A display device includes a gate electrode on a substrate of a semiconductor device, a gate insulating film over the gate electrode, an active layer comprising an oxide including indium, zinc and gallium on the gate insulating film, and overlapping the gate electrode, and a source electrode and a drain electrode that are spaced apart from each other, wherein the active layer is formed from a zinc-rich target material, and an atomic % ratio of indium, zinc and gallium in the active layer is different from an atomic % ratio of the zinc-rich target material.