The present invention relates to a semiconductor device having an improved gate leakage current. The semiconductor device includes: a substrate; a first nitride semiconductor layer, positioned above the substrate; a second nitride semiconductor layer, positioned above the first nitride semiconductor layer and having an energy band gap greater than that of the first nitride semiconductor layer; a source contact and a drain contact, positioned above the second nitride semiconductor layer; a doped third nitride semiconductor layer, positioned above the second nitride semiconductor layer and between the drain contact and the source contact; and a gate electrode, positioned above the doped third nitride semiconductor layer, where the doped third nitride semiconductor layer has at least one protrusion extending along a direction substantially parallel to an interface between the first nitride semiconductor layer and the second nitride semiconductor layer, thereby improving the gate leakage current phenomenon.