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[Super Twisted Nematic (STN)  mode]

When the requirement for displaying high information content is present, the number of pixels becomes increasingly large and it is not possible to address a single each pixel. This led to the matrix addressing in which an array of M x N pixels is driven by apply pulses to the M rows and N columns. This significantly reduce the number of electric interconnects to the external driving ICs. For example, for a 150X150 pixel LCD, only needs 150+150=300 connects for matrix driving, instead of 150x150=22,500. Such scheme which scan one row at a time is called multiplexing. The TN mode is limited to about 10 rows of pixels because of the moderate transmission-voltage curve.

In order to have more rows to address for high resolution LCD, steeper t-v curve is required based on the Alt-Pleshko law, shown in equation below:

where N is the maximum addressing rows. Von and Voff are the voltages for the black and bright state respectively.

In the early 1980's, Nehring and Scheffer, using computer simulations, came up with the super twisted nematic liquid crystal (STN) which has a twist of over 180°„, typically between 210°„ and 270°„, its t-v curve is much steeper than the 900 TN, resulting larger number of addressable rows for multiplexing. Figure 1 shows a t-v curve comparison between TN and STN modes.

Figure 1. Contrast ratio (Ton/Toff) comparison between TN and STN.

With the steeper t-v response, it is possible to achieve higher contrast with the same voltage selection ratio (Von/Voff). Therefore, the multiplexity is greatly enhanced.

Another important parameter which affects the steepness of the t-v curve is the pretilt angle at the surface.

However, the STN intends to produce blue and yellow images because of its spectral peak at yellow and blue wavelength, instead of black and white; this causes problems to produce color display. The solution lies in stacking two STN films with opposite handiness of twist, and it is called Double STN (DSTN) as shown in figure 2. As a result, a black and white display can be achieved, with implementing the color filters, a full color LCD is possible.

Figure 2. A schematic illustration of a double STN display.


Further Readings and References:

T. J. Scheffer and J. Nehring, "Investigation of the electro-optical properties of 270°„chiral nematic layers in the birefringence mode", J. Appl. Phys. Vol. 58, 3022 (1985).

T. J. Scheffer and J. Nehring, "SUPERTWISTED NEMATIC (STN) LIQUID CRYSTAL DISPLAYS", Annu. Rev. Mater. Sci. 1997. 27:555®C83

P. M. Alt and P. Pleshko, "Scanning limitations of liquid crystal displays", IEEE Trans. Electron Devices, Vol. ED-21, No. 2, 146 (1974).

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Last update: April, 2006
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