Abstract
Diamond is an attractive substrate candidate for GaN high-electron-mobility transistors (HEMT) to enhance heat dissipation due to its exceptional thermal conductivity. However, the thermal boundary resistance (TBR) at the GaN-diamond interface poses a significant bottleneck to heat transport, exacerbating self-heating and limiting device performance. In this work, TCAD simulations were employed to systematically investigate the effects of thermal boundary layer (TBL) thickness (d(TBL)) and thermal conductivity (κ(TBL)) on the electrothermal behavior of GaN-on-diamond HEMTs. Results show that increasing the TBL thickness (5-20 nm) or decreasing its thermal conductivity (0.1-1.0 W/(m·K)) leads to elevated hotspot temperatures and degraded electron mobility, resulting in a notable deterioration of I-V characteristics. The nonlinear dependence of device performance on κ(TBL) is attributed to Fourier's law, where heat flux is inversely proportional to thermal resistance. Furthermore, the co-analysis of substrate thermal conductivity and interfacial quality reveals that interface TBR has a more dominant impact on device behavior than substrate conductivity. Remarkably, devices with low thermal conductivity substrates and optimized interfaces can outperform those with high-conductivity substrates but poor interfacial conditions. These findings underscore the critical importance of interface engineering in thermal management of GaN-diamond HEMTs and provide a theoretical foundation for future work on phonon transport and defect-controlled thermal interfaces.