Thermal behavior in multi-layered semiconductor structures like the one present in a Gallium Nitride high electron mobility transistor (GaN-HEMT) is investigated for the heat-flow towards the substrate. A compact analytical model is formulated to predict the peak as well as depth dependent temperature in the structure. Modeling results are verified against detailed three-dimensional TCAD thermal simulations of GaN-on-Si structures with GaN layer thickness of 3μm and Si substrate thickness of 100μm. Excellent modeling accuracy is observed for different heat source geometries and power dissipation. In addition, the proposed model is extended to estimate the static thermal coupling factor in multifinger transistors with high level of accuracy. © 2020 IEEE.