Thermal Interface Resistance Measurements for GaN-on-Diamond Composite Substrates

The performance of high-power gallium nitride (GaN) high-electron-mobility transistors (HEMTs) is limited by self-heating effects. High thermal resistances within micrometers of the active device junction often dominate the junction temperature rise and fundamentally limit the device power handling capability. The use of high-thermal-conductivity diamond in close proximity to the transistor junction can mitigate this thermal constraint, but careful attention is required to the quality of the thermal interface between the GaN and diamond. Here we apply time-domain thermoreflectance (TDTR) to two GaN-on-diamond composite substrates with varying GaN thicknesses to measure the thermal interface resistance between the GaN and diamond (29 m² K GW^-1) as well as the thermal conductivity of the GaN buffer layer (112 W m^-1 K^-1) at room temperature. Informed by these data, we perform finite-element analysis to quantify the relative impact of the GaN-diamond thermal interface resistance, diamond substrate thermal conductivity, and a convective cooling solution on the device channel temperature rise.