A semiconductor device includes a first common source/drain and a second common source/drain spaced apart from each other in a first direction; a first channel structure between the first common source/drain and the second common source/drain, and a second channel structure between the first common source/drain and the second common source/drain and spaced apart from the first channel structure in a vertical direction; a first gate structure surrounding an upper surface, a lower surface, and side surfaces of the first channel structure; and a second gate structure surrounding an upper surface, a lower surface, and side surfaces of the second channel structure, and spaced apart from the first gate structure, wherein a level of the second channel structure is higher than a level of the first channel structure.