A method of forming an oxide semiconductor device may be provided. In the method, a substrate comprising a first major surface and a second major surface that faces away from the first major surface may be provided. An oxide semiconductor device may be formed over the first major surface to provide an intermediate device, and the semiconductor device may comprise an oxide active layer. The intermediate device may be subjected to ultraviolet (UV) light (e.g., ultraviolet ray irradiation process) for a first period, and subjected to heat (e.g., thermal treatment process) for a second period. The first and second periods may at least partly overlap.