A semiconductor device includes an active region extending in a first direction on a substrate, a plurality of channel layers vertically spaced apart from each other on the active region and including a semiconductor material, a gate structure extending in a second direction on the substrate, and a source/drain region disposed on the active region on at least one side of the gate structure. The gate structure intersects the active region and the plurality of channel layers, and surrounds the plurality of channel layers. The source/drain region contacts the plurality of channel layers and includes first impurities. In at least a portion of the plurality of channel layers, a lower region adjacent to the active region includes the first impurities and second impurities at a first concentration, and an upper region includes the first impurities and the second impurities at a second concentration lower than the first concentration.