The effect of GaN epilayer thickness on the near-junction thermal resistance of GaN-on-diamond devices

•We investigate the effect of the GaN epilayer thickness and thickness-dependent thermal conductivity on the thermal resistance of GaN power transistors heteroepitaxially integrated with diamond (GaN-on-diamond).•A typical GaN epilayer thickness of 1 µm can provide a fairly low device thermal resistance, when compared to those at other thicknesses.•We emphasize the use of the thickness-dependent, in-plane thermal conductivity of the GaN epilayer for an accurate thermal simulation of GaN devices. Gallium nitride (GaN) heteroepitaxially integrated with diamond (GaN-on-diamond) is promising for high-power electronics due to the excellent heat spreading capability of diamond. A number of past works have examined the thermal properties of GaN-on-diamond devices, particularly the diamond thermal conductivity and the thermal boundary resistance (TBR) between the GaN and diamond, as well as the impact of these two properties on the thermal resistance of GaN-on-diamond devices. Much less investigated, however, is the effect of the thickness of the GaN epilayer on the thermal resistance of GaN-on-diamond devices. Here, we examine this effect through combining finite element simulations with calculations using a semi-classical phonon transport model. The latter considers phonon scattering on defects and interfaces and is utilized here to predict the in-plane thermal conductivity of the GaN epilayer versus layer thickness. This aims at considering the thermal spreading resistance within the GaN in a more accurate manner, which also depends on the layer thickness. Our simulation results indicate that with increasing GaN layer thickness the device thermal resistance monotonically decreases until it reaches the minimum at GaN thicknesses of ∼3.6 and ∼5.8 μm for GaN/diamond TBRs of 6.5 and 30 m2 K GW–1, respectively. A typical GaN thickness of 1 µm can provide a fairly low device thermal resistance, approximately 6 and 20% higher than the minimum thermal resistances for GaN/diamond TBRs of 6.5 and 30 m2 K GW–1, respectively. Reducing the GaN thickness below 1 µm can substantially increase the device thermal resistance, particularly when the GaN/diamond TBR is high, predominantly due to the increasing contribution of the interface. Increasing the GaN thickness above 1 µm can decrease the device thermal resistance but this change is not significant. Our findings presented here can offer a more in-depth understanding of near-junction thermal transport in GaN-on-diamond trans