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In this paper, SiN<i>x</i> film deposited by plasma-enhanced chemical vapor deposition was employed as a gate dielectric of AlGaN/GaN high electron mobility transistors (HEMTs). We found that the NH₃ flow during the deposition of SiN<i>x</i> can significantly affect the performances of metal–insulator–semiconductor (MIS) HEMTs. Compared to that without using NH₃ flow, the device with the optimized NH₃ flow exhibited three orders of magnitude lower gate leakage current, two orders of magnitude higher ON/OF drain current ratio, and an increased breakdown field by 69%. In addition, an in situ N₂ plasma surface treatment prepared prior to SiN<i>x</i> deposition can further improve DC performances of MIS-HEMTs to a very low gate leakage current of 10⁻⁹ mA/mm and a high ON/OFF drain current ratio up to 10⁹ by reducing the interface state density. These results demonstrate the great potential for using PECVD-SiN<i>x</i> as a gate dielectric in GaN-based MIS-HEMTs.