Functioning of Active Matrix LCD

In a liquid crystal cell based on the TN mode, a homogeneously aligned layer of a nematic liquid crystalline material with positive dielectric anisotropy (Ac), helically twisted by 90°, is placed in an ITO-lined glass cell between crossed polarizers 

The first TN-LCD were simple, directly addressed segment displays as still used, e. g., for wrist watches. When attempts were made to increase the information content of the displays by time-sequential addressing in rows and lines (multiplexing) the limits of the TN cell were soon met. At higher multiplex ratios [14] contrast loss occurred, because of ever shorter addressing times. The development of the super-twisted nematic (STN) cell in 1984 [15] pushed the practicable limit to higher multiplex ratios, but it did not lead to a general solution of the problem. 

These efforts resulted in the first prototype of a 3-inch (7.5 cm diagonal) TFT display presented by Sharp in 1986. Because the manufacturing process of the TFT arrays is highly complex and expensive in terms of financial investment and human resources, the larger scale production of AM-LCD for use in notebook computers only started in 1989 [18]. 

Currently, innovation in LCD technology is driven predominantly by multimedia and LCD TV applications, which require displays able to show high quality moving pictures. The time span between two video frames at a rate of 25 per second (PAL standard) is 16.7 ms. This means that for a video compatible LCD a switching time below ca 15 ms has become an absolute necessity. 

For each particular application and for each display mode a nematic material with custom-tailored physical properties [23] is required. The most application-relevant of these properties are the temperature range of the nematic phase (i. e. the tern- 

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