A semiconductor device includes a back interlayer insulating film; a plurality of first channel patterns on the back interlayer insulating film and spaced apart from each other in a vertical direction; a plurality of second channel patterns on the back interlayer insulating film and spaced apart from each other in the vertical direction; a source/drain pattern between the first channel patterns and the second channel patterns; and a source/drain contact connected to the source/drain pattern, wherein the source/drain pattern includes a first layer which comes into contact with the first channel patterns and the second channel patterns, a second layer on or below the first layer, and a third layer on or above the second layer, wherein a width of the first layer in the vertical direction decreases and then increases, along a direction from the first channel patterns to the second channel patterns.