AlN/GaN high electron mobility transistors (HEMTs) were irradiated with 5 MeV protons at fluences from 2x10(11) to 2x10(15) protons/cm(2). Changes from 10% to 35% of the saturation drain current and the source-drain resistances were observed for the HEMTs exposed to the proton irradiations due to radiation-induced carrier scattering and carrier removal. Both forward and reverse bias gate currents were increased after proton irradiation and affected the drain current modulation in the positive gate bias voltage range. There was almost no gate-lag observed for the HEMT exposed to 2x10(11) protons/cm(2) irradiation and minimal changes for the higher doses, which implied that few surface traps were created by the high energy proton irradiation. (C) 2010 American Vacuum Society. [DOI: 10.1116/1.3482335]