In TFT LCD generally use backlight technology, display by many can send out any color of light pixels, as long as the control of each pixel display corresponding color can achieve the purpose. In order to precisely control the color and brightness of each pixel, it is necessary to install a shutter-like switch after each pixel. LCD has three major components, TFT-LCD display, driver and control circuit, backlight, and components, as shown in the figure.



I. TFT-LCD screen structure.

In simple terms, TFT LCD mainly consists of three parts: backplane module, liquid crystal layer, and frontline module. Two glass substrates sandwiched between a layer of liquid crystal. A color filter is applied to the front LCD panel and a thin-film transistor (TFT) is made on the back TFT panel. When a voltage is applied to the transistor, the liquid crystal turns and light passes through it to produce pixels on the front panel. This is shown in the figure.



(1) Backplane module part

After the plate module refers to the back part of the liquid crystal layer, mainly by the post-polarization plate, after the glass plate, pixel unit (pixel electrode, TFT tube), after the directional film and other components.

In the rear glass plate substrate distributed in many horizontal and vertical arrangement and each other insulated lattice of transparent metal film wire, the rear glass substrate is separated into many tiny lattices, called pixel unit (or sub-pixel); each lattice (pixel unit) and a piece of transparent metal film electrode and the surrounding wire insulation, called pixel electrode (display electrode). A corner of the pixel electrode, through a printed method produced on the glass substrate TFT film field effect tube, respectively, with two longitudinal and transverse wires connected to form a matrix structure, as shown.




TFT FET's gate and the horizontal line is connected, the horizontal line is called the gate scan line or X electrode, because of the role played by TFT select through, also known as select through-line; TFT tube's source and vertical line connection, the vertical line is called the source line or Y electrode; TFT drain that is connected with the transparent pixel electrode as one. TFT switch tube on and off.

The side where the front and back glass plates contact the liquid crystal is not smooth, but has jagged grooves, as shown in the right figure below.



Directional membrane

The main purpose of this groove is to align the liquid crystal molecules in long rods along the groove so that they will be aligned neatly. This is because if the surface is smooth, the liquid crystal molecules will not be aligned neatly. This causes scattering of light and the formation of light leaks. In the actual manufacturing process, and can not make the glass plate so groove-shaped, generally will first in the glass surface coated with a layer of PI (polyimide), and then use the cloth to do the rubbing action, so that the PI surface molecules are no longer scattered distribution but in accordance with the fixed uniform direction of the arrangement. And this layer of PI is called directional film (also known as directional film), its role is like a glass groove, to provide liquid crystal molecules were uniformly arranged interface conditions, so that the liquid crystal in accordance with the predetermined order.



(2) Liquid crystal layer part

The rear glass panel of the LCD has pixel electrodes and thin-film transistors (TFTs), while the front glass panel has a color filter affixed to it, and the liquid crystal layer is sandwiched between the front and rear glass layers.



For TFT LCD, each pixel cell from the structure can be seen as a layer of TN liquid crystal sandwiched between the pixel electrode and the common electrode, the liquid crystal layer can be equated to a liquid crystal capacitor CLC, its size is about 0.lpF; in practice, this capacitor can not hold the voltage to the next time to update the screen data, that is to say, when the TFT tube on the capacitor charged, it is not possible to hold the voltage until the next time the TFT tube charges this point again (at a typical 60Hz screen update frequency, it needs to be held for about 16ms). In this way, there is a change in voltage, the displayed grayscale will not be correct, so the panel is usually designed with another storage capacitor Cs (generally formed by the pixel electrode and common electrode alignment), the value of which is about 0.5pF, so that the charged voltage can be maintained until the next time the screen update moment.

From the drive mode, the TFT LCD screen will all the row electrodes as a scan line connected to the gate driver, all the column electrodes as a column signal end connected to the source driver, thus forming a drive array, drive array equivalent circuit as shown below.



(3) Front panel module section

The basic structure of the color filter is composed of the glass substrate, black matrix, color layer, protective layer, ITO conductive film.



On the front glass substrate, it is also divided into a number of small cells, each cell corresponds to a pixel electrode on the rear glass substrate, but the difference is that it does not have a separate electrode, but is only covered with a small piece of R (red), G (blue), B (green) three primary colors of transparent thin-film filter, called color filter (or RGB filter), to restore the normal color.

Red, blue and green are the so-called three primary colors, that is, using these three colors, can be mixed to produce a variety of different colors, CRT color TVs and monitors are using this principle to display colors. The three RGB colors are divided into three separate units, each with a different gray level, and then the three adjacent RGB display units are used as the basic unit of a display - the pixel - which can have different color variations.

In the diagram, the black part between each RGB point, called the black matrix (matrix block), is used to cover the parts that are not intended to transmit light, such as the pixel electrode alignment, TFT tube, etc.



Polarizer accounts for about 11% of the cost of the LCD panel, is also the main material, but also one of the most difficult areas of China's LCD panel technology localization, the polarizer is located on both sides of the LCD panel, by transmitting or blocking the light emitted from the backlight module, adjust pixel brightness and reproduction of color, so that the human eye to the colorful display image, without it, the LCD panel can not be displayed.



Two glass substrates, upper and lower.

The glass substrate is more than just two pieces of glass, it has a trench structure on the inside and is attached to a supporting film that allows liquid crystal molecules to line up neatly along the trench. TFT thin-film transistors and color filters are affixed to the top and bottom two glass sides.

The ITO transparent conductive layer.

Its role is to provide a conductive path, divided into pixel electrode (P-level) and a common electrode (M-level).

Thin-film transistor (that is, we often say TFT): often said TFT-LCD, which actually refers to this thin-film transistor, its role is similar to a switch, TFT can control the signal voltage on the IC control circuit, and send it to the liquid crystal molecules, to determine the size of the angle of deflection of liquid crystal molecules, so it is a very important component.

Liquid crystal molecular layer.

It is the most important element in changing the polarization state of a ray of light and determines its alignment and polarization state through a combination of electrical and elastic forces.



Color filters.

The light reflected by the liquid crystal molecules can only display different gray levels, but it cannot provide the three primary colors of red, green, and blue (RGB), while the color filter is composed of three types of RGB filters that adjust the color and brightness of each color by mixing the three. Each pixel in the LCD panel consists of three dots - red, green, and blue - and each color dot has a different grayscale variation.

Frame rubber.

It is to make the top and bottom two layers of the glass substrate in the LCD panel can firmly adhere together, and the whole internal system and external "isolation", to prevent dust from entering and affecting the color effect.


Ⅱ.Drive and control circuit



Ⅲ. Backlighting and components


Because the liquid crystal molecules themselves are unable to emit light, so if you want to appear screen, LCD screen needs a special light source to provide light, and then after the deflection of liquid crystal molecules to produce different colors. And the backlight source plays the role of providing light energy.


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TFT-LCD Basics