MgO or Sc2O3 passivated AlGaN/GaN high electron mobility transistors (HEMTs) were irradiated with 40 MeV protons at a dose equivalent to similar to10 years in low-earth orbit. Very little change in drain-source current, transconductance, or diode ideality factor was observed under these conditions, but the reverse breakdown increased due to a decrease in channel electron density. In addition, no significant change was observed in the drain-source current under pulsed conditions, indicating that the proton irradiation did not alter the effectiveness of the MgO and Sc2O3 in passivating surface states. (C) 2003 The Electrochemical Society.