AbstractWe have carried out a systematic study of the effects of irradiation on the electronic and optical properties of InGaN alloys over the entire composition range. High energy electrons, protons, and ⁴He⁺ were used to produce displacement damage doses (D_d) spanning over five orders of magnitude. The free electron concentrations in InN and In-rich InGaN increase with D_d and finally saturate after a sufficiently high D_d. The saturation of carrier density is attributed to the formation of native donors and the Fermi level pinning at the Fermi Stabilization Energy (E_FS), as predicted by the amphoteric native defect model. Electrochemical capacitance-voltage (ECV) measurements reveal a surface electron accumulation whose concentration is determined by pinning at E_FS.