Working at high RF power leads gallium nitride (GaN) high electron mobility transistors (HEMT) to self-heating that poses a limit to device performances and reliability. Thermal characterization is therefore of great value for proper design of heat dissipation and also for device reliability studies. The peak power dissipation in HEMT devices is located in the channel near the gate edge, which is typically buried under a field plate. This hot spot is thus inaccessible to direct temperature measurement. Therefore, any method used for temperature measurement has to be augmented by a thermal simulation to calculate the actual temperature in the channel from temperature measured elsewhere. In this work, we used thermal imaging to measure temperature during device operation in various pulsed RF conditions. To obtain the hot spot temperature, we developed a thermal simulation of AlGaN/GaN HEMT transistor on SiC substrate. The simulation estimates the channel temperature using 3D finite element method with multi-parameter input. To calibrate the simulation, we compared simulation results with IR images of a 2 mm AlGaN/GaN HEMT transistor operating at various pulsed RF conditions. The simulation is typically slow. In this work, we used the calibrated simulation to study the hot spot temperature as a function of the working conditions and formulated an approximated equation for the thermal behavior of the transistor as a function of power dissipation, base plate temperature, pulse width, and pulse duty cycle that may be used to estimate the channel temperature in real time.
- IR imaging
- thermal model