A semiconductor device includes a first transistor, a second transistor and a third transistor provided on a substrate, the first to third transistors respectively including source and drain regions spaced apart from each other, a gate structure extending in a first direction on the substrate and interposed between the source and drain regions, and a channel region connecting the source and drain regions to each other. A channel region of the second transistor and a channel region of the third transistor respectively include a plurality of channel portions, the plurality of channel portions spaced apart from each other in a second direction, perpendicular to an upper surface of the substrate, and connected to the source and drain regions, respectively. A width of a channel portion of the third transistor in the first direction is greater than a width of a channel portion of the second transistor in the first direction.