A semiconductor device is provided. The semiconductor device includes: a substrate including an active region; a gate structure intersecting the active region on the substrate; channel layers on the active region, spaced apart from each other and surrounded by the gate structure; and a source/drain region on the active region adjacent the gate structure and connected to the plurality of channel layers. The source/drain region includes: a first semiconductor layer on side surfaces of the channel layers; a diffusion barrier layer on an upper region of the first semiconductor layer and including carbon, wherein an upper surface of a first channel layer that is a lowermost channel layer among the plurality of channel layers is provided between the substrate and a lower end of the diffusion barrier layer; and a second semiconductor layer on the diffusion barrier layer and the first semiconductor layer.