Pixel switching

Active-matrix displays differ from the aforementioned displays in that they have a switch incorporated into each pixel (Tsukada 2000). This removes the limitations encountered in passive matrix displays but requires more sophisticated processing equipment to be used. The dominant pixel switch technology is the amorphous silicon thin-film transistor (TFT) on glass (Tsukada 2000), although other technology... 

FIGURE 6.4.15 A wraparound geometry can be used to reduce charge injection in the pixel switch. 

Another typical feature of a-Si TFTs is that the threshold shift under on-bias is more rapid than the shift under off-bias. However, TFTs used as pixel switches are in the on-condition for much less time than they are in the off-condition. For example, in an SXGA display, the TFTs are on for about 0.1% of the time and off for about 99.9% of the time. As a result, the direction of the net threshold shift under actual usage conditions can depend on the fabrication details of the TFT and must be determined experimentally. In any case, because pixel TFTs are left in the more unstable on-condition for a low duty-cycle and because they are subject to the compensating effect of the off-condition, the shift of an a-Si pixel TFT over a typical 10,000-hour lifetime is small, at most a few volts. This is taken into aeeount by allowing some voltage margin in the select and deselect voltages applied to the select lines. 

Huitema, E. et ah, Polymer-based transistors used as pixel switches in active-matrix displays, J. Soc. Inf. Display, 10, 195, 2002. 

From this we can see that each field component consists of a term (C2 or C3) that is modulated through a phase angle and a constant offset term (Ci or C4) that remains fixed as the pixels switch. This implies that the Fourier transform will contain some form of undiffracted DC term... 

Dyes for Color Filters. Colorhquid crystal display systems consist of LSI drivers, glass plates, polarizers, electrodes (indium—tin oxide), and microcolor filters. The iadependent microcolor filter containing dyes is placed on each Hquid crystal pixel addressed electrically and acts as an iadividual light switch. All colors can be expressed by the light transmitted through each filter layer of the three primary colors, ie, red, green, and blue (Fig. 12). 

Figure 11.10. NW smart pixels, (a) Schematic of an integrated crossed NW FET and LED and the equivalent circuit, (b) Shows SEM image of a representative device, (c) Plots of current and emission intensity of the nanoLED as a function of voltage apphed to the NW gate at a fixed bias of -6V. (d) EL intensity versus time relation when a voltage applied to NW gate is switched between 0 and +4V for a fixed bias of -6V. [Reprinted with permission from Ref. 59. Copyright 2005 Wiley-VCH Verlag.]...

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. 

A voltage-driven 3-a-Si H TFTs pixel electrode circuit was reported by Kim and Kanicki in 2002 [17] and is shown in Figure 9.3a. The pixel electrode circuit has five components Cst, a storage capacitor Tl, a switching TFT T2, an active resistor T3, a constant current driver TFT and an organic PLED. 

In 3-a-Si H TFTs 200 dpi AM-PLED the active-resistor had a channel width of 15 pm, and the driving and switching TFTs had channel widths of 105 and 30 pm, respectively, with the same channel length of 10 pm. The storage capacitance was 0.4 pF. The top and cross-section views of the AM-PLED backplane are shown in Figure 9.5. The inset shows a blow up of single pixel electrode circuit and its cross-section view. 

This set-up allows a pixel to be addressed at each intersection of a row and a column. This works line for nematic LCs in modest sized displays, i.e. up to 120 000 pixels, but beyond this size there is an increase in switching times and cross-talk between adjacent pixel elements leading to a loss in contrast. This problem can be overcome by using STN LCs, which are materials where the hehcal twist is increased to between 180° and 270°. These super twist LCs give a much sharper image than the 90° materials. This system is ideal for monochrome displays but even with these materials the response times start to get very slow with the several million pixels that are required for high contrast, full-colour displays. 

Whichever system is used, in order to produce the full colour gamut it is necessary to display one-third of the pixels as red, one-third as green and one-third as blue. This is achieved by having a colour hlter layer as one of the substrates within the display panel. A schematic of this arrangement is shown in Figure 5.6. The role of the LC material in this system is to act as a light switch. 

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