Institution Of Low-voltage Electric-double-layer Oxide-based Thin-film Transistors

Recently, wide bandgap-based oxide thin-film transistors (TFTs) have been extensively investigated because of their high electron mobility and low processing temperature, making them compatible with flexible substrates and because they can be processed at low temperatures to produce large-area displays with the potential of low production costs. However, they usually require large voltage to achieve high output currents and high mobilites. TFTs operated in low-voltage have attracted much application because they can reduce the power consumption of circuits effectively. In this dissertation, low-voltage electric-double-layer (EDL) TFTs with different channel layers have been fabricated on different substrates and different manufacturing processes by magnetic sputtering and plasma enhanced chemical vapor deposited (PECVD) methods at room temperature, and the electrical characteristics of the TFTs have been studied systematically and the results are summarized below.Firstly, bottom-gated ITO TFTs were fabricated by magnetic sputtering method on n type silicon substrates at room temperature. SiO2 gate dielectric was deposited by PECVD using SiH4 and O2 as the reactive gases. During the deposition of SiO2, the reaction by-products with protons were existed in the mesoporous SiO2 and when an electric field applied across the gate dielectric, the mobile protons moved along the external electric field. The motion of the protons is beneficial to form electric double layer. The ITO TFT worked at a low voltage of 1.5 V due to the large capacitance of 2.14μF/cm2. The saturation mobility, subthreshold swing and on/off current ratio of the EDL ITO TFT were 118 cm2/Vs,92 mV/decade,5×10⁶, respectively.Secondly, bottom-gated ZnO TTFTs were fabricated by magnetic sputtering method on glass substrates at room temperature. The ZnO TTFT worked at a low voltage of 1.5 V because ZnO TTFT was gated by EDL gate dielectric. The saturation mobility, subthreshold swing and on/off current ratio of the EDL ITO TFT were 14.9 cm2/Vs,82 mV/decade,2×10⁶, respectively. The in?uence of visible light irradiation on the electrical performance of the device was also investigated. When the light was turned on, a small decrease of the on/off ratio and the saturation electron mobility were observed.Thirdly, bottom-gated ITO TTFTs were fabricated by magnetic sputtering method on glass substrates at room temperature. The in?uence of oxygen content and the length of exposure to air atmosphere on the electrical performance and optical transmittance of the ITO TTFTs were investigated. The value of threshold voltage shifted from negative to positive with the increase of oxygen content. When the length of exposure to air atmosphere increased, the operating voltage of the TFT would increase and the value of threshold voltage shifted from negative to positive.Fourthly, a self-assembling diffraction method was developed for low-voltage coplanar homojunction TTFT fabrication. In this one-shadow-mask process, a channel layer can be simultaneously self-assembled between ITO source/drain electrodes during magnetron sputtering deposition. ITO TFTs showed an ultralow operation voltage of 1.5 V. A small subthreshold swing of 0.12 V/decade and a large on/off ratio of 106 were obtained. As the thickness of the channel layer decrease, the value of threshold voltage shifted from negative to positive direction.Finally, Flexible low-voltage EDL TFTs with a patterned ITO channel were self-assembled on paper substrates by only one shadow mask at room temperature. The operation voltage was 1.5 V when a microporous SiO₂ solid electrolyte with large EDL capacitance was used as the gate dielectric. Such flexible TFTs operated a high mobility of 25.5 cm2/V s, a low subthreshold swing of 0.16 V/decade, and a high current on/off ratio of 3×105. The influence of mechanical bending to the electrical performance of the flexible EDL TFTs wass investigated. After bending, the saturation mobility, subthreshold swing, on/off current ratio and threshold voltage of ITO TFT were 19.1 cm2/Vs,0.16 V/decade,1×105 and -0.73 V, respectively.