Thermal characteristics of GaN-on-diamond HEMTs: Impact of anisotropic and inhomogeneous thermal conductivity of polycrystalline diamond

The CVD growth of polycrystalline diamond (PCD) on GaN is a novel and effective approach to improve heat dissipation for high power GaN-based transistors. However, the growth-induced spatial variation in PCD's thermal conductivity makes it difficult to assess the thermal characteristics of GaN-on-diamond devices using bulk diamond properties. By including the depth-dependent anisotropic thermal conductivity of PCD, a finite element thermal simulation is used to study the heat spreading process within a device. The effective thermal conductivity (κ _eff ) of the PCD substrate as “seen” by the device is found to be lower than the average thermal conductivity (κ _avg ) of PCD, suggesting that the low thermal conductivity diamond film during the initial growth has a significant impact on the device thermal resistance. Moreover, κ _eff is investigated as a function of the thermal boundary resistance between GaN and diamond, the gate pitch of the device as well as the grain boundary conductance and the grain evolution rate of diamond polycrystals. The results provide reliable thermal assessment for the PCD substrate as used in a GaN-on-diamond device and have important implications for thermally optimizing the device layout and engineering the thermal conductivity of PCD heat spreaders for devices.