The band gap properties of amorphous SiInZnO (a-SIZO) thin-film transistors (TFTs) with different Si concentrations have been studied. The electronic structures of the films, engineered by controlling the Si content, have been investigated through the changes of the band gap and band edge states. Carrier generation at oxygen vacancies can modify the band gap states of oxide thin films. Si suppresses the number of oxygen vacancies - which are carrier generation sites - so shifts the Fermi energy level away from the conduction band. It is difficult to derive the electronic structures of amorphous oxide semiconductors by electrical measurements. Thus, we used a combination of ultraviolet photoelectron spectroscopy, Kelvin probe measurements, and electron energy loss spectroscopy to measure the band gap and electrical performance variations of SIZO TFTs with Si doping. To verify the versatility of Si doping in modulating electronic properties, high-performance depletion-mode inverter circuits consisting of 0.1 to 0.3 wt% Si-doped a-SIZO TFTs were fabricated. These inverter models operate through the threshold voltage difference that arises from the different Si contents. High voltage gains of ~20.62 at a supply voltage of 15 V were obtained with the two TFTs, with a strong dependence on the subthreshold swing.
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