Materials in Active-Matrix Liquid-Crystal Displays
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S BULLETIN/MARCH 1996
Thin-film transistors (TFT) are widely used as switching elements because of their superior optical performance for gray-scale output, although two terminal switching elements such as metalinsulator-metal (MIM) diodes and back-to-back (BTB) diodes have the advantage of relatively easy fabrication. A schematic diagram of the structure of a TFT is illustrated in Figure 2a. A pair of source and drain electrodes are placed on a semiconductor thin layer, with an ohmic layer at the interface for making an ohmic contact with the semiconductor. The semiconductor layer is overlaid on a thin layer of dielectric on which a gate electrode is located. The source-drain current is regulated by a shift in the Fermi level of the semiconductor near the dielectric interface, which is caused by induced charges at
Power source
Outgoing light /
Liquid crystal
Polarizer Transparent electrode on glass substrate
Incoming light Polarizer
Figure 1. Principle of liquid-crystal displays: Liquid-crystal molecules tend to align parallel to each other due to their chemical structure and orient themselves uniformly on a mechanically rubbed substrate, resulting in optical anisotropy— despite the fact that the molecules form a liquid. Their molecular orientation is very sensitive to an applied electric field. Thus a liquid-crystal cell works as a tiny optically tunable shutter that is controlled by a locally applied electric field and is sandwiched between a pair of polarizers, forming a single pixel. Each pixel is addressed by the top- and bottom-line electrodes of the cell according to information signals, producing a light image.
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Materials in Active-Matrix Liquid-Crystal Displays
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Polarizer Glass substrate Color filter Transparent electrode Aligning layer
Liquid crystal
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Materials for Thin-Film Transistors
TFT Semiconductor Source electrode
Drain electrode Aligning layer
Gate electrode
Glass substrate
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Polarizer
Source Line 1
Gate Line
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• TFT
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LC Cell
Gatel
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Source
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Figure 2. (a) Schematic structure of a thin-film transistor (TFT) in a liquid-crystal display (LCD). A TFT, which is fabricated on a glass substrate, consists of a gate electrode (a metal), a gate dielectric layer (an insulator), and a semiconductor layer with two electrodes for a source and a drain. The liquid crystal is sealed in a space between two glass substrates: one with TFT arrays and an aligned layer for orienting the liquid-crystal molecules, and another counter glass substrate on which a color filter, a transparent electrode, and the aligning layer are fabricated, (b) TFT arrays in an LCD panel. Each TFT is connected electrically to gate and source lines in an LCD panel. The number of TFTs is the same as the number of pixels in the panel: over 9 x 105 for a standard VGA panel of 640 x 480 dots.
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the interface associated with the gate bias. Thus its current-voltage characteristics are similar to those in a conventional metal-oxide-semiconductor field-effe
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