A semiconductor includes an active pattern with a lower pattern and sheet patterns spaced apart from the lower pattern in a first direction, a source/drain pattern on the lower pattern, the source/drain pattern being in contact with the sheet patterns, and gate structures on opposite sides of the source/drain pattern, the gate structures being spaced apart from each other along a second direction and including gate electrodes that surround the sheet patterns, wherein the source/drain pattern includes a first epitaxial region having at least one of antimony and bismuth, the first epitaxial region having a bottom part in contact with the lower pattern, but not with the sheet patterns, and a thickness of the bottom part increasing and decreasing away from the gate structures in the second direction, and a second epitaxial region on the first epitaxial region, the second epitaxial region including phosphorus.