A semiconductor device includes a substrate, a plurality of nanowires, a gate structure, a source/drain epitaxy structure, and a semiconductor layer. The substrate has a protrusion portion. The nanowires extend in a first direction above the protrusion portion of the substrate, the nanowires being arranged in a second direction substantially perpendicular to the first direction. The gate structure wraps around each of the nanowires. The source/drain epitaxy structure is in contact with an end surface of each of the nanowires, in which a bottom surface of the source/drain epitaxy structure is lower than a top surface of the protrusion portion of the substrate. The semiconductor layer is in contact with the bottom surface of the epitaxy structure, in which the semiconductor layer is spaced from the protrusion portion of the substrate.