Transient simulations and theoretical modeling of near-junction heat conduction in GaN-on-diamond HEMT

Under high-power working conditions, accurate predictions of the maximum temperature and thermal resistance distribution in devices during transient states are essential for monitoring the health status of the devices. In this work, the heat conduction characteristics of GaN-on-diamond high electron mobility transistors (HEMTs) near the junction region under transient conditions with thermal impedance are systematically investigated with the aid of finite element simulations and thermal impedance analyses. The effects of pulse characteristics, thermal properties of different layers on the maximum temperature and thermal resistance of the device are discussed specifically. The results show that pulse characteristics have a significant effect on the maximum temperature of the device. The shorter the pulse switching time is, the higher the maximum temperature will be. Regarding the effects of the thickness of the GaN layer, the maximum temperature initially decreases and then increases, with the lowest maximum temperature observed at 1.7 mu m. As the TBR increases uniformly from 10 to 40 m2K/GW, the temperature increases by approximately 4 % per 10 m2K/GW. The analyses on the thermal resistance show that the GaN layer occupies the largest proportion of thermal resistance, while the diamond substrate occupies the smallest. The optimal substrate thickness is obtained by analyzing the thermal penetration depth in the substrate for working conditions with various pulse widths. For diamond substrate, the optimal thickness at a pulse width of 5 mu s is about 25 mu m, and increases as the increase in pulse width. At the end, a modified Cauer RC model is proposed for predicting the transient maximum temperature based on heat spreading analysis. The difference between the maximum temperatures predicted by the model and the finite element simulations is within 8 %.