A field effect transistor includes a substrate; a first semiconductor layer, disposed over the substrate; a second semiconductor layer, disposed over the first semiconductor layer, wherein an interface between the first semiconductor layer and the second semiconductor layer has a two-dimensional electron gas; a p+ III-V semiconductor layer, disposed over the second semiconductor layer; and a depolarization layer, disposed between the second semiconductor layer and the p+ III-V semiconductor layer, wherein the depolarization layer includes a metal oxide layer.