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Frequency-dependent conductance measurements were carried out to investigate the trap states induced by reactive ion etching in AlGaN/GaN high-electron-mobility transistors (HEMTs) quantitatively. For the non-recessed HEMT, the trap state density decreases from 2.48 × 1013 cm−2·eV−1 at an energy of 0.29 eV to 2.79 × 1012 cm−2·eV−1 at ET = 0.33 eV. In contrast, the trap state density of 2.38 × 1013–1.10 × 1014 cm−2·eV−1 is located at ET in a range of 0.30–0.33 eV for the recessed HEMT. Thus, lots of trap states with shallow energy levels are induced by the gate recess etching. The induced shallow trap states can be changed into deep trap states by 350 °C annealing process. As a result, there are two different types of trap sates, fast and slow, in the annealed HEMT. The parameters of the annealed HEMT are ET = 0.29–0.31 eV and DT = 8.16 × 1012–5.58 × 1013 cm−2·eV−1 for the fast trap states, and ET = 0.37–0.45 eV and DT = 1.84 × 1013 − 8.50 × 1013 cm−2·eV−1 for the slow trap states. The gate leakage currents are changed by the etching and following annealing process, and this change can be explained by the analysis of the trap states. [ABSTRACT FROM AUTHOR]