TFT-LCD: What is it?

Put simply, a TFT liquid crystal display is a device controlled by electric signals. The liquid crystal sits between two transparent layers of conductive ITO electrodes. Liquid crystal molecules are aligned in different directions by varying the voltage applied to the ITO electrodes (See Diagram 2). The direction of the LC molecules directly affects the penetration level of the light source, which in turn creates the desired lightness and darkness in the image, also known as grayscale. Color is produced by the color filter substrate. Pixel grayscale is decided by designated voltage levels from the data driver.


Panel Size Evolution

The advancement of digital technology has been followed by the introduction of a number of new digital-application products that are bringing new standards of convenience and entertainment to daily living. All digital electronics need a display interface to present content and functionality to users. The display interface has become the focus of intense development and several new types of related technology are now vying for the dominant spot on the market. Industry players have committed significant resources to the development of new technology and applications with the aim of further enhancing our visual experience.


The development of Thin Film Transistor Liquid Crystal Displays (TFT-LCDs) has allowed for a much wider display interface and color with much higher resolution. The different generations of TFT-LCD technology are defined by the size of the glass substrate used in production. Ever increasing demand for large-size panels means that the newer the generation of production, the larger the substrate measurements, which allow manufacturers to cut a greater number of large-size panels, in effect lowering costs and improving economic returns.


Substrate Generation Sizes (Diagram 2)



Production Process for Liquid Crystal Display Panels

For TN (twisted nematic) LCD panels, which primarily use liquid crystal material with positive magnetic susceptibility, electrodes are placed on either side of the liquid crystal substrate layer. When voltage is applied and the electric field is created, the liquid crystal molecules line up parallel to the electric field, producing the inclination to align vertically. For MVA LCD’s, which primarily use liquid crystal material with negative magnetic susceptibility, electrodes are placed on either side of the liquid crystal layer. Once voltage is applied and the electric field is created, the LC molecules orient themselves 90 degrees to the electric field, and the LC molecules become inverted. For IPS LCD panels, which mainly use liquid crystal material with positive magnetic susceptibility, paired electrodes are placed on one side. When an electric field is created the liquid crystal molecules orient themselves parallel to the electric field and rotate on the same plane.


With TN LCD panels, the image produced will vary from different viewing angles; variations in brightness and even grayscale inversion are all typical of TN LCD panels. Thanks to mutual compensation alignment, MVA LCD panels produce an image that does not vary with different viewing angles – and there’s no risk of grayscale inversion. With IPS, paired electrodes are placed on the side of the substrate making the current run parallel with the material. This technology aligns the liquid crystal parallel to the front screen, thereby increasing the viewing angle.



Three Major Steps in the TFT-LCD Production Process

– The front-end Array process is similar to that for semiconductors, the only difference being that the thin-film transistors are placed on the glass substrate, not on the silicon wafer.
– The Cell process fits the Array substrate to a color-filter substrate; liquid crystal is then inserted between the two substrate layers.
Back-end Module Assembly
– Back-end Module Assembly involves taking the panel from the Cell process and bonding the LCD driver IC, and assembling backlights, metal frame and other components to make the finished product.


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