Addressing of Liquid Crystal Displays

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- 

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C. R. Stein and R. A. Kashnow, Recent Advances in Frequency Coincidence Matrix Addressing of Liquid Crystal Displays. Digest 1972 Soc. for Information Display International Symp., San Francisco, Calif, p. 64. 

On the basis of the field-effect studies the authors and colleagues at the Royal Signals and Radar Establishment (RSRE), -Malvern, proposed in 1976 the use of a-Si H field-effect devices in the addressing of liquid crystal matrix displays, as an alternative to the thin-film CdSe transistors which had... 

M. Hareng and S. Le Berre, Formation of Synthetic Images on a Laser-Beam-Addressed Smectic Liquid-Crystal Display, Elec. Letters 11 73 (1975). 

Addressing Techniques of Liquid Crystal Displays Temkar N. Rnckmoi athan... 

The problems of rms addressing of liquid crystal devices can be circumvented by incorporating a semiconductor switch at each pixel of the display. The most usual architecture uses a field effect transistor as the switch [69], while other active components such as diode networks [70] and MIM (metal-insulator-metal) switches [71] have also been used (Fig. 18). The semiconductor material is usually amorphous silicon, which can be deposited and processed at temperatures compatible with a glass substrate. Polycrystalline silicon and other semiconductors, especially cadmium selenide, are also used in special applications such as those requiring very small pixel geometries and, by virtue of their higher carrier mobility, offer the... 

The double threshold addressed matrix liquid crystal display panel uses a two-terminal device that accomplished both set and reset functions for each pixel with appropriate signal pulses from the gate electrode drivers. To realize this panel, an amorphous silicon (a-Si) PIN thin film diode (TFD) matrix construction is used. The circuit diagram of a matrix panel addressed by these diodes is shown in Fig. 1 [4], Each pixel is composed of a pair of diodes and one liquid crystal cell connected in series between a gate electrode bus and a data electrode bus. The diode pair is referred to as a diode ring, because the connection is a ring. 

The characteristics of this layer can be used to drive matrix liquid crystal display panels instead of the SiN diode device. It is called a metal-insulator-metal (MM) diode. A serious problem in fabricating the MM diode addressed matrix liquid crystal display panel which has a large number of gate electrodes is to reduce stray capacitances which are parallel to the MM diodes. 

An example image displayed on the MIM diode addressed matrix liquid crystal display panel is shown in Fig. 6 [27], The actual display area is 100x96 mm. There are 240 gate and 250 data electrodes which provide a resolution of 25 lines cm . The contrast ratio obtainable is very high. 

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)... 

As mentioned earlier, probably the most promising application of a-Si H FETs is their use in addressable liquid crystal displays, which will be discussed in the following Chapter. Here we shall review the results of some exploratory experiments on the application of the FETs to a number of logic and image-sensing circuits. 

The sharper thresholds required for the addressing of larger displays can be brought about by constructing liquid crystal cells with sharper transitions (see Fig. lc) (Boyd et al., 1982 Wilson et al., 1983), intrinsic threshold control, or by the addition of nonlinear electronic elements, extrinsic threshold control. Liquid crystal displays with gradual turn on such as DSM or GH LCDs always require extrinsic threshold control for matrix addressing. 

The last volume of this miniseries, 21, Part D, covers device applications, including solar cells, electrophotography, image pickup tubes, field effect transistors (FETs) and FET-addressed liquid crystal display panels, solid state image sensors, charge-coupled devices, optical recording, visible light... 

The pixel circuit requirements for the scanner are similar to those of the liquid crystal display. A 300 spi array scanning an image at 3 inch s must read each line in a time of 1 ms. For a matrix of 50 gate and data lines, each pixel is addressed for 20 ps. The capacitor of the sensor is about 1 pF (note that 300 spi corresponds to a pixel of about 80 pm which is smaller than in the liquid crystal example). The required TFT current of 1 pA is easily obtained. The low oflF-current of the TFT also ensures that the charge will not leak away between being read out. 

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