The present disclosure provides a semiconductor device and a fabricating method thereof, the semiconductor device including a substrate, a nucleation layer, a buffer layer, an active layer and a gate electrode. The nucleation layer is disposed on the substrate, and the buffer layer is disposed on the nucleation layer, wherein the buffer layer includes a first superlattice layer having at least two heteromaterials alternately arranged in a horizontal direction, and a second superlattice layer having at least two heteromaterials vertically stacked along a vertical direction. The at least two heteromaterials stack at least once within the second superlattice layer. The active layer is disposed on the buffer layer, and the gate electrode is disposed on the active layer.