A semiconductor device includes a stack structure on a substrate, the stack structure including interlayer insulating layers and first gate electrodes alternately stacked on each other, a semiconductor layer in an opening penetrating through the stack structure, a first dielectric layer between the semiconductor layer and the stack structure, and a lower pattern closer to the substrate than to the first gate electrodes in the stack structure, the lower pattern including a first surface facing the first dielectric layer, and a second surface facing the stack structure, the second surface defining an acute angle with the first surface, wherein the first dielectric layer includes a first portion facing the stack structure, and a second portion facing the first surface of the lower pattern, the second portion having a thickness greater than a thickness of the first portion.