Metal-insulator-semiconductor (MIS) diodes with Si3N4/Al2O3 bilayer gate dielectric films deposited on an AlGaN/GaN heterostructure were fabricated, where the Si3N4 layer played a role of etching stopped layer to protect the Al2O3 film from being damaged. Compared with traditional Schottky diodes, a distinct suppression of gate leakage current was achieved for the MIS diodes both at forward and reverse bias, and the dominant leakage current mechanism is Fowler–Nordheim tunneling. The 2DEG density extracted from C-V curves was 3~7 ́1013cm-2, in the same order of magnitude as Schottky diodes and hall measurement. Although the existence of the bilayer dielectric did not affect the 2DEG density at the interface of AlGaN/GaN, Si3N4 layer shared more gate bias and led to more gate bias required to deplete 2DEG and turn down the devices, moreover, Si3N4 layer had no effect on suppressing the forward or reverse gate leakage current due to its narrow band gap width and band bending compared with a single Al2O3 film. The experimental results provided a reference for the design of gate dielectric film structure for AlGaN/GaN high-electron-mobility transistors (HEMTs).
