A semiconductor device includes a first and second channel patterns on a substrate, each of the first and second channel patterns including vertically-stacked semiconductor patterns; a first source/drain pattern connected to the first channel pattern; a second source/drain pattern connected to the second channel pattern, the first and second source/drain patterns having different conductivity types; a first contact plug inserted in the first source/drain pattern, and a second contact plug inserted in the second source/drain pattern; a first interface layer interposed between the first source/drain pattern and the first contact plug; and a second interface layer interposed between the second source/drain pattern and the second contact plug, the first and second interface layers including different metallic elements from each other, a bottom portion of the second interface layer being positioned at a level that is lower than a bottom surface of a topmost one of the semiconductor patterns.