu Alexandru Sporea (2019)(Invited) Recent Developments in Contact-Controlled Transistors, In: Meeting abstracts (Electrochemical Society)MA2019-01(25)pp. 1256-1256

    An effective route to achieving high gain and low saturation voltage in thin-film transistors is by controlling the current by modulating the conductivity of the source contact area [1-3], potentially at the expense of operating speed. Such devices, called source-gated transistors (SGTs) are easily made using the usual techniques, in a variety of organic and inorganic semiconductor technologies, with the requirement that a potential barrier is reliably engineered at the source electrode. Most often, this barrier results from the presence of a Schottky contact [4], which has been proven beneficial in similar devices [5]. As this type of TFT gains popularity, novel ideas which leverage both the device physics and the specifics of the fabrication process are emerging [6]. In this paper we will discuss recent advances in device architecture and highlight important design decisions specific to the envisaged application. Since the contact injection area plays an important part in this type of devices, it is an essential design parameter. Traditionally, contact overlap should be minimized to reduce capacitance, but in SGTs it may be advantageous to purposely increase gate-to-source overlap. Interesting circuit behaviour results from the sizing of this parameter [7] and could be useful in designing circuit blocks which take advantage of this factor, in contrast with conventional TFT circuit design. This reasoning is especially relevant to pixel circuits where a storage capacitor is required in the circuit between these two electrical nodes: circuit optimization necessarily includes electrical characteristics and layout area. In many applications, the versatility and robustness of these contact-controlled devices outweighs their reduced operating frequency, and their ease of fabrication recommends them for high-gain circuits where uniformity of performance is critical. [1] S. D. Brotherton, Introduction to Thin Film Transistors: Physics and Technology of TFTs, Springer, 453–480, Cham (2013). [2] A. Valletta et al., J. Appl. Phys . 114 064501 (2013). [3] R. A. Sporea et al., Scientific Reports 4 4295 (2014). [4] N. Papadopoulos et al., Proc CICC 2017 (2017). [5] Lee, S. and A. Nathan, Science 354 , 302 (2016). [6] L. Wang et al., Appl. Phys. Lett. 110 , 152105 (2017). [7] J. M. Shannon et al., IEEE Trans. Electron Devices , 60 , 2444–2449 (2013). [8] R. Drury et al., Proc. CAD-TFT 2018 , Shenzhen, China (2018).

    DG Sporea, RA Sporea, C Oproiu, I Vatǎ (2005)Comparative study of gamma-ray, neutron and electron beam irradiated index-guided laser diodes, In: Proceedings of the European Conference on Radiation and its Effects on Components and Systems, RADECS

    The effect of gamma-ray, neutron and electron beam irradiation on index-guided laser diodes was investigated off-line. The laser diodes subjected to this evaluation were AlGaAs, low power (8 mW), single transversal mode lasers emitting in the near-IR. The diodes degradation was assessed up to the total gamma dose of 1.23 MGy, the total electron beam dose of 0.6 MGy, and to the neutron fluence of 1.2 × 1013 n/cm2. The electrical, optical and optoelectronic characteristics were studied. The major irradiation induced changes are related to the embedded photodiode responsivity, the laser threshold current and its external quantum efficiency.

    R. M. I. Bandara, K. D. G. I. Jayawardena, S. O. Adeyemo, S. J. Hinder, J. A. Smith, H. M. Thirimanne, N. C. Wong, F. M. Amin, B. G. Freestone, A. J. Parnell, D. G. Lidzey, H. J. Joyce, R. A. Sporea, S. R. P. Silva (2019)Tin(iv) dopant removal through anti-solvent engineering enabling tin based perovskite solar cells with high charge carrier mobilities, In: Journal of Materials Chemistry C Royal Society of Chemistry

    We report the need for careful selection of anti-solvents for Sn-based perovskite solar cells fabricated through the commonly used anti-solvent method, compared to their Pb-based counterparts. This, in combination with the film processing conditions used, enables the complete removal of unwanted Sn4+ dopants, through engineering the anti-solvent method for Sn-based perovskites. Using a Cs0.05(FA0.83MA0.17)0.95Pb0.5Sn0.5I3 perovskite, charge carrier mobilities of 32 ± 3 cm2 V−1 s−1 (the highest reported for such systems through the optical-pump terahertz probe technique) together with ∼28 mA cm−2 short circuit current densities are achieved. A champion efficiency of 11.6% was obtained for solvent extraction using toluene (an 80% enhancement in efficiency compared to the other anti-solvents) which is further improved to 12.04% following optimised anti-solvent wash and thermal treatment. Our work highlights the importance of anti-solvents in managing defects for high efficiency low bandgap perovskite materials and develops the potential for all-perovskite tandem solar cells.

    RA Sporea, S Georgakopoulos, M Shkunov, JM Shannon, SRP Silva, X Xu, X Guo (2013)Leveraging contact effects for field-effect transistor technologies with reduced complexity and superior current uniformity, In: MRS Online Proceedings Library1553

    In order to achieve high performance, the design of devices for large-area electronics needs to be optimized despite material or fabrication shortcomings. In numerous emerging technologies thin-film transistor (TFT) performance is hindered by contact effects. Here, we show that contact effects can be used constructively to create devices with performance characteristics unachievable by conventional transistor designs. Source-gated transistors (SGTs) are not designed with increasing transistor speed, mobility or sub-threshold slope in mind, but rather with improving certain aspects critical for real-world large area electronics such as stability, uniformity, power efficiency and gain. SGTs can achieve considerably lower saturation voltage and power dissipation compared to conventional devices driven at the same current; higher output impedance for over two orders of magnitude higher intrinsic gain; improved bias stress stability in amorphous materials; higher resilience to processing variations; current virtually independent of source-drain gap, source-gate overlap and semiconductor thickness variations. Applications such as amplifiers and drivers for sensors and actuators, low cost large area analog or digital circuits could greatly benefit from incorporating the SGT architecture.

    RA Sporea, X Guo, JM Shannon, SRP Silva (2009)Effects of process variations on the current in Schottky Barrier Source-Gated Transistors, In: Proceedings of the International Semiconductor Conference (CAS)2pp. 413-416

    The sensitivity of the drain current in Schottky barrier source-gated transistors to process variation is studied using computer simulations. It is shown that provided the device is designed correctly, the current is independent of source-drain separation and is insensitive to source length variations. However, uniform insulator thickness and precise control of the source barrier is needed if good current uniformity is to be obtained.

    X Guo, R Sporea, JM Shannon, SRP Silva (2009)Down-scaling of thin-film transistors: Opportunities and design challenges, In: ECS Transactions22(1)pp. 227-238

    With the ever-increasing demands for integration of advanced electronic functions into large-area electronics, down-scaling of thin-film transistors (TFTs) becomes very necessary. The key device operational issues associated with TFT scaling, including short-channel effects (SCEs) and self-heating, are considered in this paper. Device structure engineering approaches are introduced to suppress the SCEs for designing short-channel TFTs with excellent digital and analog performance. And electro-thermal simulation results show that the self-heating in TFTs will be much more significant than that in silicon metal-oxide-semiconductor field-effect transistors (MOSFETs) due to the substrate of poor thermal conductivity. Enhancing the heat dissipation by placement of metal heat pipe in the cap dielectric layers is proved to be an effective way to deal with the heating issues.

    RA Sporea, W Wright, JM Shannon, SRP Silva (2015)Bulk barrier source-gated transistors with improved drain current dynamic range and temperature coefficient, In: ECS Transactions67(1)pp. 91-96

    © The Electrochemical Society.Source-gated thin-film transistors (SGTs) have remarkable properties related to low-voltage amplification, tolerance to process variation and electrical stability. They rely on a potential barrier at the source in their operation, and usually this barrier is realized through a Schottky contact. Here, we study SGTs with source barriers made by doping the source region of the semiconductor to form bulk unipolar diodes (BUD). A BUSGT can have much higher drain current with a lower activation energy, resulting in higher switching speed and improved transconductance. Barriers made via doping also provide a wider range of barrier heights compared with Schottky contacts. We discuss design parameters for BUSGTs and compare these devices with SBSGTs.

    KDG Imalka Jayawardena, S Li, LF Sam, Christopher Smith, MJ Beliatis, KK Gandhi, MR Ranga Prabhath, TR Pozegic, S Chen, X Xu, DMR Dabera, LJ Rozanski, RA Sporea, Chris Mills, X Guo, S Silva (2015)High efficiency air stable organic photovoltaics with an aqueous inorganic contact, In: Nanoscale(34)pp. 14241-14247 The Royal Society of Chemistry

    We report a ZnO interfacial layer based on an environmentally friendly aqueous precursor for organic photovoltaics. Inverted PCDTBT devices based on this precursor show power conversion efficiencies of 6.8–7%. Unencapsulated devices stored in air display prolonged lifetimes extending over 200 hours with less than 20% drop in efficiency compared to devices based on the standard architecture.

    David M. Frohlich, Emily Corrigan-Kavanagh, Mirek Bober, Haiyue Yuan, Radu Sporea, Brice Le Borgne, Caroline Scarles, George Revill, Jan Van Duppen, Alan W. Brown, Megan Beynon (2019)The Cornwall a-book: An Augmented Travel Guide Using Next Generation Paper, In: The Journal of Electronic Publishing22(1) Michigan Publishing

    Electronic publishing usually presents readers with book or e-book options for reading on paper or screen. In this paper, we introduce a third method of reading on paper-and-screen through the use of an augmented book (‘a-book’) with printed hotlinks than can be viewed on a nearby smartphone or other device. Two experimental versions of an augmented guide to Cornwall are shown using either optically recognised pages or embedded electronics making the book sensitive to light and touch. We refer to these as second generation (2G) and third generation (3G) paper respectively. A common architectural framework, authoring workflow and interaction model is used for both technologies, enabling the creation of two future generations of augmented books with interactive features and content. In the travel domain we use these features creatively to illustrate the printed book with local multimedia and updatable web media, to point to the printed pages from the digital content, and to record personal and web media into the book.

    RA Sporea, MJ Trainor, ND Young, JM Shannon, SRP Silva (2010)Performance improvements in polysilicon source-gated transistors, In: DRC Conference Digestpp. 245-246

    The source-gated transistor (SGT) is a new type of transistor in which the current is controlled by a potential barrier at the source and by a gate which modulates the effective height of the source barrier. It is an ideal device architecture to be used with the low mobility materials typically applied to large area electronics, as it provides low saturation voltages and high output impedances. Furthermore, the high internal fields and low concentration of excess carriers lead to higher speed and better stability compared with FETs, particularly in disordered, low mobility semiconductors. As such, the SGT is especially well suited to thin-film analog circuits.

    RA Sporea, JM Shannon, SRP Silva (2011)High-resolution temperature sensing with source-gated transistors, In: Device Research Conference (DRC), 2011 69th Annualpp. 61-62 IEEE

    Source-gated transistors (SGTs) are three-terminal devices in which the current is controlled by a potential barrier at the source. The gate voltage is used primarily to modulate the effective height of the source barrier. These devices have a number of operational advantages over conventional field-effect transistors, including a potentially much smaller saturation voltage and very low output conductance in saturation, which lead to low power operation and high intrinsic gain.

    EVA BESTELINK, Olivier de Sagazan, Lea Motte, RADU A SPOREA (2021)Suppression of Hot-Carrier Effects Facilitated by the Multimodal Thin-Film Transistor Architecture, In: Advanced Electronic Materials2100533 Wiley

    Hot-carrier effects are a persistent challenge for Ohmic contact, high carrier mobility thin-film transistors. As semiconductor properties are systematically improved, such phenomena (e.g., the kink effect) are becoming apparent even in materials such as InGaZnO. Few of the past solutions are practical in these low-complexity semiconductor systems. Here, it is shown that contact-controlled devices offer robust performance under extreme biasing conditions due to their distinctive charge injection processes. The recently-developed multimodal transistor (MMT) provides further control still, by separate regulation of current flow and magnitude. Internal electric field distributions in the source and drain regions are studied via technology computer-aided design simulations, and support the formulation of operational guidelines for the MMT's channel control gate for optimal characteristics in saturation. As MMT principles are universal, these findings should inform device design and operation in all high carrier mobility material systems.

    EVA BESTELINK, Hao-Jing Teng, RADU A SPOREA (2021)Contact Doping as a Design Strategy for Compact TFT-Based Temperature Sensing, In: IEEE Transactions on Electron Devices Institute of Electrical and Electronics Engineers (IEEE)

    Contact-controlled devices, such as source-gated transistors (SGTs), deliberately use energy barriers at the source, and naturally, the positive temperature dependence (PTD) of drain current can be utilized for temperature sensing. We exploit the difference in drain current activation energy, which arises with contact doping in polysilicon n-type contact-controlled transistors, to demonstrate output current with either a PTD or negative temperature dependence (NTD). The range over which output current varies linearly with temperature, as well as the sensitivity, can be tailored by the choice of reference current magnitude and relative source contact properties within the current mirror. The sensing scheme simplifies the circuit design because it relies solely on thin-film transistors and it has inherent immunity to output voltage variation. This ability to tune the sign of temperature dependence allows facile integration in applications requiring homeostasis via feedback, e.g., electronic skin, in a minimal layout area and potentially with convenient reduction of patterning steps during fabrication.

    EVA BESTELINK, Olivier de Sagazan, Lea Motte, Max Bateson, Benedikt Schultes, S RAVI PRADIP SILVA, RADU A SPOREA (2020)Versatile Thin‐Film Transistor with Independent Control of Charge Injection and Transport for Mixed Signal and Analog Computation, In: Advanced Intelligent Systems

    New materials and optimized fabrication techniques have led to steady evolution in large area electronics, yet significant advances come only with new approaches to fundamental device design. The multimodal thin‐film transistor introduced here offers broad functionality resulting from separate control of charge injection and transport, essentially using distinct regions of the active material layer for two complementary device functions, and is material agnostic. The initial implementation uses mature processes to focus on the device's fundamental benefits. A tenfold increase in switching speed, linear input–output dependence, and tolerance to process variations enable low‐distortion amplifiers and signal converters with reduced complexity. Floating gate designs eliminate deleterious drain voltage coupling for superior analog memory or computing. This versatile device introduces major new opportunities for thin‐film technologies, including compact circuits for integrated processing at the edge and energy‐efficient analog computation.

    EVA BESTELINK, Olivier de Sagazan, Isin Surekcigil Pesch, RADU A SPOREA (2021)Compact Unipolar xnor/xor Circuit Using Multimodal Thin-Film Transistors, In: IEEE Transactions on Electron Devices Institute of Electrical and Electronics Engineers (IEEE)

    A novel compact realization of the xnor/xor function is demonstrated with multimodal transistors (MMTs). The multimodal thin-film transistors (MMT's) structure allows efficient use of layout area in an implementation optimized for unipolar thin-film transistor (TFT) technologies, which may serve as a multipurpose element for conventional and emerging large-area electronics. Microcrystalline silicon device fabrication is complemented by physical simulations.

    Eva Bestelink, Ute Zschieschang, Indrachapa Bandara Rajapakshe Mudiyanselage, Hagen Klauk, Radu A. Sporea (2021)The secret ingredient for exceptional contact-controlled transistors, In: Advanced Electronic Materials Wiley

    Contact-controlled transistors are rapidly gaining popularity. However, simply using a rectifying source contact often leads to unsatisfactory operation, merely as a thin-film transistor with low drain current and reduced effective mobility. This may cause otherwise promising experiments to be abandoned. Here, we analyse data from literature in conjunction with devices we have recently fabricated in polysilicon, organic and oxide semiconductors, highlighting the main factor in achieving good saturation, namely keeping saturation coefficient γ well below 0.3. We also discuss secondary causes of suboptimal electrical characteristics. Correct design of these alternative device structures will expedite their adoption for high gain, low-frequency applications in emerging sensor circuits.

    Radu A. Sporea, Isin Surekcigil Pesch, Eva Bestelink, Olivier de Sagazan, Adnan Mehonic (2021)Multimodal transistors as ReLU activation functions in physical neural network classifiers, In: Scientific Reports Nature Research

    Artificial neural networks (ANNs) providing sophisticated, power-efficient classification are finding their way into thin-film electronics. Thin-film technologies require robust, layout-efficient devices with facile manufacturability. Here, we show how the multimodal transistor's (MMT's) transfer characteristic, with linear dependence in saturation, replicates the rectified linear unit (ReLU) activation function of convolutional ANNs (CNNs). Using MATLAB, we evaluate CNN performance using systematically distorted ReLU functions, then substitute measured and simulated MMT transfer characteristics as proxies for ReLU. High classification accuracy is maintained, despite large variations in geometrical and electrical parameters, as CNNs use the same activation functions for training and classification.

    Eva Bestelink, S. Ravi P. Silva, Radu A. Sporea, Luca Maiolo, Francesco Maita (2019)49dB depletion-load amplifiers with polysilicon source-gated transistors, In: ESSDERC 2019 - 49th European Solid-State Device Research Conference (ESSDERC)2019-pp. 114-117 IEEE

    Two-transistor zero-VGS amplifiers made with polysilicon source-gated transistors achieve voltage gain approaching 300 (49dB). TCAD simulations reveal the effect of load and driver transistor geometry on gain and operating frequency. The SGT circuits have simultaneously superior gain and reduced layout area (two-transistor, channel length L = 3μm and width W = 10 and 30μm), relative to conventional TFT implementations. These results recommend low-complexity, compact SGT designs for flexible and printed amplifiers, such as bio- and chemical sensors.

    Salman Alfarisyi, Patryk Golec, EVA BESTELINK, Kham M. Niang, Andrew J. Flewitt, S Ravi Pradip Silva, Radu A Sporea (2023)Evidence of improved thermal stability via nanoscale contact engineering in IGZO source-gated thin-film transistors, In: IEEE transactions on electron devices IEEE

    Despite rapidly expanding interest in thin-film source-gated transistors (SGTs), the high temperature dependence of drain current (TDDC) in devices comprising Schottky source barriers is delaying wide adoption. To reduce this effect, alternative source designs have been theorized. Specifically, introducing additional nanoscale layers at the source contact should facilitate tunneling of charge carriers at the Fermi energy level with negligible TDDC. Here, we fabricate amorphous In₂Ga₂ZnO₇ tunnel-contact SGTs (TCSGTs) with three times lower TDDC than polysilicon transistors with Schottky contacts. Numerical simulations help elucidate the control mechanisms. We show that the potential profile across the semiconductor in the bulk of the source-gate overlap region determines injection current, and the introduction of a thin interfacial layer at the contact reduces the role of the contact metal work function in current control and associated temperature effects. This device architecture adds improved thermal stability to the long list of SGT benefits, including low voltage saturation, power-efficient operation, high intrinsic gain, device-to-device uniformity, and robustness to mechanical and electrical stress.

    Farjana Haque, Md Mehedi Hasan, Eva Bestelink, Radu A. Sporea, Mallory Mativenga (2023)Composition-Dependent High-Performance Phototransistors Based on Solution Processed CH3NH3PbI3/ZnO Heterostructures, In: Advanced optical materials Wiley

    Recently, hybrid organic-inorganic perovskites have emerged as promising photo-sensing materials for next-generation solution-processed phototransistors, achieving high responsivity, detectivity, and fast response. Here, a phototransistor that can detect visible light using a low-cost, solution processed methylammonium lead iodide/zinc oxide (CH3NH3PbI3/ZnO) heterostructure is reported. While typical ZnO thin-film transistors (TFTs) do not show any photocurrent under visible light illumination, CH3NH3PbI3 (MAPbI(3)) coated ZnO TFTs exhibit substantial photocurrent. Additionally, the optical, morphological, and structural characteristics of the light-absorbing layers are further controlled by altering the precursor ratio of methylammonium iodide and lead (II) iodide (MAI:PbI2), which in turn affects the photosensitivity. Stoichiometric composition (MAI:PbI2 = 1:1) of MAPbI(3) demonstrates optimal characteristics with a responsivity of 234 A/W and a high detectivity of 3.74 x 10(13) Jones under white light illumination. The high photo-responsivity and detectivity result from the combination of the suitable optoelectronic properties of the stoichiometric MAPbI(3) film and its smooth interface with the ZnO channel.

    Raymond Drury, Eva Bestelink, Radu A. Sporea (2019)Simulation study of overlap capacitance in source-gated transistors for current-mode pixel drivers, In: IEEE Electron Device Letters Institute of Electrical and Electronics Engineers (IEEE)

    Contrary to conventional design principles, currentdriven pixel drivers based on source-gated transistors (SGTs) achieve their optimal drive current and speed with a deliberate 5 -10μm gate-source overlap. Total pixel circuit area need not increase, as the additional device area can be compensated by reducing the pixel storage capacitor. Numerical simulations demonstrate the viability of SGTs for emissive pixel drivers and high gain, low power, robust circuits for emerging sensor arrays.

    E. Bestelink, T. Landers, R. A. Sporea (2019)Turn-off mechanisms in thin-film source-gated transistors with applications to power devices and rectification, In: Applied Physics Letters114(18)182103 AIP Publishing

    We describe the physics of the turn-off mechanism in source-gated transistors (SGTs), which is distinct from that of conventional thin-film field-effect transistors and allows significantly lower off currents, particularly in depletion-mode devices. The “n-type” SGT enters its off state when the potential applied across the semiconductor layer is decreased to low positive values or made negative through the applied gate bias, thus impeding charge injection from the source contact. Measurements on polysilicon devices were supported with TCAD simulations using Silvaco Atlas. Alongside the other known benefits of SGTs, including low saturation voltage, tolerance to process variations, and high intrinsic gain, the ability to efficiently block current at high negative gate voltages suggests that these devices would be ideal elements in emerging thin-film power management and rectification circuits.

    E. Bestelink, O. de Sagazan, R. A. Sporea (2021)Engineering current-voltage linearity in TFTs for analog and neuromorphic computing, In: 2021 IEEE INTERNATIONAL FLEXIBLE ELECTRONICS TECHNOLOGY CONFERENCE (IFETC)pp. 0007-0009 IEEE

    Many emerging analog and neuromorphic applications would benefit from a fully linear dependence of a transistor's output on its input for reduced distortion and facile design of linear functions. We show how a new TFT structure, the multimodal transistor, can achieve a linearly dependent drain current in saturation (constant transconductance) with direct proportionality over a large range of input voltages.

    Eva Bestelink, Jean-Charles Fustec, Olivier De Sagazan, Hao-Jing Teng, Radu A. Sporea (2022)Flexible Microcrystalline Silicon Source-Gated Transistors with Negliglible DC Performace Degradation at 2.5 mm Bending Radius, In: 2022 IEEE International Conference on Flexible and Printable Sensors and Systems (FLEPS)pp. 1-4 IEEE

    The first flexible source-gated transistors (SGTs) in microcrystalline silicon have been fabricated and characterized under bending stress. As SGTs are contact controlled devices, the channel does not modulate drain current, however its geometry has implications for operation. We show how reduced channel length in SGTs helps promote negligible threshold voltage shifts when strain is introduced with a radius of r = 2.5 mm.

    Georgios Bairaktaris, Fasihullah Khan, Radu A. Sporea (2022)Inkjet-printed flexible oxide photodetectors for scalable user interfaces, In: 2022 IEEE International Conference on Flexible and Printable Sensors and Systems (FLEPS)pp. 1-4 IEEE

    In today's world of connectivity and automation, novel user interfaces are used to incorporate technology into our lives in a seamless way. Printed electronics provide the tools for making flexible and conformable electronic systems that can augment the functionality of everyday objects. However, compared to conventional, off-the-shelf devices, their performance is insufficient to obtain a functional and robust fully printed system. In this work, we are developing purpose-built photodetectors (PDs) for their use in an augmented paper platform, the Magic Bookmark. The developed PDs exhibit an on/off ratio of 0.2, resulting in a voltage swing of 200 mV, when connected to external circuitry, indicating suitability for the Magic Bookmark within high tolerances due to the readout scheme selected.

    Ming Chen, Boyu Peng, Radu A. Sporea, Vitaly Podzorov, Paddy Kwok Leung Chan (2022)The Origin of Low Contact Resistance in Monolayer Organic Field-Effect Transistors with van der Waals Electrodes, In: Small science2(6)2100115 Wiley

    The successful commercialization of organic field-effect transistors (OFETs) for advanced integrated organic electronics requires reducing device sizes, which inevitably clashes with the constraints imposed by the contact effects. Herein, it is demonstrated that the contact resistance in OFETs based on monolayer organic semiconductors is extremely low, especially at mild biasing conditions. The contributions of the access resistance and the metal-organic interface resistance are successfully disentangled for the first time. It is shown that, contrary to the conventional view, the contact resistance of monolayer OFETs in the saturation regime exhibits a very weak dependence on the source electrode length. In the monolayer OFETs based on 2,9-didecyldinaphtho[2,3-6:2',3'-f]thieno[3,2-b] thiophene (C-10-DNTT), a gate-voltage-independent access resistivity (2.2 10(-2) Omega cm(2)) at V-DS = - 1 mV is obtained, while the interfacial metalorganic Schottky contact resistance is found to be negligible. The depletion of a diode associated with the metal-organic interface expands with increasing V-DS and eventually bottlenecks the device performance. Finally, how to overcome such a carrier depletion contact resistance bottleneck and achieve OFETs with outstanding performance are shown. These findings pave the way toward sophisticated organic electronic applications based on the use of monolayer OFETs.

    Ash Stott, Décio B de Freitas Neto, JM Rosolen, Radu A. Sporea, S. Ravi P. Silva (2021)Exploring the underlying kinetics of electrodeposited PANI-CNT composite using distribution of relaxation times, In: Electrochimica Acta139501

    The Distribution of Relaxation Times (DRT) was successfully demonstrated in the analysis of the impedance spectra of a parametric series of composite polyaniline (PANI) coated carbon nanotubes (CNT) electrodes. DRT was then applied to the measured spectra and polarization processes were separated based on their typical time constants. The main processes were identified, and their contribution quantified by analysing a set of electrodes with PANI deposited under various conditions, forming a parametric set. For the first time we have shown that DRT can be used to identify individual internal processes of complex composite electrodes without a priori knowledge of the system. This method offers a model-free approach for the study of EIS spectra and the characterization of resistive-capacitive systems. This was demonstrated by the optimisation of our PANI coated CNT electrodes which have exhibited a PANI specific capacitance of 772 F g − 1 and rate capability of 76% from 1 to 100 mV s − 1.

    Brice Le Borgne, Bo-Yan Chung, Mehmet O. Tas, Simon G. King, Maxime Harnois, Radu A. Sporea (2019)Eco-Friendly Materials for Daily-Life Inexpensive Printed Passive Devices: Towards “Do-It-Yourself” Electronics, In: Electronics8(6) MDPI

    The need for the fabrication of a new generation of devices has developed with the next generation of ‘home’ engineers, which is resulting in an ever-increasing population interested in “do-it-yourself” electronics and the Internet of Things. However, this new trend should not be done at the expense of the environment. Almost all previous studies, related to the low-temperature processing of devices, fail to highlight the extent of the impact that the synthesis of these technologies have on both the environment and human health. In addition, the substrates typically used, are also often associated with major drawbacks such as a lack of biodegradability. In this paper, we fabricate a simple RC filter using various domestically available printing techniques, utilising readily available materials such as: carbon soots (carbon black) as an electric conductor, and egg white (albumen) as a dielectric. These devices have been fabricated on both polyethylene terephthalate (PET) and paper, which demonstrated the same performances on both substrates and revealed that recyclable substrates can be used without compromise to the devices’ performance. The filter was found to exhibit a cut-off frequency of 170 kHz, which made it suitable for high-frequency reception applications.

    Q Cui, M Si, Radu Sporea, X Guo (2013)Simple Noise Margin Model for Optimal Design of Unipolar Thin-Film Transistor Logic Circuits, In: IEEE TRANSACTIONS ON ELECTRON DEVICES60(5)pp. 1782-1785 IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC
    RA Sporea, JM Shannon, SRP Silva, MJ Trainor, ND Young, X Guo (2011)Performance trade-offs in polysilicon source-gated transistors, In: Solid-State Electronics65-66(1)pp. 246-249

    Self-aligned Schottky-source source-gated transistors (SGTs) have been made in polysilicon. The structures enable a direct comparison to be made between a SGT and a standard thin-film field-effect transistor (FET) on the same device. SGTs having excellent characteristics have been fabricated, with intrinsic gains approaching 10,000. The effects of bulk doping in the polysilicon and of the source barrier modification implant are considered in the context of the electrical output characteristics. It is shown that the choice of source length is a tradeoff between device speed and variations in current output due to variability during fabrication. © 2011 Elsevier Ltd. All rights reserved.

    DG Sporea, C Oproiu, RA Sporea (2004)Degradation of heterojunction laser diodes under electron beam irradiation, In: Proceedings of SPIE - The International Society for Optical Engineering5577(PART 2)pp. 808-817

    We are reporting the investigation on the degradation of heterojunction laser diodes as they were subjected to electron beam irradiation. The research was done under the European Union's Fusion Programme, and targets the possible use of semiconductor lasers for remote sensing and robotics, under irradiation conditions. A total irradiation dose of 600 kGy was achieved at room temperature. The measurements were performed off-line using an automatic measuring set-up. Following each irradiation step, several characteristics of the laser diodes were monitored, as function of the driving current and the case temperature: the emitted optical power, the wavelength of the emitted radiation, the embedded photodiode current, the longitudinal and transversal mode structure, as well as the temporal behavior of all these parameters. For each irradiation dose, the laser diode serial resistance, threshold current, and quantum efficiency, and the photodiode responsivity were plotted for different operating conditions.

    Brice Le Borgne, Siyi Liu, Xavier Morvan, Samuel Crand, Radu Alexandru Sporea, Nanshu Lu, Maxime Harnois (2019)Water Transfer Printing Enhanced by Water‐Induced Pattern Expansion: Toward Large‐Area 3D Electronics, In: Advanced Materials Technologies4(4)1800600pp. 1-9 Wiley

    Perfectly wrapping planar electronics to complex 3D surfaces represents a major challenge in the manufacture of conformable electronics. Intuitively, thinner electronics are easier to conform to curved surfaces but they usually require a supporting substrate for handling. The water transfer printing (WTP) technology utilizes water surface tension to keep ultrathin electronics floating flat without supporting substrate, enabling their conformal transfer on 3D surfaces through a dipping process. In many cases, however, the size of the microfabricated electronics is much smaller than the target 3D surface. This work proposes that such mismatch in size can be overcome by leveraging stretchable electronics in WTP. Stretchable electronics are compliant to in‐plane stretch induced by water surface tension, hence can first self‐expand in water and then be transferred onto 3D objects. Uniaxial and biaxial expansion ranging from 41% to 166% has been achieved without any externally applied tension. The results demonstrate that expansion‐enhanced WTP is a promising fabrication process for conformable electronics on large 3D surfaces.

    RA Sporea, JM Shannon, SRP Silva, MJ Trainor, ND Young (2012)Field plate optimization in low-power high-gain source-gated transistors, In: IEEE Transactions on Electron Devices59(8)pp. 2180-2186 IEEE

    Source-gated transistors (SGTs) have potentially very high output impedance and low saturation voltages, which make them ideal as building blocks for high-performance analog circuits fabricated in thin-film technologies. The quality of saturation is greatly influenced by the design of the field-relief structure incorporated into the source electrode. Starting from measurements on self-aligned polysilicon structures, we show through numerical simulations how the field plate (FP) design can be improved. A simple source FP around 1 μm long situated several tens of nanometers above the semiconductor can increase the low-voltage intrinsic gain by more than two orders of magnitude and offers adequate tolerance to process variations in a moderately scaled thin-film SGT. © 2012 IEEE.

    The paper describes a setup for in situ monitoring of the radiation-induced optical absorption and the radiation-induced luminescence, in the UV-visible spectral range (200-800 nm), for large diameter (400 μm) optical fibers. Silica and sapphire optical fibers were irradiated, at room temperature, with gamma rays (dose rate of 0.33 kGyh, total dose of 34.5 kGy) and protons (dose rate of 100 Gys, total dose of 1.8 MGy). At several moments, the irradiation was interrupted and the annealing of the radiation-induced optical absorption was observed at room temperature. The setup also makes possible the monitoring of the optical radiation-induced recovery of the optical absorption (the effect of photobleaching), as the optical fiber can be exposed at the same time to both the ionizing radiation (gamma or protons) and to the radiation of a broadband optical source. The optical absorption and radioluminescence were measured with an optical fiber multichannel spectrometer coupled to an optical fiber multiplexer. The equipment control as well as the data collection and processing were performed using the graphical programming environment LabVIEW. The paper includes several graphs illustrating the evolution of the optical absorption and radiation-induced luminescence during gamma and proton irradiation of optical fibers. © 2005 American Institute of Physics.

    RA Sporea, T Burridge, SRP Silva (2015)Self-Heating Effects In Polysilicon Source Gated Transistors, In: SCIENTIFIC REPORTS5ARTN 1pp. ?-? NATURE PUBLISHING GROUP
    RA Sporea, MJ Trainor, ND Young, JM Shannon, SRP Silva (2010)Intrinsic gain in self-aligned polysilicon source-gated transistors, In: IEEE Transactions on Electron Devices57(10)pp. 2434-2439

    Thin-film, self-aligned source-gated transistors (SGTs) have been made in polysilicon. The very high output impedance of this type of transistor makes it suited to analog circuits. Intrinsic voltage gains of greater than one thousand have been measured at particular drain voltages. The drain voltage dependence of the gain is explained based on the device physics of the source-gated transistor and the fact that pinch-off occurs at both the source and the drain. The results obtained from these devices, which are far from optimal, suggest that, with proper design, the source-gated transistor is well suited to a wide range of analog applications.

    RA Sporea, MJ Trainor, ND Young, X Guo, JM Shannon, SRP Silva (2011)Performance trade-offs in polysilicon source-gated transistors, In: Solid-State Electronics65-66(1)pp. 246-249 Elsevier

    Self-aligned Schottky-source source-gated transistors (SGTs) have been made in polysilicon. The structures enable a direct comparison to be made between a SGT and a standard thin-film field-effect transistor (FET) on the same device. SGTs having excellent characteristics have been fabricated, with intrinsic gains approaching 10,000. The effects of bulk doping in the polysilicon and of the source barrier modification implant are considered in the context of the electrical output characteristics. It is shown that the choice of source length is a tradeoff between device speed and variations in current output due to variability during fabrication.

    Q Cui, W Liu, X Guo, RA Sporea (2014)Analytical models for delay and power analysis of zero-V load unipolar thin-film Transistor Logic Circuits, In: IEEE Transactions on Electron Devices61(11)pp. 3838-3844 IEEE

    In thin-film transistor (TFT) logic circuit applications, propagation delay and power dissipation are two key constraints to be considered in optimal circuit design and synthesis. The unipolar zero-V-load logic design is widely used for implementation of TFT digital circuits, because of the simple structure, easy processing, and relatively high gain. In this paper, the analytical models for delay and power were developed for zero-V-load inverters, which clarify the relationships between device and design parameters and the two key design constraints. The proposed models were verified by circuit simulations, and could serve as a guideline for optimal design of unipolar zero-V-load logic circuits.

    Radu A. Sporea, Luke J. Wheeler, Vlad Stolojan, S. Ravi P. Silva (2018)Towards manufacturing high uniformity polysilicon circuits through TFT contact barrier engineering, In: Scientific Reports Nature Research

    Abstract The predicted 50 billion devices connected to the Internet of Things by 2020 has renewed interest in polysilicon technology for high performance new sensing and control circuits, in addition to traditional display usage. Yet, the polycrystalline nature of the material presents significant challenges when used in transistors with strongly scaled channel lengths due to non-uniformity in device performance. For these new applications to materialize as viable products, uniform electrical characteristics on large areas will be essential. Here, we report on the effect of deliberately engineered potential barrier at the source of polysilicon thin-film transistors, yielding highly-uniform on-current (

    JM Shannon, RA Sporea, S Georgakopoulos, M Shkunov, SRP Silva (2013)Low-Field Behavior of Source-Gated Transistors, In: IEEE Transactions on Electron Devices60(8)pp. 2444-2449 IEEE

    A physical description of low-field behavior of a Schottky source-gated transistor (SGT) is outlined where carriers crossing the source barrier by thermionic emission are restricted by JFET action in the pinch-off region at the drain end of the source. This mode of operation leads to transistor characteristics with low saturation voltage and high output impedance without the need for field relief at the edge of the Schottky source barrier and explains many characteristics of SGT observed experimentally. 2-D device simulations with and without barrier lowering due to the Schottky effect show that the transistors can be designed so that the current is independent of source length and thickness variations in the semiconductor. This feature together with the fact that the current in an SGT is independent of source-drain separation hypothesizes the fabrication of uniform current sources and other large-area analog circuit blocks with repeatable performance even in imprecise technologies such as high-speed printing.

    RA Sporea, JM Shannon, SRP Silva (2012)Modes of operation and optimum design for application of source-gated transistors, In: ECS Transactions50(8)pp. 65-70

    We show that the source-gated transistor has two distinct modes of operation. In the low-field mode, the current from the reversebiased source barrier is restricted by the depleted semiconductor at the drain end of the source. In the high-field mode, the current depends on field-dependent barrier lowering in the same region of the source. In practice, both these modes usually occur: the former at low VG, the latter at high VG. It is shown that this understanding enables us to design devices in which the current is insensitive to large changes in structure and geometry. © The Electrochemical Society.

    A. S Dahiya, Radu Sporea, G. Poulin-Vittrant, D. Alquier (2019)Stability evaluation of ZnO nanosheet based source-gated transistors, In: Scientific Reports9(1)

    Semiconducting nanostructures are one of the potential candidates to accomplish low-temperature and solution-based device assembly processes for the fabrication of transistors that offer practical solutions toward realizing low-cost flexible electronics. Meanwhile, it has been shown that by introducing a contact barrier, in a specific transistor configuration, stable device operation can be achieved at much reduced power consumption. In this work, we investigate both one-dimensional ZnO nanowires (NWs) and two-dimensional nanosheets (NSs) for high performance and stable nano-transistors on conventional Si/SiO2 substrates. We have fabricated two variant of transistors based on nanoscale single-crystalline oxide materials: field-effect transistors (FETs) and source-gated transistors (SGTs). Stability tests are performed on both devices with respect to gate bias stress at three different regimes of transistor operation, namely off-state, on-state and sub-threshold state. While in the off-state, FETs shows comparatively better stability than SGTs devices, in both sub-threshold and on-state regimes of transistors, SGTs clearly exhibits better robustness against bias stress variability. The present investigation experimentally demonstrates the potential advantages of SGTs over FETs as driver transistor for AMOLEDs display circuits which require very high stability in OLED driving current.

    AS Dahiya, C Opoku, RA Sporea, B Sarvankumar, G Poulin-Vittrant, F Cayrel, N Camara, D Alquier (2016)Single-crystalline ZnO sheet Source-Gated Transistors, In: Scientific Reports619232pp. ?-? Nature Publishing Group

    Due to their fabrication simplicity, fully compatible with low-cost large-area device assembly strategies, source-gated transistors (SGTs) have received significant research attention in the area of high-performance electronics over large area low-cost substrates. While usually based on either amorphous or polycrystalline silicon (α-Si and poly-Si, respectively) thin-film technologies, the present work demonstrate the assembly of SGTs based on single-crystalline ZnO sheet (ZS) with asymmetric ohmic drain and Schottky source contacts. Electrical transport studies of the fabricated devices show excellent field-effect transport behaviour with abrupt drain current saturation (IDSSAT) at low drain voltages well below 2 V, even at very large gate voltages. The performance of a ZS based SGT is compared with a similar device with ohmic source contacts. The ZS SGT is found to exhibit much higher intrinsic gain, comparable on/off ratio and low off currents in the sub-picoamp range. This approach of device assembly may form the technological basis for highly efficient low-power analog and digital electronics using ZnO and/or other semiconducting nanomaterial.

    RA Sporea, JM Shannon, SRP Silva, M Trainor, N Young (2015)Temperature effects in complementary inverters made with polysilicon source-gated transistors, In: IEEE Transactions on Electron Devices62(5)pp. 1498-1503 IEEE

    Through their high gain and low saturation voltage, source-gated transistors (SGTs) have applications in both analog and digital thin-film circuits. In this paper, we show how we can design SGT-based logic gates, which are practically unaffected by temperature variations. We discuss design characteristics, which ensure reliable operation in spite of SGT temperature dependence of drain current, and their implications for manufacturability and large signal operation.

    RA Sporea, X Guo, JM Shannon, SRP Silva (2011)Source-Gated Transistors for Versatile Large Area Electronic Circuit Design and Fabrication, In: Y Kuo, G Bersuker (eds.), ECS Transactions37(1)pp. 57-63 The Electrochemical Society

    Source-gated transistors (SGTs) comprise a blocking contact or potential barrier at the source, which control the current. The paper describes how SGTs can be optimized for particular applications and for specific semiconductor material systems. It is shown how the saturation voltage can be designed to be an order of magnitude smaller than in equivalent FETs to give power savings of over 50% for the same current output. The SGT also achieves a better saturation regime, with lower output conductance over a larger range of drain voltages. Flat-panel lighting, remote sensing and signal processing and large-area circuits made using inexpensive but imprecise patterning techniques are some of the applications which could benefit from incorporating these devices.

    RA Sporea, X Guo, JM Shannon, SRP Silva (2010)Polysilicon source-gated transistors for mixed-signal systems-on-panel, In: ECS Transactions33(5)pp. 419-424 Electrochemical Society

    The performance benefits of using source-gated transistors (SGTs) in analog large-area electronic circuits are examined practically and via numerical simulations. In current mirror circuits made using thin-film technology, significant advantages are observed for SGT implementations. A comparison of current mirrors implemented with standard field effect transistors (FETs) and SGTs shows that the SGT version can operate at a lower voltage and has larger output dynamic range for a given device geometry. The results are explained in relation to the saturation mechanisms of the SGT and are supported by experimental measurements of polysilicon devices.

    RA Sporea, MJ Trainor, ND Young, JM Shannon, SRP Silva (2014)Source-gated transistors for order-of-magnitude performance improvements in thin-film digital circuits, In: SCIENTIFIC REPORTS4ARTN 4pp. ?-? NATURE PUBLISHING GROUP
    RA Sporea, M Overy, JM Shannon, SRP Silva (2015)Temperature dependence of the current in Schottky-barrier source-gated transistors, In: JOURNAL OF APPLIED PHYSICS117(18)ARTN 1pp. ?-? AMER INST PHYSICS
    RA Sporea, S Lygo-Baker (2016)Summer Research Placements - State-of-the-Art Science by pre-University Students, In: MRS AdvancesMRSF15-232

    Summer research placements are an effective training and research tool. Over three years, our group has hosted nine pre-university students over periods of four to six weeks. Apart from student training and skills acquisition, the placements have produced several peer-reviewed technical publications. Our approach relies on careful pre-planning of activities, frequent student interaction, coupled with independent and group learning. We explore the advantages and disadvantages of this manner of running summer placements.

    Radu A. Sporea, Kham M. Niang, Andrew J. Flewitt, S. Ravi P. Silva Silva (2019)Novel Tunnel-Contact-Controlled IGZO Thin-Film Transistors with High Tolerance to Geometrical Variability, In: Advanced Materials Wiley

    For the first time, thin insulating layers are used to modulate a depletion region at the source of a thin-film transistor. Bottom contact, staggered electrode transistors fabricated using RFsputtered IGZO as the channel layer, with a 3 nm ALD Al2O3 layer between the semiconductor and Ni source-drain contacts show behaviours typical of source-gated transistors (SGTs): low saturation voltage (VD_SAT ~ 3V), change in VD_SAT with gate voltage of only 0.12 V/V and flat saturated output characteristics (small dependence of drain current on drain voltage). The transistors show high tolerance to geometry variations: saturated current changes only 0.15x for channel lengths between 2 - 50 μm, and only 2x for sourcegate overlaps between 9 - 45 μm. A higher than expected (5x) increase in drain current for a 30K change in temperature, similar to Schottky-contact SGTs, underlines a more complex device operation than previously theorised. Optimizations for increasing intrinsic gain and reducing temperature effects are discussed. These devices complete the portfolio of contactcontrolled transistors, comprising devices with: Schottky contacts, bulk barrier or heterojunctions, and now, tunnelling insulating layers. The findings should also apply to nanowire transistors, leading to new low-power, robust design approaches as large-scale fabrication techniques with sub-nanometre control mature.

    Barbara Salonikidou, Takeda Yasunori, Brice Le Borgne, Jonathan England, Tokito Shizuo, Radu Sporea (2019)Toward Fully Printed Memristive Elements: a-TiO2 Electronic Synapse from Functionalized Nanoparticle Ink., In: ACS Applied Electronic Materials1(12)pp. 2692-2700 American Chemical Society

    Electronic devices that emulate biofunctionalities, such as synaptic plasticity, present a promising route to versatile and energy-efficient neuromorphic computing systems. As the demand for rapid prototyping and environmentally friendly fabrication of such devices rises, there are significant incentives toward finding solutions for low-cost materials and flexible deposition techniques. The development of printed electronic devices is still at an infant stage, presenting a timely opportunity to investigate material robustness and routes to overcoming fabrication obstacles toward fully printed electronic synapses. In this work, a low-power, fully printed Ag (200 nm)/a-TiO2 (80 nm)/Ag (160 nm) memristive device is demonstrated. The first electrical characterization of early devices exhibits biomimetic properties with an indication of activity-dependent plasticity. The active material is derived from a simplified nanoparticle ink formulation developed in-house. The ink characterization confirms that the formulation fulfills the criteria for efficient jetting while exhibiting a dwell time of 4 months. Additionally, the common detrimental fabrication issues of layer cracks and control over uniformity here are both overcome. The ink optimization and the investigation of the electrical framework under which the memristive element responds synaptically present a favorable approach to alternative fabrication methods for future neuromorphic electronics.

    Eva Bestelink, Hao-Jing Teng, Ute Zschieschang, Hagen Klauk, Radu A. Sporea (2023)Extraordinarily Weak Temperature Dependence of the Drain Current in Small-Molecule Schottky-Contact-Controlled Transistors through Active-Layer and Contact Interplay, In: Advanced electronic materials9(3)pp. 2201163-n/a Wiley

    Low saturation voltages and extremely high intrinsic gain can be achieved in contact-controlled thin-film transistors (TFTs) with staggered device architecture, enabled by the energy barrier introduced at the source contact. The resulting device, the source-gated transistor (SGT), is limited in its usefulness by the high temperature dependence of the drain current induced by the source energy barrier. Here, the interaction between the thermal characteristics of the source contact and the semiconductor to show drastically reduced temperature dependence for SGTs based on organic semiconductors (OSGTs) is exploited. This extraordinarily weak temperature dependence of the drain current is observed regardless of the height of the source energy barrier (27.8% in OSGTs with Ti contacts compared to 22.1% when using Au contacts, over a 34 K range). The reduction in mobility of the semiconductor offsets an increase in thermionic-field emission of charge carriers at the source. This is a first for SGTs and provides a route to removing one of the last hurdles to their wider adoption. The OSGTs with Ti contacts also demonstrate: drain-current saturation at very low drain-source voltages (saturation factor of 0.22); noteworthy stability after 70 days; and minimal drain-current variation with channel length or illumination.

    Georgios Bairaktaris, David M. Frohlich, Radu A. Sporea (2021)Flexible optical bookmark for open-page recognition in augmented paper, In: 2021 IEEE International Conference on Flexible and Printable Sensors and Systems (FLEPS)pp. 1-4 IEEE

    We demonstrate a flexible electronic bookmark designed to facilitate the augmentation of printed content with media-rich, dynamic digital content, thereby extending the utility of ordinary books. The system, produced by the hybrid integration of photosensors, detects the currently open page within a book via gestures that are natural to the book reading experience. Physical cut-outs within the page or high contrast patterns printed in graphical ink define a unique code, which is optically read by the functional bookmark.

    Yudai Hemmi, Yuji Ikeda, Radu A. Sporea, Satoru Inoue, Tatsuo Hasegawa, Hiroyuki Matsui (2023)Printed 700 V/V Gain Amplifiers Based on Organic Source-Gated Transistors with Field Plates, In: Advanced electronic materials9(6) Wiley

    Printed amplifiers are promising components for flexible and wearable devices. The circuits need to have small footprint, high amplification, and low power consumption, which is not simultaneously possible with conventional thin-film transistors because multiple stages are required and introduce sources of variability, failure, or wasted area. Source-gated transistors (SGTs) can in principle have extremely high intrinsic gain but this is not always the case because of lateral field control problems. A good solution is the use of field plate structures, which are straightforward to implement with top contacts but not so with bottom contacts. Here, a printed implementation of organic SGT with a field plate is presented, which can achieve a record high intrinsic gain of 920 V/V. The single-stage common-source amplifier with two organic SGTs also exhibits a high gain of 700 V/V. Simulations confirm the effectiveness of the printed field plate, making it a promising approach for future flexible and wearable electronics.

    Radu A. Sporea (2022)Tuning electrolyte-gated transistors to order, In: Nature electronics5(12)pp. 836-837 NATURE PORTFOLIO

    By controlling ion-dynamic capacitance, electrolyte-gated transistors can be switched between different operating modes, providing flexible neural network implementations.

    Yudai Hemmi, Yuji Ikeda, Radu A. Sporea, Yasunori Takeda, Shizuo Tokito, Hiroyuki Matsui (2022)N-Type Printed Organic Source-Gated Transistors with High Intrinsic Gain, In: Nanomaterials (Basel, Switzerland)12(24) Mdpi

    Source-gated transistors (SGTs) are emerging devices enabling high-gain single-stage amplifiers with low complexity. To date, the p-type printed organic SGT (OSGT) has been developed and showed high gain and low power consumption. However, complementary OSGT circuits remained impossible because of the lack of n-type OSGTs. Here, we show the first n-type OSGTs, which are printed and have a high intrinsic gain over 40. A Schottky source contact is intentionally formed between an n-type organic semiconductor, poly{[N,N '-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5 '-(2,2 '-bithiophene)} (N2200), and the silver electrode. In addition, a blocking layer at the edge of the source electrode plays an important role to improve the saturation characteristics and increase the intrinsic gain. Such n-type printed OSGTs and complementary circuits based on them are promising for flexible and wearable electronic devices such as for physiological and biochemical health monitoring.

    Xiaojun Guo, Yong Xu, Simon Ogier, Tse Nga Ng, Mario Caironi, Andrea Perinot, Ling Li, Jiaqing Zhao, Wei Tang, Radu A. Sporea, Ahmed Nejim, Jordi Carrabina, Paul Cain, Feng Yan (2017)Current Status and Opportunities of Organic Thin-Film Transistor Technologies, In: IEEE transactions on electron devices64(5)pp. 1906-1921 IEEE

    Attributed to its advantages of super mechanical flexibility, very low-temperature processing, and compatibility with low cost and high throughput manufacturing, organic thin-film transistor (OTFT) technology is able to bring electrical, mechanical, and industrial benefits to a wide range of new applications by activating nonflat surfaces with flexible displays, sensors, and other electronic functions. Despite both strong application demand and these significant technological advances, there is still a gap to be filled for OTFT technology to be widely commercially adopted. This paper provides a comprehensive review of the current status of OTFT technologies ranging from material, device, process, and integration, to design and system applications, and clarifies the real challenges behind to be addressed.

    Hyuna Lee, Yeo Eun Kim, Jisuk Bae, Sungyeop Jung, Radu A. Sporea, Chang-Hyun Kim (2023)High-Performance Organic Source-Gated Transistors Enabled by the Indium-Tin Oxide-Diketopyrrolopyrrole Polymer Interface, In: ACS applied materials & interfaces15(8)pp. 10918-10925 Amer Chemical Soc

    Source-gated transistors are a new driver of low-power high-gain thin-film electronics. However, source-gated transistors based on organic semiconductors are not widely investigated yet despite their potential for future display and sensor technologies. We report on the fabrication and modeling of high-performance organic source-gated transistors utilizing a critical junction formed between indium-tin oxide and diketopyrrolopyrrole polymer. This partially blocked hole-injection interface is shown to offer both a sufficient level of drain currents and a strong depletion effect necessary for source pinch-off. As a result, our transistors exhibit a set of outstanding metrics, including an intrinsic gain of 160 V/V, an output resistance of 4.6 G Omega, and a saturation coefficient of 0.2 at an operating voltage of 5 V. Drift-diffusion simulation is employed to reproduce and rationalize the experimental data. The modeling reveals that the effective contact length is significantly reduced in an interdigitated electrode geometry, eventually contributing to the realization of low-voltage saturation.

    Ash Stott, Mehmet O. Tas, Elaine Y. Matsubara, Mateus G. Masteghin, Jose M. Rosolen, Radu A. Sporea, S. Ravi P. Silva (2020)Exceptional rate capability from carbon‐encapsulated polyaniline supercapacitor electrodes, In: Energy & environmental materials (Hoboken, N.J.)3(3)pp. 389-397

    A high‐rate capability carbon‐encapsulated polyaniline (PANI) composite is fabricated by a novel electrodeposition method of polyaniline on a carbon nanotube (CNT) forest, grown on carbon paper. This is followed by coating of an amorphous carbon layer via hydrothermal carbonization (HTC) of glucose, forming a three‐layer structure. We demonstrate that a slow scan rate, voltage‐restricted electrodeposition process can be used to produce a uniform PANI coating on individual CNTs throughout the network. The CNT forest structure offers excellent electronic and structural connection for the PANI nanofiber network, while the coating of amorphous carbon reduces electrode resistance, promoting enhanced electrochemical performance and reinforced structural stability during charging and discharging. The as‐prepared CNT/PANI/HTC composite exhibited a high specific capacitance of 571 F g−1 at 1 A g−1, and 557 F g−1 at 100 A g−1,whilst demonstrating a record rate capability of 98% capacitance retention, when the current density is increased 100‐fold. This advanced rate performance indicates that a slow electrodeposition process produces an electrochemically stable three‐layer composite with enhanced diffusion kinetics. Hence, the method developed in this work establishes further control on the electrochemical deposition of energy storage materials, for high‐rate capability. Carbon‐encapsulated PANI structures show an exceptional high‐rate capability of 98% for current density of 1 A g−1 to 100 A g−1. Low scan rate electrodeposition of PANI is used to synthesize conformal coatings on CNT forests, followed by a coating of an amorphous carbon layer, to enhance energy storage performance.

    Barbara Salonikidou, Adnan Mehonic, Yasunori Takeda, Shizuo Tokito, Jonathan England, Radu A Sporea (2022)Inkjet-Printed Ag/a-TiO2/Ag Neuromorphic Nanodevice Based on Functionalized Ink, In: Advanced engineering materials Wiley

    This study aims to contribute to the burgeoning field of brain-inspired computing by expanding it beyond conventional fabrication methods. Herein, the obstacles toward the effective inkjet printing process are encountered and the electrical characteristics are explored, providing new insights into the reliability aspects of fully printed Ag/a-TiO2/Ag electronic synapses. The versatility of the approach is further enhanced by the highly stable in-house-developed a-TiO2 ink, exhibiting optimal shelf life of five months and repeatable jetting, producing layers with nanoscale thickness resolution. Most importantly, device electrical characterization reveals synaptic dynamics, leading to activity-dependent conductance state retention and adaptation characteristics, implying inherent learning capabilities. The synaptic dynamics are attained by solely adjusting the duty cycle of the applied pulsed voltage trigger, while keeping amplitude and polarity fixed, a method readily compatible with realistic applications. Furthermore, I-V analysis demonstrates a dynamic range dependence on a-TiO2 layer thickness and conduction mechanism that is akin to the conventionally developed electronic TiO2 synapses. The developed devices provide a time- and cost-effective ecologically benign alternative toward biomimetic signal processing for future flexible neural networks.

    Georgios Bairaktaris, Brice Le Borgne, Vikram Turkani, Emily Corrigan-Kavanagh, David M. Frohlich, Radu A. Sporea (2022)Augmented Books: Hybrid Electronics Bring Paper to Life, In: IEEE pervasive computing IEEE

    In today's digital world, paper's reason of being is challenged. Yet, studies suggest that books and paper-based objects have advantages ranging from the tactile sensation to information retention and indexing. We have developed a hybrid electronic device, the "a-book," that offers access to up-to-date and pertinent multimedia content as part of the ordinary interaction with a typical hardcover book. The device maintains the look and feel of a conventional book and is connected to the web through an adjunct smart device. Here, we provide a technical project summary of the electronic system for book augmentation. We outline the system's functionality and discuss its manufacturability, prospects, and limitations in the context of current and emerging flexible electronics technologies.

    Georgios Bairaktaris, Hristo Siderov, Deniz Celebiler, Corinne Kolii, David M. Frohlich, Radu A. Sporea (2021)Magic Bookmark: A Nonintrusive Electronic System for Functionalizing Physical Books, In: Advanced Intelligent Systems2100138 Wiley

    The use of physical paper is often preferred due to its unique physical properties that improve various aspects of reading. However, digital media and information are more engaging, diverse, and up to date, thereby challenging the existence of paper in our everyday life. By combining the two types of media in a seamless way, the interactivity of multimedia content can complement the reading experience, maintaining the unique feel of paper books. The current state of the art addressing this application negatively impacts the reading experience and often does not consider the manufacturability and sustainability of the proposed solutions. In this work we are introducing the Magic Bookmark, a technical solution for automatically recognizing the open page of a physical book, to provide seamless augmentation without changing the user’s behavior and experience significantly. Our implementation consists of three alternative solutions, with various degrees of ease of use, manufacturability at scale and reliability of data reading. The optimal realization is found to be a reflective optical readout array, for which we propose routes to implementation that may allow blending the graphical and functional aspects of the augmented book.

    R. A. Sporea, P. J. B. Jackson, S. Lygo-Baker (2017)Developing undergraduate teaching materials in collaboration with pre-university students, In: MRS Advances2pp. 1713-1719 Materials Research Society

    In this project we have involved four high-achieving pre-university summer placement students in the development of undergraduate teaching materials, namely tutorial videos for first year undergraduate Electrical and Electronic Engineering lab, and computer simulations of didactic semiconductor structures for an Electrical Science first year compulsory taught module. Here we describe our approach and preliminary results.

    RA Sporea, JM Shannon, SRP Silva, MJ Trainor, ND Young (2010)Performance trade-offs in polysilicon source-gated transistors, In: Proceedings of the European Solid State Device Research Conference, ESSDERC 2010pp. 222-225

    Self-aligned Schottky-source source-gated transistors (SGTs) have been made in polysilicon. The structures enable a direct comparison to be made between a SGT and a standard thin-film field-effect transistor (FET) on the same device. SGTs having excellent characteristics have been fabricated, with intrinsic gains approaching 10,000. The effects of bulk doping in the polysilicon and of the source barrier modification implant are considered in the context of the electrical output characteristics. It is shown that the choice of source length is a tradeoff between device speed and current uniformity.

    C Opoku, R Sporea, V Stolojan, R Silva, M Shkunov (2015)Si Nanowire - Array Source Gated Transistors

    Solution processed field-effect transistors based on single crystalline silicon nanowires (Si NWs) with metal Schottky contacts are demonstrated. The semiconducting layer was deposited from a nanowire ink formulation at room temperature. The devices with 230nm thick SiO2 gate insulating layers show excellent output current-voltage characteristics with early saturation voltages under 2 volts, constant saturation current and exceptionally low dependence of saturation voltage with the gate field. Operational principles of these devices are markedly different from traditional ohmic-contact field-effect transistors (FETs), and are explained using the source-gated transistor (SGT) concept in which the semiconductor under the reverse biased Schottky source barrier is depleted leading to low voltage pinch-off and saturation of drain current. Device parameters including activation energy are extracted at different temperatures and gate voltages to estimate the Schottky barrier height for different electrode materials to establish transistor performance - barrier height relationships. Numerical simulations are performed using 2D thin-film approximation of the device structures at various Schottky barrier heights. Without any adjustable parameters and only assuming low p-doping of the transistor channel, the modelled data show exceptionally good correlation with the measured data. From both experimental and simulation results, it is concluded that source-barrier controlled nanowire transistors have excellent potential advantages compared with a standard FET including mitigation of short-channel effects, insensitivity in device operating currents to device channel length variation, higher on/off ratios, higher gain, lower power consumption and higher operational speed for solution processable and printable nanowire electronics.

    RA Sporea, JM Shannon, SRP Silva, X Guo (2017)Source-gated transistors for improved current-mode pixel drivers

    Emissive displays require high-efficiency linear drivers which are stable under electrical stress and can deliver uniform performance across a large area. Owing to their low saturation voltage and flat saturation characteristic, source-gated transistors (SGTs) are ideally suited to act as power-efficient driving transistors in active matrix backplanes for lighting, low-power signage and display screens. It is shown that SGTs are also very stable during electrical stress. The technology is compatible with standard TFT fabrication allowing FET and SGT devices to be integrated in the same design and fabrication run.

    RA Sporea, AS Alshammari, S Georgakopoulos, J Underwood, M Shkunov, SRP Silva (2013)Micron-scale inkjet-assisted digital lithography for large-area flexible electronics, In: 2013 PROCEEDINGS OF THE EUROPEAN SOLID-STATE DEVICE RESEARCH CONFERENCE (ESSDERC)pp. 280-283 IEEE
    RA Sporea, JM Shannon, SRP Silva (2010)Properties of source-gated transistors in polysilicon, In: 6th Conference on Ph.D. Research in Microelectronics and Electronics, PRIME 2010

    This paper describes some of the performance characteristics of self-aligned polysilicon Schottky Source- Gated Transistors (SGTs) made on glass by laser annealing of amorphous silicon. The threshold and Schottky barrier height are tuned by varying the dose of dopants in the bulk and under the source respectively. These devices are well suited for analog applications owing to their low saturation voltage, low drain field dependence of the current and intrinsic gain which is in excess of 1000 for well designed structures. Double drain operation leads to fT ≈100MHz for non-optimized devices. Index Terms— Source-Gated Transistor, polysilicon, analog

    RA Sporea, M Trainor, N Young, JM Shannon, SRP Silva (2015)Temperature Effects in Complementary Inverters Made With Polysilicon Source-Gated Transistors, In: Electron Devices, IEEE Transactions onPP(99)pp. 1-1 IEEE

    Through their high gain and low saturation voltage, source-gated transistors (SGTs) have applications in both analog and digital thin-film circuits. In this paper, we show how we can design SGT-based logic gates, which are practically unaffected by temperature variations. We discuss design characteristics, which ensure reliable operation in spite of SGT temperature dependence of drain current, and their implications for manufacturability and large signal operation.

    GEORGIOS BAIRAKTARIS, DAVID MARK FROHLICH, RADU A SPOREA (2021)Printed Light Tags and the Magic Bookmark: Using light to augment paper objects, In: CHI '21 Extended Abstracts Association for Computing Machinery (ACM)

    Paper books offer a unique physical feel, which supports the reading experience through enhanced browsing, bookmarking, freeform annotations, memory and reduced eye strain. In contrast, electronic solutions, such as tablets and e-readers, offer interactive links, updatable information, easier content sharing, and efficient collaboration. To combine the best aspects of paper and digital information for reading, we demonstrate two mechanisms for augmenting paper with light sensors that trigger digital links on a nearby smartphone. Light Tags on every page of a book are used in a first demonstration to identify which pages are open. These are replaced with an electronic Magic Bookmark in a second demonstration, avoiding the need to instrument every page.

    DG Sporea, RA Sporea (2003)Integrated software package for laser diodes characterization, In: Proceedings of SPIE - The International Society for Optical Engineering5227pp. 464-471

    The characteristics of laser diodes (wavelength of the emitted radiation, output optical power, embedded photodiode photocurrent, threshold current, serial resistance, external quantum efficiency) are strongly influenced by their driving circumstances (forward current, case temperature). In order to handle such a complex investigation in an efficient and objective manner, the operation of several instruments (a laser diode driver, a temperature controller, a wavelength meter, a power meter, and a laser beam analyzer) is synchronously controlled by a PC, through serial and GPIB communication. For each equipment, instruments drivers were designed using the industry standards graphical programming environment - Lab VIEW from National Instruments. All the developed virtual instruments operate under the supervision of a managing virtual instrument, which sets the driving parameters for each unit under test. The manager virtual instrument scans as appropriate the driving current and case temperature values for the selected laser diode. The software enables data saving in Excel compatible files. In this way, sets of curves can be produced according to the testing cycle needs.

    Emily Corrigan-Kavanagh, David Frohlich, Miroslaw Bober, Radu Sporea, Alan Brown, George Revill, Haiyue Yuan, Megan Brown, Jan Van Duppen, Caroline Scarles (2020)A-photobooks: Bridging the Gap between Virtual and Material Worlds

    Despite the rise of digital photography, physical photos remain significant. They support social practices for maintaining social bonds, particularly in family contexts as their handling can trigger emotions associated with the individuals and themes depicted. Also, digital media can be used to strengthen the meaning of physical objects and environments represented in the material world through augmented reality, where such are overlaid with additional digital information that provide supplementary sensory context to topics conveyed. This poster therefore presents initial findings from the development of augmented photobooks to create ‘a-photobooks’, printed photobooks that are augmented by travellers with additional multimedia of their trip using a smartphone-based authoring tool. Results suggest a-photobooks could support more immersive engagement and reminiscing of holidays encounters, increasing cognitive, and emotional effects of associated experiences.