A photo transistor and a display device employing the photo transistor are provided. The photo transistor includes a gate electrode disposed on a substrate, a gate insulating layer that electrically insulates the gate electrode, a first active layer overlapping the gate electrode and including metal oxide, wherein the gate insulating layer is disposed between the gate electrode and the active layer, a second active layer disposed on the first active layer and including selenium, and a source electrode and a drain electrode respectively electrically connected to the second active layer.