A semiconductor device including a gate electrode structure on a substrate and including gate electrodes spaced apart from each other in a first direction, each gate electrode extending in a second direction; a memory channel structure extending through the gate electrode structure on the substrate, the memory channel structure including a channel extending in the first direction; a charge storage structure surrounding an outer sidewall of the channel; a first filling pattern filling an inner space formed by the channel; and a first capping pattern on the channel and the first filling pattern; and a dummy charge storage structure extending through the gate electrode structure on the substrate, the dummy charge storage structure including a second filling pattern extending in the first direction; a dummy charge storage structure surrounding an outer sidewall of the second filling pattern; and a second capping pattern on the second filling pattern.