A semiconductor device includes a channel, a first source/drain structure on a first side surface of the channel, a second source/drain structure on a second side surface of the channel, a gate structure surrounding the channel, an inner spacer layer on a side surface of the gate structure, and an outer spacer layer on an outer surface of the inner spacer layer. The first source/drain structure includes a first source/drain layer on the channel and a second source/drain layer on the first source/drain layer, and on a plane of the semiconductor device that passes through the channel, at least one of a first boundary line of the first source/drain layer in contact with the second source/drain layer and a second boundary line of the first source/drain layer in contact with the channel may be convex, extending toward the channel.