This article is about TFT technology. For thin-film-transistor liquid-crystal display, seeTFT LCD.
Athin-film transistor (TFT) is a special type offield-effect transistor (FET) where the transistor is made bythin film deposition. TFTs are grown on a supporting (but non-conducting)substrate, such asglass. This differs from the conventional bulk metal-oxide-semiconductor field-effect transistor (MOSFET), where the semiconductor material typicallyis the substrate, such as asilicon wafer.[1] The traditional application of TFTs is inTFT liquid-crystal displays.
Cross sectional diagram of 4 common thin film transistor structures
Because TFTs are grown on inert substrates, rather than on wafers, the semiconductor must be deposited in a dedicated process. A variety of techniques are used to deposit semiconductors in TFTs. These includechemical vapor deposition (CVD),atomic layer deposition (ALD), andsputtering. The semiconductor can also be deposited from solution,[12] via techniques such asprinting[13] or spray coating.[14] Solution-based techniques are hoped to lead to low-cost, mechanically flexible electronics.[15] Because typical substrates will deform or melt at high temperatures, the deposition process must be carried out under relatively low temperatures compared to traditional electronic material processing.[16]
Some wide band gap semiconductors, most notable metal oxides, are optically transparent.[17] By also employing transparent substrates, such as glass, and transparentelectrodes, such asindium tin oxide (ITO), some TFT devices can be designed to be completely optically transparent.[18] Such transparent TFTs (TTFTs) could be used to enablehead-up displays (such as on a car windshield).The first solution-processed TTFTs, based onzinc oxide, were reported in 2003 by researchers atOregon State University.[19] The Portuguese laboratory CENIMAT at theUniversidade Nova de Lisboa has produced the world's first completely transparent TFT at room temperature.[20] CENIMAT also developed the first paper transistor,[21] which may lead to applications such as magazines and journal pages with moving images.
ManyAMOLED displays use LTPO (Low-temperature Poly-Crystalline Silicon and Oxide) TFT transistors. These transistors offer stability at low refresh rates, and variable refresh rates, which allows for power saving displays that do not show visual artifacts.[22][23][24] Large OLED displays usually use AOS (amporphous oxide semiconductor) TFT transistors instead, also called oxide TFTs[25] and these are usually based on IGZO.[26]
The best known application of thin-film transistors is inTFT LCDs, an implementation ofliquid-crystal display technology. Transistors are embedded within the panel itself, reducingcrosstalk betweenpixels and improving image stability.
As of 2008[update], many colorLCD TVs and monitors use this technology. TFT panels are frequently used indigitalradiography applications in general radiography. A TFT is used in both direct and indirect capture[jargon] as a base for the image receptor inmedical 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.
The idea of a TFT-basedliquid-crystal display (LCD) was conceived byBernard J. Lechner ofRCA Laboratories in 1968.[34] Lechner, F.J. Marlowe, E.O. Nester and J. Tults demonstrated the concept in 1968 with an 18x2 matrixdynamic scattering LCD that used standard discrete MOSFETs, as TFT performance was not adequate at the time.[35] In 1973,T. Peter Brody, J. A. Asars and G. D. Dixon atWestinghouse Research Laboratories developed a CdSe (cadmium selenide) TFT, which they used to demonstrate the first CdSethin-film-transistor liquid-crystal display (TFT LCD).[31][36] The Westinghouse group also reported on operational TFTelectroluminescence (EL) in 1973, using CdSe.[37] Brody and Fang-Chen Luo demonstrated the first flatactive-matrix liquid-crystal display (AM LCD) using CdSe in 1974, and then Brody coined the term "active matrix" in 1975.[34] 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.[31]
A breakthrough in TFT research came with the development of theamorphous silicon (a-Si) TFT by P.G. le Comber, W.E. Spear and A. Ghaith at theUniversity of Dundee in 1979. They reported the first functional TFT made from hydrogenated a-Si with asilicon nitride gatedielectric layer.[31][38] The a-Si TFT was soon recognized as being more suitable for a large-area AM LCD.[31] This led to commercialresearch and development (R&D) of AM LCD panels based on a-Si TFTs in Japan.[39]
By 1982,pocket TVs based on AM LCD technology were developed in Japan.[40] In 1982,Fujitsu's S. Kawaifabricated an a-Sidot-matrix display, andCanon's Y. Okubo fabricated a-Sitwisted nematic (TN) andguest-host LCD panels. In 1983,Toshiba's K. Suzuki produced a-Si TFT arrays compatible withCMOS (complementary metal–oxide–semiconductor)integrated circuits (ICs), Canon's M. Sugata fabricated an a-Sicolor LCD panel, and a jointSanyo andSanritsu team including Mitsuhiro Yamasaki, S. Suhibuchi and Y. Sasaki fabricated a 3-inch a-SI color LCD TV.[39]
The first commercial TFT-based AM LCD product was the 2.1-inchEpson[41][42][43] ET-10[37] (Epson Elf), the first color LCD pocket TV, released in 1984.[44] In 1986, aHitachi research team led by Akio Mimura demonstrated alow-temperature polycrystalline silicon (LTPS) process for fabricatingn-channel TFTs on asilicon-on-insulator (SOI), at a relatively low temperature of 200°C.[45] AHosiden research team led by T. Sunata in 1986 used a-Si TFTs to develop a 7-inch color AM LCD panel,[46] and a 9-inch AM LCD panel.[47] In the late 1980s, Hosiden supplied monochrome TFT LCD panels toApple Computer.[31] In 1988, aSharp research team led by engineer T. Nagayasu used hydrogenated a-Si TFTs to demonstrate a 14-inch full-color LCD display,[34][48] which convinced theelectronics industry that LCD would eventually replacecathode-ray tube (CRT) as the standardtelevisiondisplay technology.[34] The same year, Sharp launched TFT LCD panels fornotebook PCs.[37] In 1992, Toshiba andIBM Japan introduced a 12.1-inch colorSVGA panel for the first commercial color laptop byIBM.[37]
TFTs can also be made out of indium gallium zinc oxide (IGZO). TFT-LCDs with IGZO transistors first showed up in 2012, and were first manufactured by Sharp Corporation. IGZO allows for higher refresh rates and lower power consumption.[49][50] In 2021, the first flexible 32-bit microprocessor was manufactured using IGZO TFT technology on apolyimide substrate.[51]
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