Thin-film transistor

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Several types of TFT constructions. Thin-film transistor variants EN.svg
Several types of TFT constructions.

A thin-film transistor (TFT) is a special type of metal–oxide–semiconductor field-effect transistor (MOSFET) [1] 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, [1] where the semiconductor material typically is the substrate, such as a silicon wafer.

Contents

Manufacture

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, [2] microcrystalline silicon, [2] or it can be annealed into polysilicon.

Other materials which have been used as semiconductors in TFTs include compound semiconductors such as cadmium selenide, [3] [4] or metal oxides such as zinc oxide [5] or hafnium oxide. An application for hafnium oxide is as a high-κ dielectric. [6] 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. [5] The Portuguese laboratory CENIMAT at the Universidade Nova de Lisboa has produced the world's first completely transparent TFT at room temperature. [7] CENIMAT also developed the first paper transistor, [8] which may lead to applications such as magazines and journal pages with moving images.

During producion, TFTs are repaired using Lasers, ink dispensers and Chemical Vapor Deposition (CVD). [9]

Applications

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, 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.

As of 2013, all modern high-resolution and high-quality electronic visual display devices use TFT-based active matrix displays. [10]

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 matrix

This picture does not include the actual light-source (usually cold-cathode fluorescent lamps or white LEDs), just the TFT-display matrix.

History

The MOSFET (metal-oxide-semiconductor field-effect transistor) was invented by Mohamed M. Atalla and Dawon Kahng at Bell Labs in 1959, [11] and presented in 1960. [12] Building on their work with MOSFETs, Paul K. Weimer at RCA developed the thin-film transistor (TFT) in 1962. [13] It was a CdS (cadmium sulfide) TFT, [14] and a type of MOSFET distinct from the standard bulk MOSFET. [1] In 1966, T.P. Brody and H.E. Kunig at Westinghouse Electric fabricated indium arsenide (InAs) MOS TFTs in both depletion and enhancement modes. [15] [16]

The idea of a TFT-based liquid-crystal display (LCD) was conceived by Bernard J. Lechner of RCA Laboratories in 1968. [17] 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. [18] 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). [14] [19] The Westinghouse group also reported on operational TFT electroluminescence (EL) in 1973, using CdSe. [20] 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. [17] 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. [14]

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. [14] [21] The a-Si TFT was soon recognized as being more suitable for a large-area AM LCD. [14] This led to commercial research and development (R&D) of AM LCD panels based on a-Si TFTs in Japan. [22]

By 1982, pocket LCD TVs based on AM LCD technology were developed in Japan. [23] 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. [22]

The first commercial TFT-based AM LCD product was the 2.1-inch Epson ET-10 [20] (Epson Elf), the first color LCD pocket TV, released in 1984. [24] 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. [25] A Hosiden research team led by T. Sunata in 1986 used a-Si TFTs to develop a 7-inch color AM LCD panel, [26] and a 9-inch AM LCD panel. [27] In the late 1980s, Hosiden supplied monochrome TFT LCD panels to Apple Computers. [14] 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, [17] [28] which convinced the electronics industry that LCD would eventually replace cathode-ray tube (CRT) as the standard television display technology. [17] The same year, Sharp launched TFT LCD panels for notebook PCs. [20] In 1992, Toshiba and IBM Japan introduced a 12.1-inch color SVGA panel for the first commercial color laptop by IBM. [20]

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. [29] [30]

Related Research Articles

Liquid-crystal display display that uses the light-modulating properties of liquid crystals

A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liquid crystals do not emit light directly, instead using a backlight or reflector to produce images in color or monochrome. LCDs are available to display arbitrary images or fixed images with low information content, which can be displayed or hidden, such as preset words, digits, and seven-segment displays, as in a digital clock. They use the same basic technology, except that arbitrary images are made from a matrix of small pixels, while other displays have larger elements. LCDs can either be normally on (positive) or off (negative), depending on the polarizer arrangement. For example, a character positive LCD with a backlight will have black lettering on a background that is the color of the backlight, and a character negative LCD will have a black background with the letters being of the same color as the backlight. Optical filters are added to white on blue LCDs to give them their characteristic appearance.

Semiconductor device fabrication manufacturing process used to create integrated circuits

Semiconductor device fabrication is the process used to manufacture semiconductor devices, typically the metal-oxide-semiconductor (MOS) devices used in the integrated circuit (IC) chips that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photolithographic and chemical processing steps during which electronic circuits are gradually created on a wafer made of pure semiconducting material. Silicon is almost always used, but various compound semiconductors are used for specialized applications.

MOSFET Transistor used for amplifying or switching electronic signals.

The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET), also known as the metal–oxide–silicon transistor (MOS transistor, or MOS), is a type of insulated-gate field-effect transistor (IGFET) that is fabricated by the controlled oxidation of a semiconductor, typically silicon. The voltage of the covered gate determines the electrical conductivity of the device; this ability to change conductivity with the amount of applied voltage can be used for amplifying or switching electronic signals. The MOSFET was invented by Egyptian engineer Mohamed M. Atalla and Korean engineer Dawon Kahng at Bell Labs in November 1959. It is the basic building block of modern electronics, and the most frequently manufactured device in history, with an estimated total of 13 sextillion (1.3 × 1022) MOSFETs manufactured between 1960 and 2018.

An active-matrix liquid-crystal display (AMLCD) is a type of flat-panel display, the only viable technology for high-resolution TVs, computer monitors, notebook computers, tablet computers and smartphones with an LCD screen, due to low weight, very good image quality, wide color gamut and response time.

A flat-panel display (FPD) is an electronic viewing technology used to enable people to see content in a range of entertainment, consumer electronics, personal computer, and mobile devices, and many types of medical, transportation and industrial equipment. They are far lighter and thinner than traditional cathode ray tube (CRT) television sets and video displays and are usually less than 10 centimetres (3.9 in) thick. Flat-panel displays can be divided into two display device categories: volatile and static. Volatile displays require that pixels be periodically electronically refreshed to retain their state. A volatile display only shows an image when it has battery or AC mains power. Static flat-panel displays rely on materials whose color states are bistable, and as such, flat-panel displays retain the text or images on the screen even when the power is off. As of 2016, flat-panel displays have almost completely replaced old CRT displays. In many 2010-era applications, specifically small portable devices such as laptops, mobile phones, smartphones, digital cameras, camcorders, point-and-shoot cameras, and pocket video cameras, any display disadvantages of flat-panels are made up for by portability advantages.

Active matrix is a type of addressing scheme used in flat panel displays. In this method of switching individual elements (pixels), each pixel is attached to a transistor and capacitor actively maintaining the pixel state while other pixels are being addressed, in contrast with the older passive matrix technology in which each pixel must maintain its state passively, without being driven by circuitry.

Television set Device for viewing computers screen and shows broadcast through satellites or cables

A television set or television receiver, more commonly called a television, TV, TV set, or telly, is a device that combines a tuner, display, and loudspeakers, for the purpose of viewing and hearing television broadcasting through satellites or cables, or viewing and hearing a computer. Introduced in the late 1920s in mechanical form, television sets became a popular consumer product after World War II in electronic form, using cathode ray tube (CRT) technology. The addition of color to broadcast television after 1953 further increased the popularity of television sets in the 1960s, and an outdoor antenna became a common feature of suburban homes. The ubiquitous television set became the display device for the first recorded media in the 1970s, such as Betamax, VHS and later DVD. It has been used as a display device since the first generation of home computers and dedicated video game consoles in the 1980s. By the early 2010s, flat-panel television incorporating liquid-crystal display (LCD) technology, especially LED-backlit LCD technology, largely replaced CRT and other display technologies. Modern flat panel TVs are typically capable of high-definition display and can also play content from a USB device.

In semiconductor manufacturing, silicon on insulator (SOI) technology is fabrication of silicon semiconductor devices in a layered silicon–insulator–silicon substrate, to reduce parasitic capacitance within the device, thereby improving performance. SOI-based devices differ from conventional silicon-built devices in that the silicon junction is above an electrical insulator, typically silicon dioxide or sapphire. The choice of insulator depends largely on intended application, with sapphire being used for high-performance radio frequency (RF) and radiation-sensitive applications, and silicon dioxide for diminished short-channel effects in other microelectronics devices. The insulating layer and topmost silicon layer also vary widely with application.

A thin-film-transistor liquid-crystal display is a variant of a liquid-crystal display (LCD) that uses thin-film-transistor (TFT) technology to improve image qualities such as addressability and contrast. A TFT LCD is an active matrix LCD, in contrast to passive matrix LCDs or simple, direct-driven LCDs with a few segments.

Organic field-effect transistor

An organic field-effect transistor (OFET) is a field-effect transistor using an organic semiconductor in its channel. OFETs can be prepared either by vacuum evaporation of small molecules, by solution-casting of polymers or small molecules, or by mechanical transfer of a peeled single-crystalline organic layer onto a substrate. These devices have been developed to realize low-cost, large-area electronic products and biodegradable electronics. OFETs have been fabricated with various device geometries. The most commonly used device geometry is bottom gate with top drain and source electrodes, because this geometry is similar to the thin-film silicon transistor (TFT) using thermally grown SiO2 as gate dielectric. Organic polymers, such as poly(methyl-methacrylate) (PMMA), can also be used as dielectric.

In electronics, a self-aligned gate is a transistor manufacturing feature whereby a refractory gate electrode region of a MOSFET is used as a mask for the doping of the source and drain regions. This technique ensures that the gate will slightly overlap the edges of the source and drain.

Indium gallium zinc oxide (IGZO) is a semiconducting material, consisting of indium (In), gallium (Ga), zinc (Zn) and oxygen (O). IGZO thin-film transistor (TFT) is used in the TFT backplane of flat-panel displays (FPDs). IGZO-TFT was developed by Hideo Hosono's group at Tokyo Institute of Technology and Japan Science and Technology Agency (JST) in 2003 and in 2004. IGZO-TFT has 20–50 times the electron mobility of amorphous silicon, which has often been used in liquid-crystal displays (LCDs) and e-papers. As a result, IGZO-TFT can improve the speed, resolution and size of flat-panel displays. It is currently used as the thin-film transistors for use in organic light-emitting diode (OLED) TV displays.

Electrically operated display devices have developed from electromechanical systems for display of text, up to all-electronic devices capable of full-motion 3D color graphic displays. Electromagnetic devices, using a solenoid coil to control a visible flag or flap, were the earliest type, and were used for text displays such as stock market prices and arrival/departure display times. The cathode ray tube was the workhorse of text and video display technology for several decades until being displaced by plasma, liquid crystal (LCD), and solid-state devices such as thin-film transistors (TFTs), LEDs and OLEDs. With the advent of metal-oxide-semiconductor field-effect transistors (MOSFETs), integrated circuit (IC) chips, microprocessors, and microelectronic devices, many more individual picture elements ("pixels") could be incorporated into one display device, allowing graphic displays and video.

In field effect transistors (FETs), depletion mode and enhancement mode are two major transistor types, corresponding to whether the transistor is in an ON state or an OFF state at zero gate–source voltage.

Polysilicon depletion effect is the phenomenon in which unwanted variation of threshold voltage of the MOSFET devices using polysilicon as gate material is observed, leading to unpredicted behavior of the electronic circuit. Polycrystalline silicon, also called polysilicon, is a material consisting of small silicon crystals. It differs from single-crystal silicon, used for electronics and solar cells, and from amorphous silicon, used for thin film devices and solar cells.

T. Peter Brody Inventor of Active Matrix technology

T. P. "Peter" Brody was a British-naturalised physicist and the co-inventor of Active Matrix Thin-Film Transistor display technology together with Fang-Chen Luo, having produced the world's first Active Matrix Liquid Crystal Display (AM-LCD) in 1972 and the first functional AM-EL in 1973 while employed by Westinghouse Electric Corporation in Pittsburgh. Brody coined the term "active matrix" and first used it in a published journal article in 1975.

Mohamed M. Atalla mechanical engineer

Mohamed Mohamed Atalla was an Egyptian–American engineer, physical chemist, cryptographer, inventor and entrepreneur. His pioneering work in semiconductor technology laid the foundations for modern electronics. Most importantly, his invention of the MOSFET in 1959, along with his earlier surface passivation and thermal oxidation processes, revolutionized the electronics industry. He is also known as the founder of the data security company Atalla Corporation, founded in 1972, which introduced the first hardware security module and was a pioneer in online security. He received the Stuart Ballantine Medal and was inducted into the National Inventors Hall of Fame for his important contributions to semiconductor technology as well as data security.

Low-temperature polycrystalline silicon (LTPS) is polycrystalline silicon that has been synthesized at relatively low temperatures compared to in traditional methods. LTPS is important for display industries, since the use of large glass panels prohibits exposure to deformative high temperatures. More specifically, the use of polycrystalline silicon in thin-film transistors (LTPS-TFT) has high potential for large-scale production of electronic devices like flat panel LCD displays or image sensors.

Douglas A. Keszler is a distinguished professor in the Department of Chemistry at Oregon State University, adjunct professor in the Physics Department at OSU and adjunct professor in the Department of Chemistry at University of Oregon. He is also the director of the Center for Sustainable Materials Chemistry, and a member of the Oregon Nanoscience and Microtechnologies Institute (ONAMI) leadership team.

Amorphous silicon non-crystalline silicon

Amorphous silicon (a-Si) is the non-crystalline form of silicon used for solar cells and thin-film transistors in LCDs.

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