Thin Film Transistor Models and Pixel Circuits for AMOLED Displays
The significant progress in Organic Light-Emitting Diodes (OLEDs) triggered the development of Active Matrix OLED (AMOLED) display technology, which is a new-generation flat panel display technology. Hydrogenated Amorphous Silicon (a-Si:H) Thin Film Transistor (TFT) has good uniformity, high on-to-off ratio, low cost, mature processing technology, and is considered as a suitable choice to implement AMOLED pixel circuit.
However, a-Si:H TFT's degradation caused by electrical stress affects the lifetime of the AMOLED displays using a-Si:H TFTs. The following aspects are critical for the pixel circuits which are designed for the improved pixel performance and lifetime: (1) a compact device model of TFT is needed for the simulation of circuit nominal performance, (2) an aging device model of TFT is needed for the simulation of circuit aging behavior, and (3) novel pixel circuit designs are needed to compensate the TFT degradations without introducing expensive costs in speed, power consumption, and pixel size.
These challenges are addressed in this thesis. A compact device model of TFT to describe the static and dynamic behaviors is presented. The model is developed based on existing models but with the following improvements: (1) the scalability of the model with respect to the channel length is improved by using empirical methods, and (2) the convergence of transient simulation is guaranteed by the improved continuity of charge-based device model. Besides, the corresponding parameter extraction methods are presented.
The primary degradation mechanism of a-Si:H TFT under electrical stress is the threshold voltage shift (?Vt) over stress time. Circuit-level aging simulation is a powerful tool for lifetime estimation, degradation analysis, and the circuit design/optimization for reliability. Therefore, an aging model of ?Vt developed for circuit-level aging simulation is presented in this thesis. The model is compatible to circuit-level simulators, and implemented by using Verilog-A language. Simulation results verify that the model can correctly predict the ?Vt of TFT under various bias conditions.
In order to improve pixel circuit lifetime, a novel voltage-programmed pixel circuit using a-Si:H TFTs for AMOLED displays is proposed. The ?Vt of the drive TFT caused by electrical stress is compensated by an incremental gate-to-source voltage generated by utilizing the change of the charge transfer, which is related to the ?Vt of the drive TFT, from the 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 the drive TFT to improve the OLED drive current. The non-ideality of ?Vt-compensation, TFT overlap capacitance, programming speed, and OLED degradation are discussed. The effectiveness of the proposed pixel circuit is verified by simulation and measurement results.