A semiconductor device may include gate electrodes and interlayer insulating layers alternately stacked on a substrate, a channel layer penetrating the gate electrodes and the interlayer insulating layers, a gate dielectric layer between the gate electrodes and the channel layer, a filling insulation that fills at least a portion of an interior of the channel layer, a charge fixing layer between the channel layer and the filling insulation and including a high-k material and/or a metal, and a conductive pad connected to the channel layer and on the filling insulation. The conductive pad may be physically separated from the charge fixing layer.