Active-Matrix-Addressed Liquid Crystal Displays

Active-Matrix-Addressed Liquid Crystal Displays 

No consensus has yet been reached as to what constitutes the optimum design of display transistors. This is not surprising, since the optimum design is not one that maximizes individual transistor performance, but rather one that simplifies processing and maximizes the number of functioning devices. 

After experimenting with inverted, noninverted, and dual-gate a-Si H TFT prepared with quasi-ohmic Ti-Al source-drain contacts, the author selected inverted TFTs with Ti-Al top contacts (see Fig. 6) for the active matrix. 

Important factors in the selection of the substrate material are planarity, ease of sealing cost, and the consideration that soda-lime glass substrates, unlike quartz, cannot be processed in a regular silicon line. 

The following section will discuss the layout of a display built in 1981 at the Hewlett-Packard Solid State Laboratory. This display includes two innovative features (i) an edge-free layout of the transistors and (ii) a photoconductive feedback mechanism that maximizes the ON current and minimizes the cumulative trapping at the dielectric-a-Si H interface. 

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V. Active-Matrix-Addressed Liquid Crystal Displays. . 127... 

Kaneko E Active matrix addressed displays Handbook of Liquid Crystals Vol 2A. Low Molecular Weight Liquid Crystals led D Demus, J Goodby, G W Gray, H-W Speiss and V Vill (New York Wiley-VCH)... 

Liquid crystal displays (AM-TFT-LCDs) with active matrix thin film transistor addressing 48000000... 

Compared to STN displays, active matrix addressing in TFTs allows enhanced sharpness and greater multiplexing, each liquid crystal pixel is addressed by a transistor, which thus primarily governs the response of the device. TFT i greater number of pixels (higher resolution) and number of colour levels than STN devices. They are widely used ii computers, although they are more expensive than STN displays. Further details can be found elsewhere [115]. 

Lueder, E. (2001). Liquid Crystal Displays Active and Passive Matrix Addressing Techniques, Wiley, New York. 

For active matrix addressing to work properly, it is necessary to have a sharp discontinuity in the transfer characteristic of the device at each pixel at the intersection of the row and column address Hnes. In some cases, the characteristics of the display device itself can be used to provide this discontinuity. The breakdown potential of a gas discharge can be used (for plasma displays) or alternatively, liquid crystals can be made to have a reasonably sharp transfer curve. 

Liquid crystal displays have been widely employed in high-information content systems and high-density projection devices. The steady progress in active-matrix Hquid crystal displays, in which each pixel is addressed by means of a thin film transistor (TFT), has led to the development of fuU-color video TFT-addressed liquid crystal light valve (LCLV) projectors. Compared with conventional CRT-based projectors. 

The most advantageous technology of display production is active matrix addressing. According to forecasts for 1995 the main type of liquid crystal screens used in personal computers, portable TVs, and measuring devices will be based on this technology [1]. We will consider matrix active addressed displays in more detail. 

Ferroelectric liquid crystal displays have potential as very fast displays and also do not require active matrix addressing technology. Due to their fast response times, they also have potential applications as high-speed electro-optical shutters or spatial light modulators. However, due to fabrication... 

The ITO layers on both glass substrates are patterned to generate conductive areas in which the liquid crystal orientation can be modified by an applied electric field. These active areas are separated by nonconductive areas. Low information content displays for watches and calculators are typically addressed by the so-called direct-addressing method. Higher information content displays for PDAs, mobile telephones, laptops and monitors require matrix addressing. In TFT displays an active matrix is used to drive the pixels while in STN displays ad-... 

Many liquid crystal display modes have been devised, but the versatility and balance of properties offered by the twisted nematic (TN) device have proved very difficult to beat. It superseded the nematic djmamic scattering display used in early displays. To improve its performance, the TN device has been developed into new displays, i.e., STN and active matrix-addressed TN. Displays using dichroic dyes find a niche market in large information displays (airport displays), and, recently, devices using liquid crystals in conjunction with polymeric materials have been discovered. 

Figure 5.27 Schematic of an element of a thin-film transistor active matrix display. Each liquid crystal pixel is addressed directly by a transistor element in a matrix...

There are two types of matrix addressing schemes— passive and active. The passive matrix (PM) addressing scheme requires the row and coluiim electrodes to address each individual pixel. This scheme still promises well in the area of bistable device such as ferroelectric liquid crystal (FLC) display and bistable twisted nematic (BTN) display because they do not need a control unit for gray-scale capability. The active matrix (AM) addressing scheme is the most developed and widely adopted one in cmrent LC displays. In this scheme, each pixel is cormected to a small electronic switch or TFT made with o-Si, poly-Si, or CdSe. This switch not only enables the pixel to hold the video information until it can be refreshed, but also prevents cross talk among neighboring addressed pixels. 

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