A semiconductor device and a manufacturing method thereof are provided. The semiconductor device includes first and second epitaxial layers, and first and second semiconductor layers. The second epitaxial layer is disposed on the first epitaxial layer. The first semiconductor layer extends from above the second epitaxial layer to a top surface of the second epitaxial layer. A vertically extending region of the first semiconductor layer has a body portion and an extending portion extending from a bottom end of the body portion to the second epitaxial layer. A width of the body portion is greater than a width of the extending portion. The second semiconductor layer is disposed on the second epitaxial layer, and laterally surrounds the vertically extending region of the first semiconductor layer. A portion of the second semiconductor layer extends between and overlaps with the body portion of the first semiconductor layer and the second epitaxial layer.