Liquid crystal display (LCD):Normally white and normally black

Normally white and normally black

The LC display may be used in two different modes: normally white (or bright) and normally black (or dark). The former allows the backlight through while the latter blocks the backlight when the voltage across the LC cell is zero. The arrangement in Figure 11.6 is that for the more popular normally white LCD. A normally black LCD would have only one polarising plate.

There are several types of TNLC cells depending on the angular twist the molecules are subjected to. In the simple TN type, the molecules are twisted by 90º resulting in a drop in contrast when used with large screens. The Super Twist-Nematic (STN) has its molecules twisted from 180 to 260º to improve the contrast ratio. Finally, the Film Super Twist-Nematic (FSTN) twists the molecules by 360º. This is used for very high quality black and white LCDs.

Passive- and active-matrix LCDs

There are two matrix LCD technologies: passive-matrix LCD (PMLCD) and active-matrix LCD (AMLCD). In the PMLCD, pixels are addressed directly with no switching devices involved in the process as illustrated in Figure 11.7). The effective voltage applied to the LC must average the signal volt- age pulses over several frame times, which results in a slow response time greater than 150 ms and a reduction of the maximum contrast ratio. The addressing of a PMLCD also produces a kind of cross-talk resulting in blurred images because non-selected pixels are driven through a second- ary signal-voltage path. This places a limit to the number of pixels that may be used in a display and with it a limit on the maximum resolution.

In the AMLCDs, on the other hand, a switching device is used to apply the voltage across the LC (Figure 11.8) and hence a better response time becomes possible. In contrast to PMLCDs, the active type, AMLCDs has no inherent limitation in the number of pixels, and they present fewer cross- talk problems.

There are several kinds of AMLCD depending on the type of switching device used. Most use transistors made of deposited thin films, which are accordingly called thin-film transistors (TFTs). The most common TFT semiconductor material is made of amorphous silicon (a-Si). a-Si TFTs are amenable to large-area fabrication using glass substrates in a low temperature (300–400°C).

An alternative TFT technology, polycrystalline silicon, normally known as polysilicon or p-Si is costly to produce and especially difficult to fabricate when manufacturing large-area displays. Nearly, all TFT LCDs are made from a-Si because of the technology’s economy and maturity, but the elec- tron mobility of a p-Si TFT is 100 times better than that of an a-Si TFT. This makes the p-Si TFT a good candidate for a TFT array containing integrated drivers, which is likely to be an attractive choice for small, high definition displays such as view finders and projection displays.