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Structure Characteristics Of TFT-LCD
Apr 03, 2018

Unlike the TN technology, the TFT display uses "back - through" irradiation - the imaginary light source path is not from up to down like the TN liquid crystal, but from down to up. Such a way is to set special light tube on the back of the LCD. When the light source is irradiated, it will go upward through the lower polarizer. As the electrode of the upper and lower interlayer is changed into the FET electrode and the common electrode, the performance of the liquid crystal molecules will change when the FET electrode is connected. The purpose of the display can be achieved through the light shading and light transmittance, and the response time is greatly increased to about 80ms. Because it has higher contrast and richer color than TN-LCD, the screen refresh rate is faster, so TFT is commonly called "real color".  Compared to DSTN, the main feature of TFT-LCD is to configure a semiconductor switch device for each pixel. Each pixel can be directly controlled by point pulse. Therefore, each node is relatively independent and can be continuously controlled. This design method not only improves the response speed of the display screen, but also can control the display grayscale accurately, which is the reason why the TFT color is more realistic than the DSTN. In TFT-LCD, the function of TFT is equivalent to a switch tube. The commonly used TFT is the three terminal device. Generally, semiconductor layer is made on glass substrate, and there are two source and drain electrodes connected to them. The grid insulating film is opposite to the semiconductor phase and has a grid. The current applied to the grid is used to control the current between the source and the leakage pole. For the display, each pixel can be simplified from the structure, and a liquid crystal is sandwiched between the pixel electrode and the common electrode. More importantly, it can be regarded as capacitance from the perspective of electricity. The equivalent circuit is shown in Figure 1. To charge J row I column pixel P (I, J), we must switch t (I, J) to connect the signal line D (I) to the target voltage. When the pixel electrode is fully charged, even if the switch is disconnected, the charge in the capacitor is kept, and the liquid crystal molecules between the electrodes continue to have a voltage field effect.  The role of data (column) driver is to impose target voltage on signal line, and the function of gate driver is to switch on and disconnect. Because the display voltage added to the liquid crystal layer can be stored in the storage capacitance of each pixel, the liquid crystal layer can work stably. The display voltage can also be rewritten in a short time through TFT, so even in the high resolution LCD, the image quality requirements can be met. The key to display image is also the molecular orientation of liquid crystal under electric field.  Generally, through the orientation processing of the inner side of the substrate, we can make the arrangement of liquid crystal molecules produce desired structural deformation to achieve different display modes. Under the action of the electric field, the orientation changes of the liquid crystal molecules, and the intensity of the incident light changes after the liquid crystal layer. Thus the image display is realized. In a word, TFT-LCD is different from the simple matrix of passive TN-LCD and STN-LCD. It has a thin film transistor (TFT) on every pixel of the LCD screen, which can effectively overcome the crosstalk of non - selection, and make the static characteristics of the display LCD independent of the number of scanning lines, thus greatly improving the image quality. The characteristic of switch unit (TFT) is to meet the requirement of low on state resistance and very large closed state resistance.

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