Abstract
In recent years, amorphous indium-gallium-zinc oxide thin-film transistors (a-IGZO TFTs) have gained considerable attention because they have better large-area uniformity, lower manufacturing cost and lower leakage currents than low-temperature polysilicon (LTPS) TFTs. Also, a-IGZO TFTs have higher mobility, better electrical stability and better transparency to visible light than do hydrogenated amorphous silicon (a-Si:H) TFTs. However, the threshold voltage shifts under gate voltage stress must be considered in design of gate driver circuits for active-matrix liquid crystal displays (AMLCDs) and pixel circuits for active-matrix organic light-emitting diode (AMOLED) and blue-phase (BP) LCDs, because they affect the lifetime of those circuits.This thesis proposes three circuits that are based on a-IGZO TFTs, whose feasibility is verified by HSPICE simulations. The first circuit is a gate driver that comprises four TFTs and two capacitors and is suitable for high-resolution and narrow-bezel TFT-LCD applications. Three-phase overlapping clock signals with a 40% duty cycle are used to suppress the threshold voltage shifts of the a-IGZO TFTs. Furthermore, both the input TFT and the driving TFT provide paths for charging and discharging, reducing the number of components of the gate driver. Also, the input TFT can be made to enhance the stability of the output waveform by using an appropriate period of overlap of the clock signals. Consequently, the structure of the circuit is simplified and the resulting single-stage layout area is 400 μm × 126 μm. The second circuit is a pixel circuit that is composed of three TFTs and one capacitor and is for use in large-size and high-resolution three-dimensional (3D) AMOLED displays. The simultaneous emission (SE) driving scheme is utilized to provide enough time for shutter glasses to switch and reduces left-right crosstalk. The proposed pixel circuit for detecting the threshold voltage of the driving TFT is a source-follower structure to compensate for the threshold voltage shifts in both normally-off and normally-on a-IGZO TFTs. According to the simulation results obtained with ±1 V threshold voltage shifts of the driving TFT, the relative error rates of the OLED currents are less than 4.46% for the entire range of possible data voltages. The third circuit is a pixel circuit which consists five TFTs, two storage capacitors and one equivalent capacitor of the BPLC for high-resolution and high-frame-rate BP-LCDs. The voltage across the BPLC can be maintained by two driving TFTs when the BPLC suffers from the frequency effect. Moreover, the gate-source voltage in one of the driving TFTs is smaller than zero to ameliorate the threshold shifts of the a-IGZO TFTs and extend the lifetime of the proposed BPLC pixel circuit. Simulation results indicate that this circuit can operate with both positive polarity and negative polarity of the BPLC.
| Date of Award | 2015 |
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| Original language | Chinese (Traditional) |
| Supervisor | Chih-Lung Lin (Supervisor) |