A semiconductor device including a substrate; a first active pattern on the substrate and extending in a first direction, an upper portion of the first active pattern including a first channel pattern; first source/drain patterns in recesses in an upper portion of the first channel pattern; and a gate electrode on the first active pattern and extending in a second direction crossing the first direction, the gate electrode being on a top surface and on a side surface of the at least one first channel pattern, wherein each of the first source/drain patterns includes a first, second, and third semiconductor layer, which are sequentially provided in the recesses, each of the first channel pattern and the third semiconductor layers includes silicon-germanium (SiGe), and the first semiconductor layer has a germanium concentration higher than those of the first channel pattern and the second semiconductor layer.