Active matrix LCDs pixel

Fig. 4. (Left) Schematic representation of an active matrix LCD display, showing single transistors driving capacitive pixel elements. (Right) OLED displays, on the other hand, require current-based driving, and therefore, multi-transistor pixel architectures are more common.

Figure 4.7 Set-up of a typical active matrix LCD Exploded" view of twelve pixels (left) and cross-section of three sub-pixels in the basic colors (right) (PI = polyimide, TFT = thin film transistor). The spacers are used to adjust the cell gap to typically 5-6 pm.

Supertwlsted nematic displays may be passive-matrix LCDs, containing no active (switching) electronic components. Nowadays much higher performance, especially for colour displays. Is obtained from active-matrix LCDs. In this construction, a thin-film transistor Is added to each pixel to ensure an adequate and constant drive is maintained between refresh cycles. This [Pg.466]

By far the most important application area of liquid crystals per se is of course in display devices. Who can fail to be amazed at the technological triumph of an active matrix LCD flat projection panel capable of displaying 1024 x 768 pixels X 256 colors Perhaps the insights derived from PLCs will have some impact on displays, or even modulators, nonlinear optical devices or data storage devices. 

One way in which the sharpness of an LCD can be increased is to fabricate the display with an active device such as a thin film transistor at each pixel. This is shown in Figure 13.12, where it can be seen that the voltages are applied to the electrodes and transistor in such a way that the response is determined mostly by the transistor rather than the display itself A properly designed transistor can increase the sharpness of a LCD a great deal, allowing for highly multiplexed displays. Such an active matrix LCD is used in laptop computers (see Plate 8). 

Active-Matrix LCDs. Increase in FPD size along with demand for video response equivalent to the CRT made it necessary to avoid the high level of cross talk between adjacent pixels in passive displays. The nonlinear response of the liquid crystals was no longer suffieient and it became apparent that a switch was needed at each pixel. In principle, several switching technologies could be utilized since they all could be fabricated with films and photolithography. Metal-insulator-metal (MIM) devices, diodes, and transistors have all been tried. Thin-fihn transistors (TFTs) have performed the best and as a result have been adopted for most active-matrix applications. 

Many LCDs are based on active-matrix addressing, in which an active device circuit containing one or more TFTs is connected to each pixel. The TFT circuit at each pixel effectively acts as an individual electrical switch that provides the means to store display information on a storage capacitor for the entire frame time, such that the pixel can remain emitting during this entire time rather than for a small fraction of time, as is the case in passive addressing. 

R. Hattori, Y. Kuroki, and J. Kanicki, Analog-circuit simulation of the current-programmed active-matrix pixel electrode circuits based on poly-Si TFT for organic light-emitting displays, Proc. AM-LCD, 223-226, 2001. 

Amorphous Si H has an effective-field drift mobility about 0.2 cm2 V-1 sec-1. To gain some perspective on how this material might fit into active matrix display applications, the requirements on a TFT designed to drive a 1-mm2 GH LCD pixel will be discussed. In the linear regime, the transistor can be modeled as a (gate-voltage-controlled) resistor, and the minimum ON resistance Rmi is approximately given by the estimate... 

Direct, multiplex and active matrix addressing are the three electronic drive methods used to generate the appropriate voltage at a particular pixel of an LCD, see Figures 2.11-2.14. The size, shape and pattern of electrodes on LCD substrates are fashioned to be compatible with the chosen method of addressing. In directly addressed LCDs the desired pattern of pixel electrodes is created by etching on one surface. A non-pattemed back electrode on the second surface provides the electrical contact. LCDs with multiplexed addres-... 

OLEDs can be addressed in a similar fashion to LCDs, see Chapter 2, i.e. directly with segmented electrodes, see Figure 4.8, by multiplexed addressing with rows and columns of electrode strips, see Figure 4.9 and by active matrix addressing with one transistor at each pixel, see Figure 4.10. The major... 

An active matrix screen works in a similar manner to the LCD watch. The screen is made up of several individual LCD pixels. A transistor behind each pixel, when switched on, activates two electrodes that align the crystals and turn the pixel dark. This type of display is very crisp and easy to look at. 

Active matrix displays have a thin-film transistor (TFT) switching circuit embedded in the area of each individual pixel. Although the TFT backplanes needed for polymer emissive displays are similar to those developed for liquid crystal displays (LCDs), the TFT circuits must be capable of switching much higher currents than are required for LCDs. For active matrix displays, the luminescent semiconducting polymer and cathode, etc. are deposited directly onto the premanufactured TFT backplane (the anode for the LED pixel is built onto the TFT circuit). In an active matrix display, the pixels are held at constant brightness by the TFT circuit and the image is refreshed at video rates (e. g. 60 Hz). 

The essential principle of an active matrix display is that each pixel has associated with it a semiconductor device that is used to control the operation of that pixel. It is this rectangular array of semiconductor devices (the active matrix) that is addressed by the drive circuitry. The devices, which are fabricated by thin-film techniques on the inner surface of a substrate (usually glass) forming one wall of the LCD cell, may be either two-terminal devices (Fig. 6) or three terminal devices (Fig. 7). Various two-terminal devices have been proposed ZnO varistors, MIM devices, and several structures involving one or more a-Si diodes. Much of the research effort, however, has concentrated on the three-terminal devices, namely thin-film, insulated-gate, field-effect transistors. The subject of thin-film transistors (TFTs) is considered elsewhere in this volume suffice it to say that of the various materials that have been suggested for the semiconductor, only a-Si and poly-Si appear to have serious prospects of commercial exploitation. 

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