A semiconductor device including an active structure on a substrate, the active structure including silicon germanium patterns and silicon patterns alternately and repeatedly stacked in a vertical direction perpendicular to an upper surface of the substrate; a semiconductor layer on sidewalls of the active structure that face in a first direction parallel to the upper surface of the substrate, the semiconductor layer being a source/drain region; and a gate structure on a surface of the active structure and the substrate, the gate structure extending in a second direction that is perpendicular to the first direction, wherein the silicon germanium patterns are silicon rich-silicon germanium.