What is The Structure of TFT (Thin-film Transistor)?

   TFT LCD screen is mainly composed of three parts: rear panel module, LCD layer and front panel module. A layer of liquid crystal is sandwiched between two glass substrates. A color filter is attached 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 passing through the liquid crystal produces pixels on the front panel. As shown in the figure:

rear panel module

   The rear plate module refers to the part behind the liquid crystal layer, which is mainly composed of the rear polarizer , the rear glass layer, the pixel unit (pixel electrode, TFT tube), the rear directional film, etc.
   The rear glass substrate is divided into many tiny grids, called pixel units (or sub-pixels), by a number of transparent metal film wires arranged horizontally and vertically and insulated from each other. Each cell has a transparent metal film electrode, called the pixel electrode, which is insulated from the surrounding wire. One corner of the pixel electrode is connected with two vertical and horizontal wires through a TFT thin film field effect tube made on glass substrate by printing method to form a matrix structure:
   The gate of the TFT field effect tube is connected with the horizontal line, the horizontal line is called the gate scan line or the X electrode, because it plays the role of TFT pass selection, also called the pass selection line; The source pole of the TFT tube is connected with the vertical line, which is called the source line or Y electrode. The drain of the TFT is integrated with the transparent pixel electrode. The function of the TFT tube is a switch tube, using the gate voltage applied to the TFT switch tube, can control the conduction and cut-off of the TFT switch tube.
   The side of the front and rear glass plates that contact the liquid crystal is not smooth, but has a zigzag groove, as shown below:
   The main purpose of this groove is to hope that long rod-like liquid crystal molecules will line up along the groove so that they will be neat. Because if it's a smooth plane, the liquid crystal molecules don't line up neatly. Cause the scattering of light, the formation of light leakage phenomenon. In the actual manufacturing process, it is not possible to make the glass plate into such a groove. Generally, a layer of PI (polyimide) is coated on the surface of the glass plate first, and then the cloth is used to do the friction action, so that the PI surface molecules are no longer scattered distribution, but in accordance with the fixed uniform direction. This layer of PI is called Alignment Layer (also known as directional film). It acts like the grooves of glass, providing interface conditions for the liquid crystal molecules to be evenly arranged, so that the liquid crystals can be arranged in a predetermined order.

LCD layer

   The LCD screen has a pixel electrode and a thin film transistor (TFT) on the rear glass plate, and a color filter on the front glass plate. The liquid crystal layer is sandwiched between the front and back glass layers.
   For TFT LCD screen, each pixel unit can be regarded as a layer of TN liquid crystal sandwiched between the pixel electrode and the common electrode. The liquid crystal layer can be equivalent to a liquid crystal capacitor (CLc), whose size is about 0.1pF. In practice, this capacitor cannot hold the voltage until the next time the picture data is updated, that is, when the TFT tube is fully charged to this capacitor, it cannot hold the voltage until the next time the TFT tube is charged to this point (at the usual 60Hz picture update frequency, it needs to hold the voltage for about 16ms). As a result, if the voltage changes, the gray scale will be incorrect. Therefore, when designing the panel, a storage capacitor Cs (usually formed by the wiring of the pixel electrode and the common electrode) will be added, with a value of about 0.5pF, so that the charged voltage can be maintained until the next image updated.

front panel module

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

   In the front glass substrate, is equally divided into many small grid, each grid with glass substrate after a pixel electrode, but the difference is that it has no independent electrode, and is covered with a piece of R (red), G (blue) and B (green), three colors of transparent thin film filter, called a color filter (or RGB color film), To restore the normal color.
   Red, blue, and green are the so-called three primary colors. That is to say, with these three colors, different colors can be mixed together. The three RGB colors are divided into three independent units, each of which has different gray-scale changes. Then the three adjacent RGB display units are taken as a basic display unit — pixel, and the pixel can have different color changes.
   In the figure, the black part between each RGB point, called the black-matrix, is mainly used to cover the part that is not intended to transmit light, such as the pixel electrode wire, TFT tube, etc.
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