A semiconductor device includes a plurality of channel structures on a substrate, each channel structure extending in a first direction perpendicular to the substrate, a common source extension structure including a first semiconductor layer having an n-type conductivity and a gate insulating layer between the substrate and the channel structures, a plurality of gate electrodes on the common source extension structure 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 structure and including a second semiconductor layer having an n-type conductivity. An upper portion of the common source extension structure has a first width, and a lower portion of the common source extension structure has a second width smaller than the first width.