This paper proposes a method of repairing interface defects by supercritical nitridation technology,in order to suppress the threshold voltage shift of AlGaN/GaN metal-insulator-semiconductor high-electron-mobility transistors (MIS-HEMTs).We find that supercritical NH3 fluid has the characteristics of both liquid NH3 and gaseous NH3 simultaneously,i.e.,high penetration and high solubility,which penetrate the packaging of MIS-HEMTs.In addition,NH2-produced via the auto coupling ionization of NH3 has strong nucleophilic ability,and is able to fill nitrogen vacancies near the GaN surface created by high temperature processes.After supercritical fluid treatment,the threshold voltage shift is reduced from 1 V to 0V,and the interface trap density is reduced by two orders of magnitude.The results show that the threshold voltage shift of MIS-HEMTs can be effectively suppressed by means of supercritical nitridation technology.