A semiconductor device includes a plurality of channel structures on a substrate, each channel structure extending in a first direction perpendicular to the substrate, and having a gate insulating layer and a channel layer, a common source extension region including a first semiconductor layer having an n-type conductivity between the substrate and the channel structures, a plurality of gate electrodes on the common source extension region and spaced apart from each other on a sidewall of each of the channel structures in the first direction, and a common source region on the substrate in contact with the common source extension region and including a second semiconductor layer having an n-type conductivity, wherein the gate insulating layer of each of the channel structures extends to cover an upper surface and at least a portion of a bottom surface of the common source extension region.