A semiconductor device includes: a semiconductor substrate having a first surface and a second surface, wherein the substrate has a first conductive type; a first trench extending from the first surface of the semiconductor substrate in a depth direction; and an epitaxial semiconductor layer having a second conductive type, wherein the epitaxial semiconductor layer is disposed in the first trench. The first trench includes an inner wall as an interface between the semiconductor substrate and the epitaxial semiconductor layer so that the interface provides a PN junction. The first trench has an aspect ratio equal to or larger than 1.