High power-added efficiency microwave power amplifier results are reported for AllnAs/GalnAs on InP HEMTs operated at relatively low power supply voltages (2.5-3V). C-band power amplifiers are reported with power-added efficiencies as high as 67%, and output powers between 200 and 300 mW. This excellent performance at low power supply voltages is attributed to the high gain and low access resistances of the devices, which leads to a high drain efficiency despite the low power supply voltage. © 1993, The Institution of Electrical Engineers. All rights reserved.