A semiconductor device includes a semiconductor substrate having a conductive type, a source metal layer, a gate metal layer, at least one transistor device, a heavily doped region having the conductive type, a capacitor dielectric layer, a conductive layer. The source metal layer and the gate metal layer are disposed on the semiconductor substrate. The transistor device is disposed in the semiconductor substrate under the source metal layer. The heavily doped region, the capacitor dielectric layer and the conductive layer constitute a capacitor structure, disposed under the gate metal layer, and the capacitor structure is electrically connected between a source and a drain of the transistor device.