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The characteristic energies of traps in InAlN/AlN/GaN high-electron mobility transistor structures on Si(111) substrates formed after irradiation with 75 MeV S-ions is studied by means of c-lattice parameter analysis, vertical IV-characteristics, micro-photoluminescence (µ-PL), photocurrent (PC) and thermally stimulated current (TSC) spectroscopy. From the lattice parameter analysis, point defect formation is concluded as the dominant source of defects upon irradiation. A strong compensation effect mani-fests itself through enhanced resistivity of the devices as found in vertical IV- measure-ments. The defect formation is detected optically by an additional PL-band within the green spectral region while defect states with threshold energies at 2.9 eV and 2.65 eV were observed by PC spectroscopy. TSC spectra exhibit two defect-related emissions between 300 K and 400 K with thermal activation energies of 0.78-0.82 eV and 0.91-0.98 eV, respectively. The data further supports the formation of Ga vacancies (VGa) and related complexes acting mainly as acceptors compensating the originally undoped n-type GaN buffer layers after irradiation.