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A thin-film transistor (TFT) is a type of field-effect transistor that is usually used in a liquid crystal display (LCD). This type of display features a TFT for each individual pixel. These TFTs act as individual switches that allow the pixels to change state rapidly, making them turn on and off much more quickly. Because these TFTs are arranged in a matrix, they are called "active-matrix" TFT.
Thin-film transistors are built by layering the thin films of an active semiconductor, hence the name, as well as a dielectric layer and some metallic contacts on a glass substrate. Glass is used because it is nonconductive with excellent optical clarity; it is also nonreactive to the chemicals used in semiconductor processing. In contrast, in the construction of a typical transistor, the substrate used is a semiconductor material, usually a silicon wafer.
Thin-film transistors are primarily used in LCD displays, which is why glass is used as the substrate. TFT technology is also used in both direct and indirect capture digital radiography detectors used in medical radiography. Active-matrix organic light-emitting diode (AMOLED) screens also have a TFT layer.