A semiconductor device including an active region defined in a substrate; at least one channel layer on the active region; a gate electrode intersecting the active region and on the active region and surrounding the at least one channel layer; and a pair of source/drain regions adjacent to both sides of the gate electrode, on the active region, and in contact with the at least one channel layer, wherein the pair of source/drain regions includes a selective epitaxial growth (SEG) layer, and a maximum width of each of the pair of source/drain regions in a first direction is 1.3 times or less a width of the active region in the first direction.