A semiconductor device includes a first III-V compound layer on a substrate, a second III-V compound layer on the first III-V compound layer, in which a material of the first III-V compound layer is different from that of the second III-V compound layer, a gate metal stack disposed on the second III-V compound layer, a source contact and a drain contact disposed at opposite sides of the gate metal stack, a gate field plate disposed between the gate metal stack and the drain contact, an anti-reflective coating (ARC) layer formed on the source contact and the drain contact, and an etch stop layer formed on the ARC layer.