Accurate estimation of thermal properties for AlGaN/GaN HEMTs on diamond

For accurate estimation of the thermal characteristics of AlGaN/GaN high electron mobility transistors on diamond, the anisotropic thermal conductivities of polycrystalline diamond (PCD) and GaN with their thickness dependences are included in the finite element simulation model, of which the PCD thermal conductivity kPCD is calculated through detailed analyses of the grain sizes in the directions parallel and vertical to the interface and relative phonon-grain boundary scatterings. In addition, the GaN thermal conductivity kGaN is the result of an approximate solution of the phonon Boltzmann transport equation. To investigate the effects of anisotropic GaN and PCD thermal conductivity, four cases of thermal conductivity models are compared: anisotropic kGaN and kPCD, anisotropic kGaN with constant kPCD, constant kGaN with anisotropic kPCD, and constant kGaN and kPCD. Then the maximum channel temperature simulated from the four above-mentioned models and the errors relative to the case of anisotropic kGaN and kPCD are investigated with respect to thermal boundary resistance, GaN layer thickness, gate pitch, heat source width, and power dissipation. The results show that the combination of anisotropic kPCD and constant kGaN overestimates the maximum channel temperature rise, and using constant kPCD and anisotropic kGaN underestimates the maximum channel temperature rise for almost all the simulations in the text. However, the channel temperature stemming from constant kPCD and kGaN is generally closer to the case with anisotropic PCD and GaN thermal conductivity.