A method of forming a semiconductor device is provided. A gate electrode is formed within an insulating layer that overlies a substrate. A gate dielectric layer is formed over the gate electrode. A first oxide semiconductor layer is formed over the gate dielectric layer. A dielectric layer is formed over the first oxide semiconductor layer. The dielectric layer and the first oxide semiconductor layer are patterned, so as to form first and second openings that expose portions of the gate dielectric layer. An interfacial layer is conformally formed on sidewalls and bottoms of the first and second openings. A second oxide semiconductor layer is formed over the interfacial layer in the first and second openings. A metal layer is formed over the second oxide semiconductor layer in the first and second openings.