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 AIN microchannel coolers exhibit good performance at lower power densities (1000-1200W/cm(2) over 3 x 5 mm 2 to 2 x 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 x 5 mm(2) active areas with 120 degrees 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. (c) 2007 Elsevier Ltd. All rights reserved.