Thin-film transistor
Template:Short description Template:About Template:Use American English A thin-film transistor (TFT) is a special type of field-effect transistor (FET) where the transistor is made by thin film deposition. TFTs are grown on a supporting (but non-conducting) substrate, such as glass. This differs from the conventional bulk metal-oxide-semiconductor field-effect transistor (MOSFET), where the semiconductor material typically is the substrate, such as a silicon wafer.<ref>Template:Cite book</ref> The traditional application of TFTs is in TFT liquid-crystal displays.
Design and manufactureEdit
TFTs can be fabricated with a wide variety of semiconductor materials. Because it is naturally abundant and well understood, amorphous or polycrystalline silicon were (and still are) used as the semiconductor layer. However, because of the low mobility of amorphous silicon<ref>Template:Cite journal</ref> and the large device-to-device variations found in polycrystalline silicon,<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> other materials have been studied for use in TFTs. These include cadmium selenide,<ref name="Brody1">Template:Cite journal</ref><ref name="Brody2">Template:Cite journal</ref> metal oxides such as indium gallium zinc oxide (IGZO) or zinc oxide,<ref>Template:Cite journal</ref> organic semiconductors,<ref>Template:Cite journal</ref> carbon nanotubes,<ref>Template:Cite journal</ref> or metal halide perovskites.<ref>Template:Cite journal</ref>
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 include chemical vapor deposition (CVD), atomic layer deposition (ALD), and sputtering. The semiconductor can also be deposited from solution,<ref>Template:Cite journal</ref> via techniques such as printing<ref>Template:Cite journal</ref> or spray coating.<ref>Template:Cite journal</ref> Solution-based techniques are hoped to lead to low-cost, mechanically flexible electronics.<ref>Template:Cite journal</ref> 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.<ref name=":0">Template:Cite book</ref>
Some wide band gap semiconductors, most notable metal oxides, are optically transparent.<ref>Template:Cite journal</ref> By also employing transparent substrates, such as glass, and transparent electrodes, such as indium tin oxide (ITO), some TFT devices can be designed to be completely optically transparent.<ref>Template:Cite journal</ref> Such transparent TFTs (TTFTs) could be used to enable head-up displays (such as on a car windshield).The first solution-processed TTFTs, based on zinc oxide, were reported in 2003 by researchers at Oregon State University.<ref name="WagerZnO">Wager, John. OSU Engineers Create World's First Transparent Transistor Template:Webarchive. College of Engineering, Oregon State University, Corvallis, OR: OSU News & Communication, 2003. 29 July 2007.</ref> The Portuguese laboratory CENIMAT at the Universidade Nova de Lisboa has produced the world's first completely transparent TFT at room temperature.<ref name="Fortunato20015"> Template:Cite journal</ref> CENIMAT also developed the first paper transistor,<ref name="Furtado2008">Template:Cite journal</ref> which may lead to applications such as magazines and journal pages with moving images.
Many AMOLED 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.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref> Large OLED displays usually use AOS (amporphous oxide semiconductor) TFT transistors instead, also called oxide TFTs<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> and these are usually based on IGZO.<ref>Template:Cite book</ref>
ApplicationsEdit
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.
Template:As of, 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 captureTemplate:Technical inline as a base for the image receptor in medical radiography.
Template:As of, all modern high-resolution and high-quality electronic visual display devices use TFT-based active matrix displays.<ref>Template:Cite book</ref>
AMOLED displays also contain a TFT layer for active-matrix pixel addressing of individual organic light-emitting diodes.
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 matrixEdit
This picture does not include the actual light-source (usually cold-cathode fluorescent lamps or white LEDs), just the TFT-display matrix.
Template:Col-begin Template:Col-2
- 1 – Glass plates
- 2/3 – Horizontal and vertical polarisers
- 4 – RGB colour mask
- 5/6 – Horizontal and vertical command lines
- 7 – Rubbed polymer layer
- 8 – Spacers
- 9 – Thin-film transistors
- 10 – Front electrode
- 11 – Rear electrodes
HistoryEdit
In February 1957, John Wallmark of RCA filed a patent for a thin film MOSFET in which germanium monoxide was used as a gate dielectric. Paul K. Weimer, also of RCA implemented Wallmark's ideas and developed the thin-film transistor (TFT) in 1962, a type of MOSFET distinct from the standard bulk MOSFET. It was made with thin films of cadmium selenide and cadmium sulfide. In 1966, T.P. Brody and H.E. Kunig at Westinghouse Electric fabricated indium arsenide (InAs) MOS TFTs in both depletion and enhancement modes.<ref>Template:Cite book</ref><ref>Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="Kuo">Template:Cite journal</ref><ref>Template:Cite book</ref><ref>Template:Cite journal</ref>
The idea of a TFT-based liquid-crystal display (LCD) was conceived by Bernard J. Lechner of RCA Laboratories in 1968.<ref name="Kawamoto">Template:Cite journal</ref> 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.<ref>Template:Cite book</ref> 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).<ref name="Kuo"/><ref>Template:Cite journal</ref> The Westinghouse group also reported on operational TFT electroluminescence (EL) in 1973, using CdSe.<ref name="Souk">Template:Cite book</ref> 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.<ref name="Kawamoto"/> 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.<ref name="Kuo"/>
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.<ref name="Kuo"/><ref>Template:Cite journal</ref> The a-Si TFT was soon recognized as being more suitable for a large-area AM LCD.<ref name="Kuo"/> This led to commercial research and development (R&D) of AM LCD panels based on a-Si TFTs in Japan.<ref name="Castellano">Template:Cite book</ref>
By 1982, pocket TVs based on AM LCD technology were developed in Japan.<ref>Template:Cite journal</ref> 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 (complementary metal–oxide–semiconductor) 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.<ref name="Castellano"/>
The first commercial TFT-based AM LCD product was the 2.1-inch Epson<ref>Template:Cite patent</ref><ref>Template:Cite patent</ref><ref>Template:Cite journal</ref> ET-10<ref name="Souk"/> (Epson Elf), the first color LCD pocket TV, released in 1984.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> 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 200Template:Nbsp°C.<ref>Template:Cite journal</ref> A Hosiden research team led by T. Sunata in 1986 used a-Si TFTs to develop a 7-inch color AM LCD panel,<ref>Template:Cite journal</ref> and a 9-inch AM LCD panel.<ref>Template:Cite journal</ref> In the late 1980s, Hosiden supplied monochrome TFT LCD panels to Apple Computer.<ref name="Kuo"/> 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,<ref name="Kawamoto"/><ref>Template:Cite book</ref> which convinced the electronics industry that LCD would eventually replace cathode-ray tube (CRT) as the standard television display technology.<ref name="Kawamoto"/> The same year, Sharp launched TFT LCD panels for notebook PCs.<ref name="Souk"/> In 1992, Toshiba and IBM Japan introduced a 12.1-inch color SVGA panel for the first commercial color laptop by IBM.<ref name="Souk"/>
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.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref><ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In 2021, the first flexible 32-bit microprocessor was manufactured using IGZO TFT technology on a polyimide substrate.<ref name="Nature">Template:Cite journal</ref>