MgO/GaN metal oxide semiconductor diodes were irradiated with 40 MeV protons at a fluence of 5 x 10(9) cm(-2), simulating long-term (10 yr) exposure in space-born applications. The result of the proton irradiation was a decrease in device capacitance, consistent with the creation of deep electron traps that reduce the effective channel doping and also a decrease in breakdown field from similar to10(6) V cm(-1) in control devices to 0.76 x 10(6) V cm(-1) in devices irradiated with the gate metal in place. The capacitance of the device irradiated with the contacts in place recovers to the same value as the contact diodes. The D-it values are decreased by the H-2 anneal in both the unirradiated and irradiated devices. (C) 2002 The Electrochemical Society.