A semiconductor device includes a first active pattern including a first lower pattern and first sheet patterns; a second active pattern including a second lower pattern and second sheet patterns, a height of the second lower pattern being smaller than a height of the first lower pattern; a first gate structure on the first lower pattern; a second gate structure on the second lower pattern; a first source/drain pattern on the first lower pattern and connected to the first sheet patterns; and a second source/drain pattern on the second lower pattern and connected to the second sheet patterns, wherein a width of an upper surface of the first lower pattern is different from a width of an upper surface of the second lower pattern, and wherein a number of first sheet patterns is different from a number of second sheet patterns.