A semiconductor device includes: a substrate including a plurality of first active regions and a plurality of second active regions; a plurality of first gate structures formed above the first active regions, respectively, and a plurality of second gate structures formed above the second active regions, respectively; and a plurality of first source/drain layers corresponding to the first gate structures, respectively, and a plurality of second source/drain layers corresponding to the second gate structures, respectively, wherein a width of each of the first source/drain layers is smaller than a width of each of the second source/drain layers.