Active Matrix Organic Light-Emitting Displays: Novel Amorphous Silicon Thin-Film Transistors and Pixel Electrode Circuits.

Abstract

Today active-matrix organic light-emitting displays (AM-OLEDs) are considered as next generation flat panel display. In this thesis, several hydrogenated amorphous silicon (a-Si:H) thin-film transistor (TFT) technologies have been developed to accelerate the AM-OLED development. Among others to address the charging time delay issue of the conventional current-driven a-Si:H TFT pixel electrode circuit, a non-linear current scaling-function by cascaded-capacitors connected to the driving TFT was investigated. To enhance the performance of fabricated pixel electrode circuit, novel design of a-Si:H pixel circuit with cascaded storage capacitors was investigated based on the current-mirror structure. The electrical and thermal stability of the proposed a-Si:H TFT pixel electrode circuits were also explored in comparison to the conventional current-driven circuit. To address the inherent electrical stability issue of the a-Si:H TFT, two novel a-Si:H TFT structures were proposed: Corbino and Hexagonal TFTs. It was shown that both a-Si:H TFT structures have the asymmetric electrical characteristics under different drain bias conditions. To extract the electrical device parameters, asymmetric geometric factors were developed for different drain bias conditions. By using multiple Hexagonal TFT structure, the output current of Hexagonal a-Si:H TFT connected in parallel increases linearly with their number within a given pixel circuit. Current-voltage measurements indicate that a high ON-OFF current ratio and a low sub-threshold slope can be maintained for multiple Hexagonal TFTs connected in parallel while the field-effect mobility and threshold voltage remain identical to a single HEX a-Si:H TFT. Due to a unique device geometry, enhanced electrical stability and larger pixel aperture ratio can be achieved in the multiple a-Si:H HEX-TFT in comparison to standard single a-Si:H TFT having same channel width. Lastly, the dynamic responses of different a-Si:H TFT structures with various storage capacitor size were explored for AM-OLEDs. The effect of dofferent storage capacitors and overlap capacitors of TFTs on the charging time and feed-through voltage characteristics of the a-Si:H switching TFT were explored. Feed-through voltage behavior of Corbino a-Si:H TFT was also discussed in comparison to normal rectangular a-Si:H TFT as a switching TFT for AM-OLEDs.