The thermal design of vertically integrated multichip modules (MCMs) based on GaN high electron mobility transistor (HEMT) power amplifiers (PAs) on SiC substrates with back-side heat-sink/antenna and Si modulators bonded to the common ground plane and PA chip using polydimethylsolixane (PDMS) is reported. The heat transfer in the integrated structure was estimated using finite element simulation for different PA power density, HEMT gate finger pitch, Si thickness, the presence or absence of the PDMS layer, and the thickness of dielectric isolation interlayers. The maximum temperature in the integrated antenna approach occurs near the gates of the HEMTs and hence the gate pitch has a strong effect on the temperature distribution. The presence of the PMDS has a major effect on the operating temperature of the PA and Si modulator, especially at high power densities, and also influences the temperature distribution within the MCM. (c) 2006 The Electrochemical Society.