Disclosed is a semiconductor device comprising gate stack structures on a substrate, separation structures extending in a first direction on the substrate and separating the gate stack structures, and vertical structures penetrating the gate stack structures. Each gate stack structure includes cell dielectric layers and electrodes including upper electrodes, a barrier layer extending between the electrodes and the cell dielectric layers, a separation dielectric pattern extending in the first direction and penetrating the upper electrodes to separate each upper electrode into pieces that are spaced apart from each other in a second direction intersecting the first direction, and capping patterns between the separation dielectric pattern and the upper electrodes. The capping patterns are on sidewalls of each upper electrode and spaced apart from each other in a third direction perpendicular to a top surface of the substrate. Each capping pattern is on a sidewall of the barrier layer.