A semiconductor device includes an active pattern disposed on a substrate; a gate structure disposed on the active pattern; channels disposed on the substrate and that are spaced apart from each other in a vertical direction substantially perpendicular to an upper surface of the substrate; a first epitaxial layer disposed on a portion of the active pattern adjacent to the gate structure; and a contact plug disposed on the first epitaxial layer. The contact plug includes a lower portion; a middle portion disposed on the lower portion, where the middle portion has a width that increases from a bottom to a top thereof along the vertical direction; and an upper portion disposed on the middle portion.