The present disclosure relates to a semiconductor device and a fabrication method thereof. The semiconductor device includes a substrate, a first nitride semiconductor layer disposed on the substrate, a second nitride semiconductor layer disposed on the first nitride semiconductor layer and having a bandgap greater than that of the first nitride semiconductor layer. The semiconductor device further includes a first gate conductor disposed on a first region of the second nitride semiconductor layer, a first source electrode disposed on a first side of the first gate conductor, a first field plate disposed on a second side of the first gate conductor; and a capacitor having a first conductive layer and a second conductive layer and disposed on a second region of the second nitride semiconductor layer. Wherein the first conductive layer of the capacitor and the first source electrode have a first material, and the second conductive layer of the capacitor and the first field plate have a second material.