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The gate leakage reduction mechanism of AlGaN/GaN metal-insulator-semiconductor (MIS) heterostructure field-effect transistors (HFETs) is investigated and compared with those of three types of HFET, namely; a conventional HFET, a standard MIS-HFET and a specially prepared MIS-HFET with a metal interlayer. It is found that the resistance of the AlGaN layer with an insulator deposited on its surface is much higher than that of the AlGaN layer with a metal. From the fitting of transconductance-frequency characteristics, the resistance of the insulator-deposited AlGaN layer is about 5 orders of magnitude higher than that of the metal-deposited AlGaN layer. From dc measurements, the resistance of the insulatordeposited AlGaN layer at a negative gate bias is higher than that of the insulator layer. From these results, it can be concluded that gate leakage is suppressed by the high resistance of the AlGaN layer in the MIS-HFET at a negative gate bias.