Abstract
For the pixel circuits of active-matrix organic light-emitting diode (AMOLED) displays, thin-film transistors (TFTs) are used as the driving and switching devices. However, variations in the electrical characteristics of TFTs, such as threshold voltage variations and shifts, cause non-negligible deviations in OLED currents. Furthermore, the luminance of OLED devices degrades over time, which shortens the life-time of AMOLED displays. Therefore, several compensating pixel circuits have been developed to improve the image quality as well as extend the life-time of the displays. Recently, the amorphous indium-gallium-zinc-oxide (a-IGZO) TFTs are regarded as one of the candidates employed in AMOLED displays for superior electrical characteristics. While they may be normally-off or normally-on depending on the fabrication process, the normally-on characteristic of the a-IGZO TFTs is a critical issue for the design of the pixel circuits.This thesis proposes three new pixel circuits and verifies their performance by HSPICE simulations. The first proposed 3T2C pixel circuit has immunity against the threshold voltage variations of the low temperature polycrystalline-silicon (LTPS) driving TFT for small-size and high-resolution AMOLED displays. Based on simulation results, relative current error rates among the entire data voltage range all fall below 5% with ±0.5 V threshold voltage variations of the driving TFT. However, this pixel circuit is incapable of compensation for the OLED luminance drop. Therefore, the second proposed 5T1C pixel circuit adopting a-IGZO TFTs is designed to compensate for both the threshold voltage shift of the driving TFT and luminance drop of the OLED by internally detecting the threshold voltages of the driving TFT and the OLED. The electrical characteristics of the driving a-IGZO TFT are experimentally measured and the simulation parameters are extracted for HSPICE simulations. The simulation results demonstrate that the fluctuations of OLED currents resulting from the threshold voltage shifts of the normally-off and normally-on driving TFTs are suppressed below 5%. Furthermore, the maintenance of the normalized luminance is also achieved by increasing the driving current for OLED. Nevertheless, the structure of the proposed 5T1C pixel circuit is sensitive to the parasitic capacitance of the TFTs, which easily affects the stored voltage in the storage capacitor and therefore reduces the compensation performance of the proposed 5T1C pixel circuit.
To improve circuit stability, the third 5T2C pixel circuit adopting a-IGZO TFTs is proposed with compensations for the threshold voltage shifts of the driving TFT and the luminance drop of OLED. According to simulation results, the current error rates are within 3.5% for the proposed 5T2C pixel circuit, with 1V shift in the threshold voltage of normally-off and normally-on driving TFTs. Moreover, the deviations of the normalized luminance of the proposed pixel circuit fall 5% for different data voltages. Thus, the proposed 5T2C pixel circuit has high immunity against the VTH shifts of a-IGZO driving TFTs and the luminance drop of OLED devices.
| Date of Award | 2013 |
|---|---|
| Original language | Chinese (Traditional) |
| Supervisor | Chih-Lung Lin (Supervisor) |