What is the Structure and Working Principle of TFT LCD?

    LCD displays can be seen everywhere in our life, such as TV screens at home, control switch displays, computer displays, roadside advertising LCD screens we see when walking on the street, signs in shopping malls, etc. Then, what is the structure and work principle of TFT LCD displays?

    Definition of TFT LCD

    TFT (Thin Film Transistor) LCD, refers to thin film transistor (color active matrix). TFT is a technology for LCD.

    The technology field has different technologies, TFT and TN, which have great differences. The viewing angle of TFT is wider, and the viewing angle of TN is narrower. In terms of color, the color of TFT is more vivid and rich, because the display of TFT adopts “back-through” illumination from LED light, which is to set a special light pipe on the back of the LCD. When the FET electrodes are turned on, the molecules of the LCD change, and the purpose of the display is achieved by shading and transmitting light, which improves the response time of the TFT LCD.

    Compared with TFT and TN, TFT‘s contrast is higher, the color is richer, and the picture update frequency is faster, so TFT is also called “true color”. At the same time, TFT-LCD configures a semiconductor switching device for each pixel, so that each pixel can be directly controlled by a single-point pulse, so each node is relatively independent and can be controlled continuously. This is one of the reasons why TFT has more obvious advantages than TN.

    There is no doubt that the products of notebook/PC monitor manufacturers use TFT-LCD. At present, with the improvement of intelligent life, TFT LCD is also applied to many products. Smart home products, such as refrigerators, coffee machines, kitchen range hoods, ovens, rice cookers, etc., as well as home central controllers, temperature and humidity controllers, smart speakers, etc., in the automotive field, automotive speed displays, smart front drives Display, carriage advertising screen, etc., various industrial displays in the industrial field, large industrial machines, computers, etc. TFT LCD products have very wide range of uses in industrial, medicine, automotive and daily life.

    Moreover, with the continuous development of technology, the yield rate has been continuously improved, and the emergence of new technologies has made TFT-LCD a great improvement in response time, contrast, brightness, and viewing angle. With the development of science and technology, people’s demand for large touch screens is also increasing, which is a huge progress in human interaction, and it is also the performance of TFT LCD’s continuous expansion of application fields.

     

    TFT type liquid crystal display structure

    The usual a-Si TFT is mainly composed of a glass substrate, a gate electrode, a gate insulating layer, a semiconductor active layer a-Si, an ohmic contact layer n+a-Si, a source-drain electrode and a protective film, among which the gate insulating layer and the protective film SiN is generally used.

    The structure of a-Si TFT can be divided into four typical structures: the source, drain and gate electrodes are located on the same side of the semiconductor active layer a-Si as a planar structure, and the source, drain and gate electrodes are located on the upper side of the a-Si layer. It is called a positive gate plane structure, and the source, drain, and gate electrodes are located on the lower side of the a-Si layer, which is called an inverted gate plane structure; the source, drain, electrode and gate electrodes are located on both sides of the a-Si layer. On the upper side of the a-Si layer, the source and drain electrodes on the lower side of the a-Si layer are called positive gate staggered structure or top gate structure, the gate is on the lower side of the a-Si layer, and the source and drain electrodes are on the upper side of the a-Si layer. It is called inverted gate staggered structure or bottom gate structure.

    From the point of view of the manufacturing process, the three layers (or two layers) of SiN, a-Si and n+a-Si in the staggered structure can be deposited continuously, which is suitable for pipeline operation and can reduce cross-contamination. Now, the staggered structure has become the mainstream, it can not only operate continuously for a-Si, SiN., n+a-Si, but also the inverted gate can also be used as a light shielding layer (no need for another light shielding layer), which is very suitable for a-Si TFT. is important because a-Si is sensitive to light, and once light flows in and causes leakage current to increase, image quality will deteriorate.

    The working principle of TFT LCD

    From the sectional structure diagram of TFT-LCD, it can be seen that the LCD is composed of two layers of glass substrate sandwiching the liquid crystal to form a parallel plate capacitor. The capacitor and the built-in storage capacitor are charged through the TFT embedded on the lower glass substrate to maintain each The voltage required for one image until the next image is updated. The TFT LCD itself does not emit light by itself, so a backlight is required, and a backlight panel must be added to the LCD to provide a light source with high brightness and uniform brightness distribution. The LCD actually turns on the light from the light source behind it to represent its colors, but most of the backlight we can see is a white LED light source.

     

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