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In this Letter, the effects of trap states in AlN/GaN superlattice channel HEMTs (high electron mobility transistors) under total ionizing dose with γ-irradiation have been systematically investigated. After 1 Mrad γ-irradiation with a dose rate of 50 rad/s, negative drifts in threshold voltage and C–V characteristics are observed. Simultaneously, the two-dimensional electron gas sheet density of the upper channel increases from 5.09 × 1012 to 5.47 × 1012 cm−2, while that of the lower channel decreases from 4.41 × 1012 to 3.86 × 1012 cm−2, respectively. Furthermore, frequency-dependent capacitance and conductance measurements are adopted to investigate the evolution of trap states in an electron channel. The trap state density (DT = 0.21–0.88 × 1013 cm−2 eV−1) is over the ET range from 0.314 to 0.329 eV after irradiation for the upper channel, while the trap state in the lower channel decreases from 4.54 × 1011 cm−2 eV−1 at ET = 0.230 eV to 2.38 × 1011 cm−2 eV−1 at ET = 0.278 eV. The density (1.39–1.54 × 1011 cm−2 eV−1) of trap states with faster τT (0.033–0.037 μs) generated in a lower channel is located at shallower ET between 0.227 and 0.230 eV. The results reveal the mechanism of trap states in the channel, affecting the performance of HEMTs, which can provide a valuable understanding for hardening in space radiation. [ABSTRACT FROM AUTHOR]