A plurality of gate electrodes is stacked on an upper surface of a substrate in a direction perpendicular to an upper surface of the substrate. A channel region penetrates through the plurality of gate electrodes to extend perpendicularly to the upper surface of the substrate. A gate dielectric layer includes a tunneling layer, a charge storage layer and a blocking layer that are sequentially disposed between the channel region and the plurality of gate electrodes. The charge storage layer includes a plurality of doping elements and a plurality of deep level traps generated by the plurality of doping element. A concentration distribution of the plurality of doping elements in a thickness direction of the charge storage layer is non-uniform.