AlGaN/GaN high electron mobility transistors with either MgO or Sc2O3 surface passivation were irradiated with 40 MeV protons at a dose of 5 x 10(9) cm(-2). While both forward and reverse bias current were decreased in the devices as a result of decreases in channel doping and introduction of generation-recombination centers, there was no significant change observed in gate lag measurements. By sharp contrast, unpassivated devices showed significant decreases in drain current under pulsed conditions for the same proton dose. These results show the effectiveness of the oxide passivation in mitigating the effects of surface states present in the as-grown structures and also of surface traps created by the proton irradiation. (C) 2003 Published by Elsevier Science Ltd.