Low temperature polysilicon thin film transistors (Poly-Si TFT) have recently been used to design integrated driver circuits for active matrix liquid crystal display. Although the performance of Poly-Si TFT is inferior to the conventional single-crystal MOSFET, they have the important advantage of being compatible with fabrication on glass substrate. This can greatly reduce the system cost and increase the long-term reliability. At the present time, there are digital driver circuits realized with gray scale up to 6 bits or 8 bits resolution. This means the number of gray levels for each RGB pixel is limited to 256 levels only. In order to have a full gray scale colour display, analog driver circuit is a good candidate....[ Read more ]

Low temperature polysilicon thin film transistors (Poly-Si TFT) have recently been used to design integrated driver circuits for active matrix liquid crystal display. Although the performance of Poly-Si TFT is inferior to the conventional single-crystal MOSFET, they have the important advantage of being compatible with fabrication on glass substrate. This can greatly reduce the system cost and increase the long-term reliability. At the present time, there are digital driver circuits realized with gray scale up to 6 bits or 8 bits resolution. This means the number of gray levels for each RGB pixel is limited to 256 levels only. In order to have a full gray scale colour display, analog driver circuit is a good candidate.

One of the major building blocks in analog driver circuit is the operational amplifier (opamp). However, the large threshold voltage variation of Poly-Si TFT over a wafer creates the problem of non-uniformity in opamp. In addition, the poor saturation characteristic called “kink effect” also makes it difficult to design of an opamp with good performance. In this research, a new biasing method is introduced to eliminate the effect of threshold voltage variation in the performance of opamp. With the use of a novel “kink free” Poly-Si TFT structure, good performance opamp and other building blocks for analog driver circuit are designed. In order to predict the performance of the driver circuit, circuit simulation model is needed. However, there is no standardized Poly-Si TFT Spice model available. Therefore, a modified HSPICE model is also developed based on the characteristic of the Poly-Si TFT structure being used.