A finite-element simulation was used to quantitatively estimate the heat transfer in a three-dimensional multichip module (MCM) consisting of a GaN power amplifier with solder-bump-bonded Si modulator and integrated antenna on a high-resistivity SiC substrate under various conditions of power density and substrate and epi thicknesses via wire thickness and effective heat transfer coefficient. The maximum temperature in the integrated-antenna approach occur-red in the center of the MCM. At a GaN power amplifier power level of 3 W/mm, a steady-state temperature of similar to 125 degrees C was reached in similar to 20 s. Bulk GaN substrates were also found to provide good thermal transfer characteristics, while sapphire produced an increase in temperature almost a factor of 3 higher than for SiC. At a power density of 10 W/mm, the steady-state operating temperature was similar to 400 degrees C even with SiC substrates. (c) 2006 American Vacuum Society.