A semiconductor device includes first and second active patterns protruding upward from a substrate, a gate electrode crossing the first and second active patterns and extending in a first direction, a first source/drain region on the first active pattern and on at least one side of the gate electrode, and a second source/drain region on the second active pattern and on at least one side of the gate electrode. The first and second source/drain regions have a conductivity type different from each other, and the second source/drain region has a bottom surface in contact with a top surface of the second active pattern and at a lower level than that of a bottom surface of the first source/drain region in contact with a top surface of the first active pattern. The first active pattern has a first width smaller than a second width of the second active pattern.