Thermal Spreading Resistance in Ballistic-Diffusive Regime for GaN HEMTs

Abstract

To develop an efficient thermal design for gallium-nitride (GaN) high-electron-mobility transistors (HEMTs) that usually hold a super-high-power density, it is essential to accurately predict the junction temperature. In GaN HEMTs, the heat transfer process is dominated by thermal spreading resistance. Moreover, the phonon mean free paths (MFPs) of GaN are comparable with the channel layer thickness and the heat spot width. Thus, a ballistic effect emerges, resulting in the invalidity of Fourier’s heat conduction law. Therefore, Fourier’s law-based thermal resistance model should be reexamined and modified for this case. In this paper, we used the phonon Monte Carlo (MC) method to investigate the thermal spreading resistance in a ballistic-diffusive regime for GaN HEMTs. Our simulation results indicate that the ballistic effect significantly altered the temperature distributions within channel layers and resulted in a dramatic increase in the thermal resistance when compared with Fourier’s law-based predictions. Furthermore, a semiempirical thermal resistance model with a fitting parameter was derived. This model can accurately address the issues of thermal spreading and the ballistic effect. This paper can provide a more in-depth understanding of the thermal spreading resistance in a ballistic-diffusive regime, and it can be useful for the prediction of junction temperatures and for the thermal management of HEMTs.