This brief reports, for the first time, an oxide-passivated AlGaN/GaN high electron mobility transistor by using the hydrogen peroxide treatment. Characterizations by using electron spectroscopy for chemical analysis and transmission electron microscopy have been performed to verify the formation of surface oxide on the AlGaN barrier layer. The present design has demonstrated superior improvements of 41% in the maximum drain/source current density , 39% in the drain/source saturation current density at zero gate bias , 47% in the maximum extrinsic transconductance , 53.2% in the two-terminal gate/drain breakdown voltage , 36% in the cutoff frequency , and 20% in the maximum oscillation frequency , as compared with an unpassivated conventional device.