AlGaN-GaN high electron mobility transistors (HEMTs) are most suitable for commercial and military applications requiring high voltage, high power, and high efficiency operation. In recent years, leading AlGaN HEMT manufacturers have reported encouraging reliability of these devices, but their long-term reliability especially in the space environment still remains a major concern. In addition, degradation mechanisms in AlGaN HEMT devices are still not well understood, and a large number of traps and defects present both in the bulk and at the surface lead to undesirable characteristics. Study of reliability and radiation effects of AlGaN-GaN HEMTs is therefore necessary before GaN HEMT technology is successfully employed in satellite communication systems. For the present study, we investigated electrical characteristics of AlGaN-GaN HEMTs and AlGaN Schottky diodes irradiated with protons. We studied two types of MOCVD-grown AlGaN HEMTs on semi-insulating SiC substrates (HEMT-1 and HEMT-2) as well as MOCVD-grown Al0.27Ga 0.73N Schottky diodes on conducting SiC substrates. Our HEMT-1 structure consisted of a GaN cap, AlGaN/AlN barrier, and 2 μm GaN buffer layers. Our HEMT-2 structure consisting of undoped AlGaN barrier and GaN buffer layers grown on an AlN nucleation layer showed a charge sheet density of ~1013/cm2 and a Hall mobility of ~1500 cm2 /V.sec. Our HEMT-1 devices had a Pt-Au Schottky gate length of 0.2 μm, a total gate width of 200-400 μm periphery, and SiNx passivation. Electrical characteristics of AlGaN-GaN HEMTs and AlGaN Schottky diodes were compared before and after they were proton irradiated with different energies and fluences. Current-mode deep level transient spectroscopy (DLTS) and capacitance-mode DLTS were employed to study pre-proton irradiation trap characteristics in the AlGaN-GaN HEMTs and AlGaN Schottky diodes, respectively. Focused ion beam (FIB) was employed to prepare both cross-sectional and plan view TEM samples for defect analysis using a high resolution TEM. In addition, electrical characteristics of GaAs MESFETs used as reference devices were compared before and after they were proton irradiated.