We report record high breakdown voltages up to 340 and 230 V realized on unintentionally doped (1.5 &mgr;m gate length) and Si doped (1 &mgr;m gate length) AlGaN/GaN modulation doped field effect transistors (MODFETs), respectively. The devices also have large transconductances up to 140 mS/mm and a full channel current of 150–400 mA/mm. The Si doped MODFET sample demonstrated a very high room temperature mobility of 1500 cm2/Vs. With these specifications, GaN field effect transistors as microwave power devices are practical. ©1996 American Institute of Physics.