Study On Amorphous Indium-Gallium-Zinc Oxide Thin Film Transistors And Basic Circuits

Currently flat panel displays have two major kinds of driver circuit solutions. One is based on hydrogenated amorphous silicon (a-Si:H) thin film transistors (TFTs), and the other is based on low temperature polycrystalline silicon (LTPS) TFTs. At present, the backplane of mainstream flat panel displays, such as active matrix liquid crystal displays (AM-LCD) and active matrix organic light emitting displays (AM-OLED), commonly consist of arrays of a-Si:H TFTs or LTPS TFTs. In rapidly developing modern information society, the above two driver circuit solutions can not meet the even increasing requirements for high performance displays, including high operation speed, large area, low cost, and low power consumption.Transparent oxide-based TFTs have been attracting much attention recently due to their excellent electrical and optical characteristics for display applications. In particular, amorphous indium-gallium-zinc oxide (a-IGZO) TFTs are intensively investigated as a replacement for silicon-based TFTs in active matrix displays as they could simultaneously offer high channel carrier mobility, high optical transparency, low off-state leakage and low process cost. Although a-IGZO TFTs have shown good static characteristics, it is necessary to study dynamic characteristics of a-IGZO TFTs for fabricating large size panel pixel driving circuits and peripheral driving circuits. In this thesis, we focus on fabrication and characterization of some basic logic circuits based on a-IGZO TFTs. The main results are highlighted as below:1. Prototype inverters with an enhancement-mode load based on bottom-gate a-IGZO TFTs are fabricated on glass substrate. The element TFT has shown reasonably good performance, such as field-effect mobility of～5 cm2/V·s, turn-on voltage of～3.0 V and sub-threshold swing of～0.5 V/dec. The inverter consists of a load transistor and a driver transistor with identical channel lengths but different channel widths. The voltage transfer curve of inverter shows a fairly large swing range from～1 V to～27 V, corresponding to nearly 85% of the full swing range. Meanwhile, a voltage gain of>-6 V/V is extracted, which represents good logic level conversion.2. Seven-stage ring oscillators based on a-IGZO TFTs are fabricated on glass substrates. The seven-stage ring oscillator circuit consists of seven delay stages and an output buffer inverter. The element inverter exhibits good logic level conversion. The dynamic performance of the ring oscillators is evaluated as a function of supply voltage and at different gate lengths. It is found that the oscillation frequency increases with increasing supply voltage, while the propagation delay time decreases. A maximum oscillation frequency of 1.47 MHz is obtained at a supply voltage of 40 V and a channel length of 4um, corresponding to a propagation delay of less than 50 ns/stage.