A heterojunction semiconductor device (200) comprising a substrate (202) and a multilayer structure disposed on the substrate. The multilayer structure comprising a first layer (204), which comprises a first semiconductor disposed on top of the substrate, and a second layer (206), which comprises a second semiconductor disposed on top of the first layer to define an interface between the first layer and the second layer. The second semiconductor is different from the first semiconductor such that a Two-Dimensional Electron Gas (220) forms adjacent to the interface. The multilayer structure also comprising a passivation layer, which comprises a semiconductor passivation layer (208) disposed on top of the second layer. The heterojunction semiconductor device also includes a first terminal (210) electrically coupled to a first area of the heterojunction semiconductor device; and a second terminal (212) electrically coupled to a second area of the heterojunction semiconductor device. The second terminal (212) is electrically coupled to the semiconductor passivation layer such that electric charge can flow into the second terminal (212) from the semiconductor passivation layer (208).