A semiconductor device is provided. The semiconductor includes a plurality of interlayer insulating layers and a plurality of gate electrodes alternately stacked in a first direction on a substrate. The plurality of interlayer insulating layers and the plurality of gate electrodes constitute a side surface extended in the first direction. A gate dielectric layer is disposed on the side surface. A channel pattern is disposed on the gate dielectric layer. The gate dielectric layer includes a protective pattern, a charge trap layer, and a tunneling layer. The protective pattern includes a portion disposed on a corresponding gate electrode of the plurality of gate electrodes. The charge trap layer is disposed on the protective pattern. The tunneling layer is disposed between the charge trap layer and the channel pattern. The protective pattern is denser than the charge trap layer.