Summary

The effects of multi-channel structure on the device uniformity of LTPS TFTs and the output performance of source follower analog buffer are investigated in this chapter. It is clear that variations of the threshold voltage and subthreshold swing are reduced as channel stripes increasing, while no such tendency can be seen in the transconductance and leakage current.

The mechanism of the improving uniformity of multi-channel structure is studied in this chapter. It is proposed that probability effect is the most possible cause of improving uniformity of multi-channel structure. In conclusion, better uniformity of threshold voltage and the subthreshold swing of poly-Si TFTs can be obtained with channel slicing due to the more uniform grain boundaries densities between transistors according to the probability distribution. While the variations of the mobility and leakage current can not be reduced due

to the remained tail states variations of multi-channel devices. At last, the multi-channel structure is also employed to the conventional source follower to study the multi-channel effect on the output performance of buffer circuits. It is obvious that the output offset voltage variations of source follower with multi-channel structure in the driving TFT are reduced to about half of that in conventional source follower. It proves that the variations of poly-Si TFTs surely can be reduced by the multi-channel structure.

Chapter 4

Proposed Source-Follower Type Analog Buffer Circuits

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4.1 Introduction

Low-temperature poly-Si thin-film transistors (LTPS TFTs) have attracted a considerable attention for integrating driver circuits of TFT-based flat panel displays such as AMLCD and AMOLED [4.1]-[4.4]. In a poly-Si TFT-LCD, poly-Si TFT is used to implement the driving circuits with the pixel array on a single glass substrate. Such integration reduces the number of external components and the connections to display which allows us to reduce system cost and make compact, high reliable displays.

For the integrated data driver employing LTPS TFTs, output buffers are indispensable to drive the large load capacitance of data lines. When the digital to analog converter is insufficient for driving the large loading of data line, the output buffer is applied to enhance its driving capability. The main requirements of an output buffer for the data driver of flat pane display including high output voltage accuracy, high driving capability, small layout area, and low power consumption.

Analog buffer can be simply employed by a typical two-stage operational amplifier (op-amp) or a simple source follower. However, large output variations will exist in these simplest circuits due to the electrical characteristic variations of LTPS-TFTs such as threshold

voltage variation, carrier mobility variation, and subthreshold swing variations. That is mainly caused by the random grain boundaries distributed in the channel region of LTPS TFTs between devices across the glass substrate resulting from pulse-to-pulse variation of excimer laser energy density and non-uniform laser beam profile. Moreover, LTPS-TFTs have much poor subthreshold characteristic compared with single crystal Si transistor. Such poor electrical characteristics and non-uniformity of LTPS-TFTs will results to large output offset of the analog buffer employing LTPS-TFTs and lead to the wrong gray scale. Since thousands of output buffers are necessary for a poly-Si TFT-LCD, it is necessary to design an analog buffer with high immunity to the non-uniformity of LTPS-TFTs to eliminate the output deviation as possible. Several researches on LTPS TFTs analog buffer have been tried to carry out a buffer with high immunity to the device variations [4.5]-[4.18]. These compensating circuits can be classified into operational amplifier type (op-amp type) analog buffer [4.5]-[4.8] and source-follower type analog buffers [4.9]-[4.18] according to their circuit architecture. Operational amplifier is usually used as the output buffer in single crystal silicon integrated circuits. However, the complicated circuit configuration, high power dissipation and the huge output voltage variation of op-amp type analog buffer using LTPS TFT make it not suitable for system-on-panel application. Source-follower type analog buffer with simple configuration, low power consumption and higher immunity to the device variation of LTPS TFT is considered an excellent candidate for system-on-panel application.

In this chapter, a new simple source-follower type analog buffer using low-temperature polycrystalline silicon thin-film transistors (LTPS-TFTs) for the integrated data driver circuits of AMLCD and AMOLED is proposed. In additional to the electrical properties variations of driving TFT, LTPS TFTs have much larger subthreshold current compared with single crystal Si transistors, and this poor electrical characteristic will results to large output offset and cause the output voltage not to be constant with time. This output voltage unsaturated phenomenon will also make more difficulty in buffer circuit design. In order to overcome

these problems, a new compensated analog buffer composed of two n-type thin film transistors, one storage capacitor and four switches is proposed. An active load is added to the buffer circuit to suppress the unsaturated phenomenon of output voltage. The output characteristic of conventional source-follower analog buffer circuit is discussed at the first in this chapter. Next, the circuit configuration and detail operation principle of the proposed buffer circuit are shown and discussed. In order to study the performance of the proposed analog buffer, simulation and measurement results are shown and compared with the conventional one. Both the simulated and measured results exhibit that the proposed buffer circuit is capable of minimizing the variation from both the signal timing and the device characteristics.