AlGaN/GaN high electron mobility transistors (HEMTs) with GaN cap layers were irradiated with high-energy (17 MeV) protons to doses up to 2 x 10(16) cm(-2). There was no significant degradation in dc electrical parameters such as drain-source current (I-DS) and extrinsic transconductance (gm) of the HEMTs up to a fluency of 7.2 x 10(13) protons/cm(2). At the highest dose of 2 x 10(16) protons/cm(2) there was a decrease of 43% in I-DS and a 29% decrease in g(m). The data are consistent with the introduction of defect centers in the HEMT structure by the high-energy protons leading to a reduction in carrier concentration and mobility in the two-dimensional electron gas channel. These results show that AlGaN/GaN HEMTs are attractive for space-based applications where high-energy proton fluxes are present. (C) 2008 The Electrochemical Society.