A thin-film transistor (TFT) is a special type of MOSFET (metal-oxide-semiconductor field-effect transistor) made by depositing thin films of an active semiconductor layer as well as the dielectric layer and metallic contacts over a supporting (but non-conducting) substrate. A common substrate is glass, because the primary application of TFTs is in liquid-crystal displays (LCDs). This differs from the conventional bulk MOSFET transistor, where the semiconductor material typically is the substrate, such as a silicon wafer.
TFTs can be made using a wide variety of semiconductor materials. A common material is silicon. The characteristics of a silicon-based TFT depend on the silicon's crystalline state; that is, the semiconductor layer can be either amorphous silicon, microcrystalline silicon, or it can be annealed into polysilicon.
Other materials which have been used as semiconductors in TFTs include compound semiconductors such as cadmium selenide, or metal oxides such as zinc oxide or hafnium oxide. An application for hafnium oxide is as a high-κ dielectric. TFTs have also been made using organic materials, referred to as organic field-effect transistors or OTFTs.
By using transparent semiconductors and transparent electrodes, such as indium tin oxide (ITO), some TFT devices can be made completely transparent. Such transparent TFTs (TTFTs) can be used for construction of video display panels. Because conventional substrates cannot withstand high annealing temperatures, the deposition process must be completed under relatively low temperatures. Chemical vapor deposition and physical vapor deposition (usually sputtering) are applied. The first solution-processed TTFTs, based on zinc oxide, were reported in 2003 by researchers at Oregon State University. The Portuguese laboratory CENIMAT at the Universidade Nova de Lisboa has produced the world's first completely transparent TFT at room temperature. CENIMAT also developed the first paper transistor, which may lead to applications such as magazines and journal pages with moving images.
The best known application of thin-film transistors is in TFT LCDs, an implementation of liquid-crystal display technology. Transistors are embedded within the panel itself, reducing crosstalk between pixels and improving image stability.
As of 2008[update], many color LCD TVs and monitors use this technology. TFT panels are frequently used in digital radiography applications in general radiography. A TFT is used in both direct and indirect capture[jargon] as a base for the image receptor in medical radiography.
The most beneficial aspect of TFT technology is its use of a separate transistor for each pixel on the display. Because each transistor is small, the amount of charge needed to control it is also small. This allows for very fast re-drawing of the display.
Structure of a TFT-display matrix
The MOSFET (metal-oxide-semiconductor field-effect transistor) was invented by Mohamed M. Atalla and Dawon Kahng at Bell Labs in 1959, and presented in 1960. Building on their work with MOSFETs, Paul K. Weimer at RCA developed the thin-film transistor (TFT) in 1962. It was a CdS (cadmium sulfide) TFT, and a type of MOSFET distinct from the standard bulk MOSFET. In 1966, T.P. Brody and H.E. Kunig at Westinghouse Electric fabricated indium arsenide (InAs) MOS TFTs in both depletion and enhancement modes.
The idea of a TFT-based liquid-crystal display (LCD) was conceived by Bernard J. Lechner of RCA Laboratories in 1968. Lechner, F.J. Marlowe, E.O. Nester and J. Tults demonstrated the concept in 1968 with an 18x2 matrix dynamic scattering LCD that used standard discrete MOSFETs, as TFT performance was not adequate at the time. In 1973, T. Peter Brody, J. A. Asars and G. D. Dixon at Westinghouse Research Laboratories developed a CdSe (cadmium selenide) TFT, which they used to demonstrate the first CdSe thin-film-transistor liquid-crystal display (TFT LCD). The Westinghouse group also reported on operational TFT electroluminescence (EL) in 1973, using CdSe. Brody and Fang-Chen Luo demonstrated the first flat active-matrix liquid-crystal display (AM LCD) using CdSe in 1974, and then Brody coined the term "active matrix" in 1975. However, mass production of this device was never realized, due to complications in controlling the compound semiconductor thin film material properties, and device reliability over large areas.
A breakthrough in TFT research came with the development of the amorphous silicon (a-Si) TFT by P.G. le Comber, W.E. Spear and A. Ghaith at the University of Dundee in 1979. They reported the first functional TFT made from hydrogenated a-Si with a silicon nitride gate dielectric layer. The a-Si TFT was soon recognized as being more suitable for a large-area AM LCD. This led to commercial research and development (R&D) of AM LCD panels based on a-Si TFTs in Japan.
By 1982, pocket LCD TVs based on AM LCD technology were developed in Japan. In 1982, Fujitsu's S. Kawai fabricated an a-Si dot-matrix display, and Canon's Y. Okubo fabricated a-Si twisted nematic (TN) and guest-host LCD panels. In 1983, Toshiba's K. Suzuki produced a-Si TFT arrays compatible with CMOS integrated circuits (ICs), Canon's M. Sugata fabricated an a-Si color LCD panel, and a joint Sanyo and Sanritsu team including Mitsuhiro Yamasaki, S. Suhibuchi and Y. Sasaki fabricated a 3-inch a-SI color LCD TV.
The first commercial TFT-based AM LCD product was the 2.1-inch Epson ET-10 (Epson Elf), the first color LCD pocket TV, released in 1984. In 1986, a Hitachi research team led by Akio Mimura demonstrated a low-temperature polycrystalline silicon (LTPS) process for fabricating n-channel TFTs on a silicon-on-insulator (SOI), at a relatively low temperature of 200°C. A Hosiden research team led by T. Sunata in 1986 used a-Si TFTs to develop a 7-inch color AM LCD panel, and a 9-inch AM LCD panel. In the late 1980s, Hosiden supplied monochrome TFT LCD panels to Apple Computers. In 1988, a Sharp research team led by engineer T. Nagayasu used hydrogenated a-Si TFTs to demonstrate a 14-inch full-color LCD display, which convinced the electronics industry that LCD would eventually replace cathode-ray tube (CRT) as the standard television display technology. The same year, Sharp launched TFT LCD panels for notebook PCs. In 1992, Toshiba and IBM Japan introduced a 12.1-inch color SVGA panel for the first commercial color laptop by IBM.
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