Amorphous Silicon Thin Film Transistor Models and Pixel Circuits for AMOLED Displays
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Hydrogenated amorphous Silicon (a-Si:H) Thin Film Transistor (TFT) has many advantages and is one of the suitable choices to implement Active Matrix Organic Light-Emitting Diode (AMOLED) displays. However, the aging of a-Si:H TFT caused by electrical stress affects the stability of pixel performance. To solve this problem, following aspects are important: (1) compact device models and parameter extraction methods for TFT characterization and circuit simulation; (2) a method to simulate TFT aging by using circuit simulator so that its impact on circuit performance can be investigated by using circuit simulation; and (3) novel pixel circuits to compensate the impact of TFT aging on circuit performance. These challenges are addressed in this thesis. A compact device model to describe the static and dynamic behaviors of a-Si:H TFT is presented. Several improvements were made for better accuracy, scalability, and convergence of TFT model. New parameter extraction methods with improved accuracy and consistency were also developed. The improved compact TFT model and new parameter extraction methods are verified by measurement results. Threshold voltage shift (∆Vt) over stress time is the primary aging behavior of a-Si:H TFT under voltage stress. Circuit-level aging simulation is very useful in investigating and optimizing circuit stability. Therefore, a simulation method was developed for circuit-level ∆Vt simulation. Besides, a ∆Vt model which is compatible to circuit simulator was developed. The proposed method and model are verified by measurement results. A novel pixel circuit using a-Si:H TFTs was developed to improve the stability of OLED drive current over stress time. The ∆Vt of drive TFT caused by voltage stress is compensated by an incremental gate voltage generated by utilizing a ∆Vt-dependent charge transfer from drive TFT to a TFT-based Metal-Insulator-Semiconductor (MIS) capacitor. A second MIS capacitor is used to inject positive charge to the gate of drive TFT to improve OLED drive current. The effectiveness of the proposed pixel circuit is verified by simulation and measurement results. The proposed pixel circuit is also compared to several conventional pixel circuits.