Dr Christian Matei

For reliability predictions, gallium nitride transistors require accurate estimations of the peak operating temperatures within the device. This paper presents a new application of thermoreflectance-based temperature measurements performed on a gallium nitride high electron mobility transistor. The submicron spatial and nanosecond temporal resolutions of the measurement system enables for the first time, the dynamic temperature measurement of a transistor operating up to 5 MHz. The GaN transistor is first biased in class-A and excited with a 1 MHz AC signal to demonstrate the dynamic temperature measurement. The transistor is then incorporated in a 20–40 V DC/DC boost converter to measure the dynamic temperature distributions across the semiconductor die operating under real loading conditions at 1 and 5 MHz switching frequencies. This technique captures the temperature variations that occur during the switching of the transistor and the recorded peak temperatures are 7.4

The ever-increasing power density of semiconductors used in monolithic microwave integrated circuits (MMICs) and power amplifiers (PAs) mandates good thermal management and accurate temperature measurements are needed to study device reliability, assess temperature gradients, and to validate nonlinear electrothermal model performance. Thermoreflectance based temperature measurements have been recently applied to high-power microwave semiconductors. The technique determines the temperature by measuring the change in reflectance from a surface due to changing in the index of refraction of the sample. Since the measurement technique uses visible light, thermoreflectance imaging can achieve a spatial resolution of 290 nm and sub-nanosecond temporal resolution over a wide field of view. This makes the method well suited to study the thermal dynamics within these transistors. This presentation will review the capabilities of thermoreflectance measurement, explain the theory behind the thermoreflectance phenomenon, and demonstrate the usefulness of the method and the insights that can be gained by examining several recent measurements.