A semiconductor device including a substrate, a heterojunction body, a passivation layer, a source contact, a drain contact, and a gate contact. The heterojunction body disposed on or above the substrate includes a first semiconductor layer, a mask layer, a regrowth layer, and a second semiconductor layer. The first semiconductor layer is disposed on or above the substrate. The mask layer is disposed on or above a portion of the first semiconductor layer. The regrowth layer disposed on the first semiconductor layer and adjacent to the mask layer includes a main portion and at least one inclined portion. The second semiconductor layer is disposed on the mask layer and the regrowth layer. The passivation layer is disposed on the second semiconductor layer. The gate contact is disposed on the passivation layer, between the source contact and the drain contact, and at least above the inclined portion of the regrowth layer.