Provided is a semiconductor device including: a fin structure on a substrate including a negative channel field-effect transistor (nFET) region and a positive channel field-effect transistor (pFET) region; a gate structure on the fin structure; and a source/drain structure adjacent to the gate structure, wherein the source/drain structure formed in the nFET region is an epitaxial layer including an n-type impurity at a concentration of about 1.8×1021/cm3 or more, includes silicon (Si) and germanium (Ge) on an outer portion of the source/drain structure, and includes Si but not Ge in an inner portion of the source/drain structure, wherein an inclined surface contacting an uppermost surface of the source/drain structure forms an angle of less than about 54.7° with a top surface of the fin structure.