The Research Of Low Voltage Oxide-based Thin Film Transistors

Recently, thin film transistors (TFTs) based on wide bandgap oxide semiconductors have been extensively investigated because of their high electron mobility and low processing temperature, making them compatible with flexible substrates and opening the potential for low production cost. However, they usually require large voltage to achieve high mobilites and on/off ratios. Oxide-based thin-film transistors capable of low-voltage operation have much potential in portable applications because it can reduce the power consumption of the device and circuit effectively. Finding new type of low voltage TFTs has become one of hot points in recent years'research.Among various kinds of methods to reduce the operation voltage, developing a gate dielectric which produces large capacitive coupling between the gate and channel should be the optimum and the most effective approach, it can improve the drain current, lower the operation voltage. This paper chose two kinds of dielectrics deposited at room temperature to be studied, and make them as two types thin film transistor devices, confirm the possibility for achieve low voltage thin film transistors by test the thin film and device characteristics.The first dielectric was a high permittivity (εr=37) Ba0.4Sr0.6TiO3 (BST) film. The device used n+type Si (100) substrate and also as common bottom gate, indium-tin-oxide (ITO) as channel layer and electrode. The channel length and width of the TFTs were 80μm and 1000μm, respectively. The test result show:the operation voltage of the TFT is found be to 5.0 V, and operated in the n type depletion mode with a threshold voltage of -3.7 V, a field effect mobility of 3.2 cm2/Vs, a subthreshold swing of 0.5 V/decade and current on/off ratio of 1.4×104.The second dielectric was mesoporous SiO2 insulator with electric double layer (EDL) characteristics. This device used glass substrate, ITO bottom gate, Al-doped ZnO nanocrystal thin film as active channel layer and Al electrode contact with channel length and width of 100μm and 1200μm, respectively. The 4μm-thick mesoporous SiO2 dielectric shows a huge gate capacitance of 2.3μF/cm2. The device was operated at 2.0 V, with a threshold voltage of 0.4 V. The TFTs exhibit high-performance n-type transistor characteristics with the field effect mobility, subthreshold gate voltage swing and current on/off ratio of 4.5 cm2/Vs,140 mV/decade and 1.5×105, respectively.The results demonstrate that TFTs based on BST and mesoporous SiO2 dielectrics are both have characteristics of low operation voltage. They are very promising for the application of the next generation low voltage and low power consumption electronic devices.