A semiconductor device includes; gate layers stacked on a substrate, a channel layer extending through the gate layers, a string select gate layer disposed on the channel layer and a string select channel layer extending through the string select gate layer to contact the channel layer. The string select channel layer includes a first portion below the string select gate layer including a first protruding region, a second portion extending through the string select gate layer, and a third portion above the string select gate layer including a second protruding region.