The reduction of gate leakage current in AlGaN/GaN metal-insulator-semiconductor (MIS) heterostructure field effect transistors (HFETs) is influenced by the resistance of the insulating layers and even more so by the increase in AlGaN layer resistance. Electrical performance of AlGaN/GaN HFETs with and without electron beam evaporated SiO chi gate insulator layers indicates that the AlGaN layer resistance is increased seven orders of magnitude by oxide evaporation at negative gate bias. Also, the transconductance frequency dispersion of the MIS-HFET can be explained with a model in which the insulator is more conductive than the AlGaN layer, though they are both highly resistive. These results indicate that the deposition of the oxide reduces electron leakage through the AlGaN layer in the direction from the AlGaN surface to the GaN layer. (c) 2005 Elsevier Ltd. All rights reserved.