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What Are Thin Film Transistors?

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  • Written By: G.W. Poulos
  • Edited By: Daniel Lindley
  • Last Modified Date: 21 September 2014
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Thin film transistors are transistors created by depositing layers of semiconductor materials as thin films onto a substrate of some kind to form field effect transistors. Normally, a transistor’s substrate is the transistor’s semiconductor material itself. In the case of thin film transistors, however, the substrate is nonconductive and serves only to support the films that make up the physical structure of the transistors. Often, these substrates are made of glass or a similar type of stable composite. Thin film transistors find use almost exclusively in digital displays used for computers, televisions, and similar devices.

When used for displays, the substrate for thin film transistors serves as the back of the screen and is, itself, built of a number of layers. The layers on the substrate usually include an insulating layer, a polarizing layer, and then a glass layer. Atop this layer, the rear electrodes for the display are added as well as a series of horizontal and vertical command lines, which will be used to activate the thin film transistors. Then the transistors themselves are added.

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A series of films are placed on the substrate in specific locations to construct the transistors. First, a thin film of electrically charged semiconductor material is laid down to form the body of the transistors. Next, another film of semiconductor is laid down in specific locations that are electrically charged opposite to the first layer. A film that acts as a dielectric is then added, which prevents current from directly conducting through the transistors but allows electromagnetic fields to pass. Finally, another layer of semiconductor is added, which is charged to match the layer forming the transistors’ bodies.

Once the transistors are fully constructed on the substrate, a series of small metal contacts are added, which connects the operational sections of the transistors to the horizontal and vertical command lines already present on the substrate. To finish forming the display, a tough layer of polymer is added, followed by the front electrode. Finally, the RGB mask is added, which is what allows the display to show colors, followed by another layer of glass and another polarizing layer.

The completed display screen operates by activating the thin film transistors through the command lines. When they begin to conduct, the transistors create electromagnetic fields, which interact with the rest of the substrate to create a series of colored lights that serve as the display’s pixels. Typically, thin film transistor liquid crystal displays (TFT LCD) have one transistor for each color in each pixel. Very fast and stable, and requiring very small amounts of power to operate, these transistors allow for thin, light, durable displays that have high refresh rates and crisp definition between the individual pixels.

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