A semiconductor device and a clamping circuit including a substrate; a first semiconductor layer, arranged on the substrate and composed of a III-nitride semiconductor material; a second semiconductor layer, arranged on the first semiconductor layer and composed of a III-nitride semiconductor material; a power transistor structure, including a gate structure, a drain structure and a source structure arranged on the second semiconductor layer; the first transistor structures, arranged on the second semiconductor layer; and the second transistor structures, arranged on the second semiconductor layer in series. One end of the first transistor structures and one end of the second transistor structures are jointly electrically connected to the drain structure of the power transistor structure, and the other end of the first transistor structures and the other end of the second transistor structures are jointly electrically connected to the source structure of the power transistor structure.