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This paper presents finite-element thermo-mechanical simulation studies of microchannel-based techniques to cool AlGaN/GaN high electron mobility rf transistors grown on SiC substrates. A number of problems are considered, including standard thickness dies on both oxygen-free-high-conductivity (OFHC) copper and AlN microchannel coolers, as well as thinned dies on a hybrid diamond/silicon microchannel cooler. The active device sizes and cooling strategies selected are relevant to X-band (∼10 GHz) amplifiers dissipating 50-100 W of steady-state waste heat. The effects of die attach materials on device temperature and mechanical stresses are studied. The plastic yielding behaviors of the die attach material and other metallic portions of the package are incorporated into the analysis. The removal of 100 W of steady-state waste heat in an example X-band compatible device is found to be consistent with 140-185°C maximum transistor junction temperatures and tolerable mechanical stresses.