A semiconductor device including: a lower semiconductor substrate; an upper semiconductor substrate overlapping the lower semiconductor substrate, the upper semiconductor substrate including a first surface and a second surface opposite to the first surface; an upper gate structure on the first surface of the upper semiconductor substrate; a first interlayer insulation film which covers the upper gate structure, wherein the first interlayer insulation film is between the lower semiconductor substrate and the upper semiconductor substrate; and an upper contact connected to the lower semiconductor substrate, wherein the upper contact is on a side surface of the upper gate structure, wherein the upper contact includes a first portion penetrating the upper semiconductor substrate, and a second portion having a side surface adjacent to the side surface of the upper gate structure, and a width of the first portion decreases toward the second surface.