GaN HEMT Junction Temperature Dependence on Diamond Substrate Anisotropy and Thermal Boundary Resistance

Abstract

A thermal model of Gallium Nitride High Electron Mobility Transistors (GaN HEMTs) examines the impact of diamond substrate parameters on device thermal performance. These parameters include substrate thickness, GaN-substrate thermal boundary resistance (TBR), and a simplified anisotropic substrate thermal conductivity. Diamond substrates appear to only provide thermal improvement over GaN-on-Silicon Carbide (SIC) devices when the corresponding GaN-on-diamond TBR is not substantially larger than the GaN-on-SiC range, independent of the degree of substrate anisotropy. The reduced lateral conductivity due to substrate anisotropy also proves to be of less significance to substrate thermal resistance when vertical conductivity is very high, but decreased spreading in the substrate can significantly impact cold plate temperature rise. Finally, for any degree of anisotropy, substrates thinner than 150μm are shown to significantly increase cold plate temperature rise as they restrict heat spreading and impose higher heat fluxes to downstream components.