TFT LCD Liquid Crystal Display Technology Analysis

- May 08, 2020-

The basic display principle of LCD is to control the change of light by controlling the arrangement state of liquid crystal molecules under each primary color of the color filter, so as to achieve the primary colors with different brightness to simulate various colors in nature. At present, a variety of working modes are derived from the basic principles of liquid crystal display, mainly including TN mode, STN mode, FLC mode and liquid crystal-polymer mode. Currently, twisted nematic liquid crystal (TN) is about to be eliminated, and super twisted nematic (STN) and active matrix (TFT) have matured and become popular.


Twisted Nematic (TN) LCD


In twisted nematic, the liquid crystal is nematic liquid crystal with a 90o twist. The twisted nematic liquid crystal display appeared earlier. In addition to the basic characteristics of liquid crystal display, it also has the advantages of high contrast, simple production technology and low cost. TN screens are widely used in portable calculators, instruments and clocks. At present, domestic LCD manufacturers mostly produce such products.


Thin film transistor (TFT) liquid crystal display is an active matrix display formed by introducing a thin film transistor switch in a twisted nematic (TN) liquid crystal display, which overcomes the disadvantages of passive matrix display cross-interference, less information, and slow writing speed. Greatly improve the display quality, making it applicable to computer high-resolution full-color display and other fields. The thin-film transistor (TFT) currently used is based on the structure of an amorphous silicon thin-film transistor (α-Si TFTAM-LCD).


There is a TFT array on the lower glass substrate, the ITO electrode of each pixel is connected with the TFT drain electrode, the gate is connected with the scan bus, and the original power supply is connected with the signal bus. When the scanning signal voltage is applied, the storage capacitor is charged, and the signal voltage of the storage capacitor is applied to the liquid crystal pixel within the frame frequency to make it in the gated state. When addressing again, the signal voltage is charged or discharged. Each pixel is isolated by a thin-film transistor switching element, which prevents cross-interference and ensures the response speed of the liquid crystal. At the same time, the gray level is obtained according to the size of the stored information. At present, the gray level can reach 256 levels, and 16.7 million colors can be obtained, which is almost available Full color display.


At present, the production line of thin-film transistor (TFT) liquid crystal displays has developed from the first generation to the sixth generation. The area of the substrate glass has been greatly increased every time it is replaced, and the output is continuously improved and the cost is continuously reduced. The glass substrate size of the seventh-generation thin-film transistor (TFT) liquid crystal display production line will reach 1870 * 2200mm. The LCD TV screen that can be made at present is 94cm (37inch), the maximum size of the laptop screen is 38.1cm (15inch), the monitor screen The maximum size is 63.5cm (25inch). The development of thin-film transistor (TFT) liquid crystal display also has the trend of thinning, lightening and low power consumption. Based on the development of new materials, the innovation of manufacturing process technology, the improvement of equipment accuracy and automation, and the advancement of software technology, the speed of replacement of thin-film transistor (TFT) liquid crystal display products is very fast.


LCD display faces huge challenges


In recent years, as display technologies such as OLED have been on the market one after another, taking advantage of its own advantages over liquid crystal in some respects has created huge challenges for liquid crystal displays. Because various displays have different advantages and disadvantages and their own characteristics, it is completely realistic to use a certain part of itself to replace or impact another type of display device. In the future, LCD displays should strive to make major breakthroughs from the following aspects:


1. Liquid crystal display requires a variety of methods such as reflective display technology and improved backlight to increase the aperture ratio and polarizer transmittance to improve display brightness and contrast.


2. Improve materials, device structure, process, and response speed of liquid crystal display. Develop some fast-response new LCD display modes to meet market video needs.


3. The narrow working temperature range is a major defect of the liquid crystal material, so the liquid crystal material should be improved. At present, liquid crystal materials that can work at minus 50 degrees to minus 90 degrees have been developed.


4. In order to realize the large screen display, the liquid crystal display develops the projection display. On the basis of the original transmissive amorphous silicon TFT projection display, in recent years, it has transitioned to polysilicon TFT projection display. Although polysilicon can increase the aperture ratio by more than 10% to 15%, which greatly improves the display brightness and clarity, it is not ideal. In order to compete with large-screen displays such as PDPs, in recent years, liquid crystal displays have developed a "liquid crystal on silicon" LCOS. The large-scale integrated circuit is used as a substrate, and the liquid crystal is assembled into a reflective micro-liquid crystal display. Large-screen display from 50 inches to 100 inches or more can be realized through reflective projection of external light sources.