A semiconductor device includes an active region on a substrate extending in a first direction, the active region having an upper surface and sidewalls, a plurality of channel layers above the active region to be vertically spaced apart from each other, a gate electrode extending in a second direction to intersect the active region and partially surrounding the plurality of channel layers, and a source/drain region on the active region on at least one side of the gate electrode and in contact with the plurality of channel layers, and extending from the sidewalls of the active region having a major width in the second direction in a first region adjacent to a lowermost channel layer adjacent to the active region among the plurality of channel layer.