A semiconductor device including a substrate, an epitaxial layer, a first sinker, a transistor, a diode unit, a first buried layer, and a second buried layer is provided. When the semiconductor device is operated at the high voltage, the highly large substrate current due to the external load is avoided through the diode unit disposed in the semiconductor device of an embodiment consistent with the invention. Furthermore, according to the design of the semiconductor device, the issue of the narrow input voltage range is improved, and interference of the semiconductor device with the other semiconductor devices is prevented.