A semiconductor device includes a substrate; a channel pattern on the substrate, the channel pattern having sidewalls extending in a vertical direction perpendicular to a surface of the substrate and a lower portion connecting lower portions of two sidewalls facing each other in a horizontal direction; a gate insulation layer pattern and a first conductive layer pattern sequentially stacked laterally on an inner sidewall of the channel pattern; and a second conductive layer pattern contacting at least an uppermost surface and an upper outer sidewall of the channel pattern, the second conductive pattern being spaced apart from the first conductive layer pattern.