A semiconductor device includes an active pattern on a substrate, a plurality of source/drain patterns in a first direction on the active pattern, a first channel structure between a pair of source/drain patterns, a second channel structure between another pair of source/drain patterns, a first gate electrode extending in a second direction perpendicular to the first direction, and a second gate electrode intersecting the second channel structure and extending in the second direction. The first gate electrode includes a first portion between a bottom surface of the first channel structure and a top surface of the active pattern, and the second gate electrode includes a first portion between a bottom surface of the second channel structure and the top surface of the active pattern. A thickness of the first portion of the second gate electrode is greater than a thickness of the first portion of the first gate electrode.