A semiconductor device may include semiconductor patterns, a gate structure, a first spacer, a first semiconductor layer and a second semiconductor layer. The semiconductor patterns may be formed on a substrate, and may be spaced apart from each other in a vertical direction perpendicular to an upper surface of the substrate and may overlap in the vertical direction. The gate structure may be formed on the substrate and the semiconductor patterns. At least portion of the gate structure may be formed vertically between the semiconductor patterns. The first spacer may cover opposite sidewalls of the gate structure, the sidewalls opposite to each other in a first direction. The first semiconductor layer may cover the sidewalls of the semiconductor patterns in the first direction, and surfaces of the first spacer and the substrate. The first semiconductor layer may have a first concentration of impurities. The second semiconductor layer may be formed on the first semiconductor layer, and may have a second concentration of impurities different from the first concentration of impurities. The semiconductor device may have good characteristics and high reliability.