  We will describe the thermal performance of power semiconductor module, which
consists of hetero-junction bipolar transistors (HBTs), for mobile
communication systems. We calculate the thermal resistance between the HBT
fingers and the bottom surface of a multi-layer printed circuit board (PCB)
using a finite element method (FEM). We applied a steady state analysis to
evaluate the influence of design parameters on thermal resistance of the
module. We found that the thickness of GaAs substrate, the thickness of
multi-layer circuit board, the thermal conductivity of bonding material under
GaAs substrate, and misalignment of thermal vias between each layer of PCB are
the dominant parameter in thermal resistance of the module.
