A semiconductor device includes a stack comprising insulating patterns vertically stacked on a substrate and gate patterns interposed between the insulating patterns, an active pillar passing through the stack and electrically connected to the substrate and a charge storing layer interposed between the stack and the active pillar. The charge storing layer includes a first portion between the active pillar and one of the gate patterns, a second portion between the active pillar and one of the insulating patterns, and a third portion joining the first portion to the second portion and having a thickness less than that of the first portion.