A heterostructure semiconductor device includes a first active layer and a second active layer disposed on the first active layer. A two-dimensional electron gas layer is formed between the first and second active layers. A sandwich gate dielectric layer structure is disposed on the second active layer. A passivation layer is disposed over the sandwich gate dielectric layer structure. A gate extends through the passivation layer to the sandwich gate dielectric layer structure. First and second ohmic contacts electrically connected to the second active layer. The first and second ohmic contacts are laterally spaced-apart, with the gate being disposed between the first and second ohmic contacts.