2.1.1 Material and Display Theory of Liquid Crystal [4], [5]
Liquid crystal is a phase of matter whose order is intermediate between that of a liquid and that of a crystal. Fig. 2.1 shows the phase variation of liquid crystal in different temperature. Most liquid crystals consist of molecules shaped like the rod. Rod-shaped molecules are referred to as calamitic. Calamitic (Bahaur, 1990; Demus et al., 1998a,b) liquid crystal has many applications. One characteristic of the phase variation of liquid crystal materials is ―the twice melting‖ showing in Fig. 2.1. Below the melting point (Tm) they are solid, crystalline and anisotropic, when above the clearing point (Tc) they are clear and isotropic liquid. The material has the appearance of a milky liquid between Tc and Tm but still exhibit the ordered phases.
The phases during Tc and Tm can roughly be divided into smectic phase and nematic phase by its molecules arrangement. The molecules are ordered in two dimensions in smectic phase and appeared with only a one-dimensional order in nematic order. Most LCD materials‘
nematic phase is the basis and widely used as Twisted Nematic (TN) cell with active matrix addressing. Because the twist of liquid crystals can be controlled by the electric field that is applied across it, liquid crystals are used as a switch that passes or blocks the light.
The polarizer can block or pass the specific light by changing the phase of the polarizer.
In general, the first polarizer of a couple of polarizers is called polarizer and the second polarizer of these is called analyzer. The light can be blocked by a couple of polarizers with 90° phase shift, is shown in Fig. 2.2 (a). If we twist the liquid crystal molecule by applying the specific electric field across it, the light still can pass the polarizer. This is because the direction of liquid crystal molecules varies with electric field and it can guide the light along the long axis, shown in Fig. 2.2 (b).
Fig. 2.3 (a) shows a pixel of a transmissive twisted nematic LC-cell with no voltage applied. The white backlight f passes the polarizer. The light leaves it linearly polarized in the direction of the lines in the polarizer, and passes the glass substrate, the transparent electrode out of Indium-Tin-Oxide (ITO) and the transparent orientation layer. In this case, the analyzer is crossed with polarizer. The light can pass the analyzer without applied voltage due to the twisted nematic LC-cell and the pixel appears white. If a voltage VLC of the order of 10 V is applied across the cell, as shown in Fig. 2.3 (b), all molecules aligned parallel to the electric field. In this state, the wave that reaches the crossed analyzer is polarized in the same direction as at the input. Therefore, the analyzer blocks the light and the pixel appears black.
This operation is termed the normally white (NW) mode. On the contrary, if the analyzer is rotated by 90°, paralleled with polarizer, the light is blocked in the analyzer. The pixel is black.
This is called the normally black (NB) mode. The transmitted luminance, also termed transmittance, of the light. Fig. 2.4 shows the transmitted luminance versus the normalized voltage (VLC/V0) across the LC cell for the normally white mode and the normally black mode, respectively
2.1.2 Liquid Crystal Display Module Structure
The cross section structure of TFT-LCD panel is shown in Fig. 2.5 particularly. It can be roughly divided into two part, TFT array substrate and color filter substrate, by liquid crystal filled in the center of LCD panel. We still need a backlight module including an illuminator and a light guilder since liquid crystal molecule cannot light by itself. However it usually consumes the most power of the system, some applications such as mobile communications try to exclude or replace it from the system. In TFT array substrate, we need a polarizer, a glass substrate, a transparent electrode and an orientation layer. In color filter substrate, we also need an orientation layer, a transparent electrode, color filters, a glass substrate and a polarizer. Most transparent electrodes are made by ITO, and they can control the directions of liquid crystal molecules in each pixel by voltage supplied from TFT on the glass substrate.
Color filters contain three original colors, red, green, and blue (RGB). As the degree of light, named ―gray level‖, can be well controlled in each pixel covered by color filer, we will get more than million kinds of colors.
2.1.3 Equivalent Model of Dot in each Pixel Cell
One dot is the most fundamental unit of LCD panel and each dot can express one kind of original color. Because one full color should be mixed with three original colors, each pixel composed of three dots. Fig. 2.6 shows the basic layout and cross section of the AMLCD sub-pixel. The equivalent circuit of a TFT sub-pixel with voltages, currents, and parasitic capacitances is shown as Fig. 2.7 [4].
Fig. 2.8 and Fig. 2.9 show the layout and equivalent circuit of each sub-pixel, including two major structures, the CS on common mode and CS on gate mode. The right-down region of the sub-pixel layout is the TFT switch, and the region of each sub-pixel area excluding TFT switch and storage capacitor (CS) is called aperture region, which is the largest window for
light passing. So the larger ratio of aperture region to pixel area is the better performance of the TFT-LCD panel. In Fig. 2.9, the MS is a thin film transistor as a switch. The Clc is the effective capacitor of liquid crystals, and CS is the storage capacitor used to maintain the voltage level of liquid crystals during the hold time of frame transitions. The Cgd is the parasitic capacitor between gate line and effective liquid crystal capacitor. The structure, CS on gate, which connects the bottom of the storage capacitor to the previous row of the gate line has some benefits. By this structure, we can compensate the unstableness of voltage level due to the clock feed-through effect from Cgd. Furthermore, this structure also has larger aperture ratio. But the trade-off with the CS on gate method is an increase in the RC time constant of the gate line, which reduces the TFT switching performance.