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AlGaN/GaN HFET's were cooled to ~85 K and irradiated to a fluence of 1012 n/cm2 (1 MeV equivalent) and gate currents measured. The observed increased gate leakage current was studied by curve-fitting a 4-parameter thermionic trap assisted tunneling model to the experimental measurements. The model parameters were constrained and a least-squares fitting routine was applied to best fit the gate current's voltage and temperature dependence. The results were used to provide an interpretation of the physical changes to the HFET gate following neutron irradiation. The fitting showed that Schottky barrier lowering plays a minor role in post irradiation gate leakage, and trap density and mean trap energy were the dominant post irradiation gate leakage parameters. The ~25% increase in trap density and ~7 mV increase in mean trap energy indicated that neutron elastic collisions produce additional non-native defects within the AlGaN gate region.