Thin Film Transistors in Liquid Crystal Displays

Understanding More about TFTs

A thin-film transistor (TFT) is a field-effect transistor that is built by layering thin films on a glass substrate. This differs from the traditional diffusion into a silicon substrate more common for integrated circuits such as microprocessors. The TFT in LCDs controls each individual pixel in the display, as determined by the display driver’s integrated circuits. The TFT sets the level of electric field across a liquid crystal capacitor (three per pixel) which controls the polarization of the liquid crystal material.    The extent of the liquid crystal polarization determines the amount of light that reaches the color filter. The speed at which the TFT switch effect can be updated for changes in the picture is referred to as the refresh rate, measured in changes per second as Hertz (Hz).  

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Types of TFTs Used in Display Manufacturing

Amorphous Silicon TFT: Amorphous silicon (also a-silicon or a-Si) is a form of silicon that has no crystalline structure. a-Si is critical to producing TFTs because it is without any crystal structure that allows TFTs to be vapor-deposited onto large substrates. So, amorphous silicon is ideal to use as thin film transistors on nearly all active matrix LCD screens.

a-silicon is also preferred in display manufacturing due to its low temperature (as low as 75 degrees Celsius) and relatively inexpensive production process compared to other similar, better performing materials. 

However, the elimination of the crystal structure significantly compromises its performance as a semiconductor material. This is the crux of the issue with TFTs in displays: processing the semiconductor so it can be vapor deposited onto glass degrades its semiconductor abilities, while enhancing its sensitivity to light and temperature.

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Polysilicon TFT: Polysilicon (P-Si) is a material consisting of small silicon crystals. P-Si is an attempt to overcome the poor performance of Amorphous Silicon TFT by introducing a low level of crystalline structure into the semiconductor material. A drawback to polysilicon TFT is that it is manufactured using laser annealing, an expensive but necessary processing step.

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Metal Oxide TFT: Metal oxide TFT is a transistor used for amplifying or switching electronic signals, and the most common transistor used in both digital and analog circuits. For use in display manufacturing, metal oxide TFT combines the best aspects of a-silicon and polysilicon. It is, however, still a TFT and hence retained its drawbacks. Metal oxide TFT offers:

·         Better semiconductor performance than a-Si , but not as good as low-temperature polysilicon.

·         Low temperature fabrication, coupled with the potential for relatively cheap large-scale production.

·         Smaller pixel sizes than a-silicon, and hence higher resolution displays.

·         A larger aperture ratio compared to a-silicon per pixel, allowing for increased backlight transmission through the backplane. This can reduce the power consumption of the backlight, or to increase brightness on large-format TVs and 3D TVs.

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