I. Introduction
Thin-Film Transistors (TFTs) are extensively used as switching devices in active-matrix liquid-crystal displays. Hydrogenated amorphous Si TFTs are the current industry standard. Among the candidate materials for next-generation TFTs, ZnO-based materials such as ZnO [1]–[4], [5], [6], [7], [8], and amorphous In–Ga–Zn–O [9]–[12] have attracted considerable attention due to their high electron mobility and low processing temperatures. These characteristics make ZnO-related TFTs suitable for driving high-resolution liquid-crystal displays and organic light-emitting diode pixels [1], [13].