GaN/AlGaN epilayers were grown on semi-insulating silicon carbide by MOCVD and fabricated into high electron mobility transistors (HEMTs) with 0.45-mum gate lengths. Standard 0.25-mm HEMTs with an Al molar fraction of 16% demonstrated an extremely high power-added efficiency of 60% and 3.7 W/mm when measured under deep Class AB conditions at 10 GHz. High power densities were maintained even on HEMTs with multi-mm gate peripheries, where a 2-mm-wide device has demonstrated a total output power of 8.3 W (4.15 W/mm) at 10 GHz with a PAE of 33% when measured under Class AB conditions. HEMTs with gate peripheries of 6-mm were demonstrated for the first time, which exhibited typical saturated drain currents of 5.2 A and extrinsic transconductances of 1 S.