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 C higher compared with conventional measurement and simulation approaches. Index Terms-Thermoreflectance measurement, boost converter, gallium nitride, power transistor.

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We present for the first time a measurement system that is capable of directly detecting and identifying the physical location of an oscillation within RF and microwave power amplifiers. The method uses a combined external electrooptic, non-linear vector network analyzer, and vector load-pull measurement system, which allows the measurement of crossfrequency phase-coherent multi-harmonic vector electric fields above the transistor with an 8 µm spatial resolution and 20 MHz-40 GHz bandwidth. Raster scans above the amplifier allow the time-domain electric fields to be animated and superimposed on top of the amplifier image enabling immediate identification of any oscillations by direct inspection. The method is first demonstrated on a low power amplifier composed of two parallel 0.1-W pHEMT transistors that is intentionally designed to have an odd-mode oscillation. The applicability of the method is further demonstrated by measuring and animating in-package parametric odd-mode oscillations within a 260-W laterally diffused metal-oxide-semiconductor (LDMOS) transistor operating at 2.2 GHz under pulsed RF conditions with 10 µs pulses and 10% duty cycle. The measurement and identification technique is applicable to all semiconductor devices as the external electric field is non-invasively measured above the amplifier.

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