A semiconductor device includes; a first fin vertically protruding from a substrate and extending in a first horizontal direction, a second fin vertically protruding from the substrate, an isolation layer contacting side surfaces of the first fin and the second fin, a first lower barrier layer on the first fin, a second lower barrier layer on the second fin, source/drain regions spaced apart in the first horizontal direction on the first lower barrier layer, channel layers disposed between the source/drain regions and vertically spaced apart on the first barrier layer, a gate structure intersecting the first lower barrier layer, surrounding each of the channel layers, and extending in a second horizontal direction, an upper barrier layer on the second lower barrier layer, and first semiconductor layers and second semiconductor layers stacked on the upper barrier layer.