Abstract Experiments on removing high heat fluxes from GaN-on-SiC semiconductor dies using microchannel coolers are described. The dies contain an AlGaN/GaN heterostructure operated as a direct current resistor, providing a localized heat source. The active dimensions of the heat source are sized to represent the spatially-averaged heat flux that would appear in microwave power amplifiers. A wide variety of microchannel materials and configurations are investigated, allowing a comparison of performance and the resulting GaN temperatures. Silicon and AlN microchannel coolers exhibit good performance at lower power densities (1000–1200 W/cm 2 over 3 × 5 mm 2 to 2 × 5 mm 2 active areas). Polycrystalline chemical vapor deposited (CVD) SiC microchannel coolers are found to be extremely promising for higher power densities (3000–4000 W/cm 2 over 1.2 × 5 mm 2 active areas with 120 °C GaN temperature). A hybrid microchannel cooler consisting of low-cost CVD diamond on polycrystalline CVD SiC exhibits moderately better performance (20–30%) than polycrystalline CVD SiC alone.