A semiconductor device includes: an active fin disposed on a substrate and protruding from an upper surface of the substrate; a gate structure disposed on the active fin; a source/drain layer disposed on a portion of the active fin adjacent to the gate structure; an ohmic contact pattern on the source/drain layer; and a contact plug disposed on an upper surface of the ohmic contact pattern, wherein the contact plug includes: a conductive structure including a metal; and a barrier pattern covering a lower surface and a sidewall of the conductive structure, wherein a thickness of the barrier pattern is equal to or less than about 10 Å, and wherein a maximum diameter of a grain of the metal included in the conductive structure is in a range of about 8 nn to about 15 nm.