The authors report the proton energy dependence of the degradation of AlGaN/GaN high electron mobility transistors (HEMTs) with GaN cap layers from 5 to 15 MeV at a fixed dose of 5 × 1015 cm-2. All the samples degraded after proton irradiation. However, higher damage in dc electrical properties was observed at lower proton energies. Saturation currents at VDS = 6 V and VGS = 0 V reduced by 47% after proton irradiation at 5 MeV energy, but the reduction was less by 25% and 9% at 10 and 15 MeV, respectively. Similar trends were observed in other electrical properties [transconductance (gm) and gate leakage currents]. This energy dependence from 5 to 15 MeV can be explained by the energy-dependent penetration depth of the proton. Protons with higher kinetic energy can penetrate deeper while creating less numbers of defects at shallow depths where the active layers of the HEMTs are located. These results are in good agreement with stopping and range of ions in matter results. The optimization of the AlGaN/GaN HEMT structure will be critical for space-borne applications where high fluxes of protons are encountered.