A semiconductor device includes a substrate including an active pattern, a channel pattern on the active pattern, the channel pattern including a first semiconductor pattern, a second semiconductor pattern, and a third semiconductor pattern sequentially stacked and vertically spaced apart, a source/drain pattern on the active pattern, and a gate electrode on the first semiconductor pattern, the second semiconductor pattern, and the third semiconductor pattern, where the source/drain pattern includes a buffer layer and a main layer on the buffer layer, the main layer includes silicon that is doped with an impurity, an impurity concentration of the main layer is a first atomic fraction at a first level corresponding to the first semiconductor pattern, and the impurity concentration of the main layer is a second atomic fraction at a second level corresponding to the second semiconductor pattern.