A semiconductor device, includes, a first active pattern includes a first lower portion and a first channel layer spaced apart from the first lower portion, a second active pattern includes a second lower portion and a second channel layer spaced apart from the second lower portion, a channel semiconductor layer on the first lower portion and the first channel layer, a first gate pattern on the channel semiconductor layer, a second gate pattern on the second lower portion and the second channel layer, a first source/drain pattern connected to the first lower portion, the first channel layer, the channel semiconductor layer, and the first gate pattern, and a second source/drain pattern connected to the second lower portion, the second channel layer, and the second gate pattern, a lattice constant of the channel semiconductor layer being greater than a lattice constant of each of the first and second active patterns.