A semiconductor device including a semiconductor substrate including first regions and second regions, at least one of the first regions being disposed between adjacent second regions; a plurality of first gate structures on the first regions of the semiconductor substrate; and a plurality of second gate structures on the second regions of the semiconductor substrate, wherein each of the first and second gate structures includes a lower gate structure including a recess region defined by sidewalls and a bottom connecting the sidewalls; and an upper gate structure including a gap-fill metal pattern that fills the recess region of the lower gate structure, wherein the bottom of the lower gate structure included in the first gate structure has a thickness different from a thickness of the bottom of the lower gate structure included in the second gate structure, and wherein the gap-fill metal patterns of the first and second gate structures have top surfaces at substantially a same level.