Thin Film Transistors Based On Oxide Semiconductors

Thin film transistors (TFT) based on ZnO and In2O3 semiconductors are demonstrated in this dissertation to meet the demands for switching devices in active matrix Surface-conduction Electron-emitter Display (SED) and flexible electronics, respectively.ZnO TFT was fabricated on glass substrate, and only conventional thin film processing technology was used. Bottom-gated structure with reactive sputtered Ta2O5 insulator layer and rf sputtered ZnO semiconductor layer was adopted in the device. The performance of ZnO TFT was effectively improved by shielding the substrates with a metal mesh and tuning the Ar/O2 flow ratio during sputtering deposition of ZnO layer. Field effect mobility of 3.8cm2/Vs and on-off ratio of 105 was obtained. On-state current of the device with width/length ratio of 5 reached 160μA when the device worked in the linear region of the output characteristics. Magnitude of the current meets the demands in application of SED. The devices showed stable performance below the breakdown voltage, and no threshold voltage shift was found. Field effect mobility of the device was further improved by annealing in atmosphere. Devices annealed at 400°C had a field effect mobility of 24cm2/Vs. Levinson expression is successfully employed to simulate the transfer characteristics of the device for low drain voltage. Electron mobility and donor concentration in ZnO crystallite are obtained by matching the curve parameters.In2O3 is one of the semiconductor materials that can be treated at lower temperature and shows higher carrier mobility. In2O3-TFT with the whole fabrication process carried out at temperature lower than 140°C is demonstrated. Reactive sputtered Ta2O5 film was adopted as insulator layer of the device, and the semiconductor layer was deposited by reactive dc sputtering of an indium target. Stability of the device was greatly improved by passivating the carrier traps at the channel interface by diffusing bismuth into the channel. For bottom-gated structure In2O3 TFT, field effect mobility of 12cm2/Vs and On-Off ratio of 103 was obtained. For top-gated structure In2O3 TFT, field effect mobility of 5cm2/Vs and On-Off ratio of 104 was obtained.The results show that by using oxide semiconductor as the semiconductor layer in thin film transistors, high field effect mobility and stable performance can be obtained. The devices are expected to be used in the next generation active matrix flat panel display. Applications such as "flexible electronics" can be developed based on devices fabricated with low temperature processing technology.