Neutron irradiation effects were compared for AlGaN/AlN/GaN high electron mobility transistor (HEMT) structures with Al composition in the AlGaN barrier ranging from 20% to 50%, “standard” Al0.25Ga0.75N/GaN HEMTs and for InAlN/GaN HEMTs with InAlN barrier lattice matched to GaN (17% In in the barrier). These samples were exposed to fast reactor neutrons with average energy ∼2 MeV and fluence of 1–3 × 1015 cm-2. The main effect of irradiation was the decrease of two-dimensional electron gas (2DEG) mobility and a positive shift in the threshold voltage corresponding to 2DEG depletion in capacitance–voltage characteristics. For the highest fluences, there was a decrease in both 2DEG concentration and accumulation capacitance, with the effect being strongest for AlGaN/AlN/GaN HEMTs with the highest Al composition and for InAlN/GaN HEMTs. The results correlate with the increase in concentration of deep negatively charged traps in the AlGaN or InAlN barrier with neutron dose. For applications in which tolerance to neutron irradiation is a concern, current high Al content AlGaN/AlN/GaN HEMTs and also InAlN/GaN structures are less radiation hard than low-Al-composition AlGaN/AlN/GaN and Al0.25Ga0.75N/GaN HEMTs.