Thermal characterization of GaN-on-diamond substrates for HEMT applications

High-power operation of AlGaN/GaN high-electron-mobility transistors (HEMTs) requires efficient heat removal through the substrate. GaN composite substrates including high-thermal-conductivity diamond are promising, but high thermal resistances at the interfaces between the GaN and diamond can offset the benefit of a diamond substrate. We report on measurements of the thermal resistances at the GaN-diamond interfaces for two generations (1^st and 2^nd) of GaN-on-diamond substrates using a combination of picosecond time-domain thermoreflectance (TDTR) and nanosecond transient thermoreflectance (TTR) techniques. Two flipped-epitaxial samples are presented to determine the thermal resistances of the AlGaN/AlN transition layer. For the 2^nd generation samples, electrical heating and thermometry in nanopatterned metal bridges confirms the TDTR results. This paper demonstrates that the latter generation samples, which reduce the AlGaN thickness by 75%, result in a strongly-reduced thermal resistance between the GaN and diamond. Further optimization of the GaN-diamond interfaces should provide an opportunity for improved cooling of HEMT devices.