AlInN/GaN heterostructure field-effect transistors (HFETs) grown on silicon withstand irradiation with 75-MeV sulfur ions up to fluences of 5.5 × 10¹³ ions/cm². The static transistor operation characteristics of the devices exhibit a shift of the threshold voltage and a decrease in the saturation and the OFF-state current. Microphotoluminescence spectroscopy reveals a decrease in the electron carrier density in the channel region. Simulations were performed to model the damage caused to the devices assuming the generation of acceptor-like defects upon irradiation. It turns out that the degradation depends on the thickness of the buffer layer. Therefore, we propose the reduction in the thickness of the buffer layer as a way to increase the radiation tolerance of HFETs.