Thin-film transistor

Source: Wikipedia, the free encyclopedia.
This article is about TFT technology. For thin-film-transistor liquid-crystal display, see TFT LCD.

A thin-film transistor (TFT) is a special type of

TFT liquid-crystal displays
.

Design and manufacture

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[2] and the large device-to-device variations found in polycrystalline silicon,[3][4][5] other materials have been studied for use in TFTs. These include cadmium selenide,[6][7] metal oxides such as indium gallium zinc oxide (IGZO) or zinc oxide,[8] organic semiconductors,[9] carbon nanotubes,[10] or metal halide perovskites.[11]

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 include chemical vapor deposition (CVD), atomic layer deposition (ALD), and sputtering. The semiconductor can also be deposited from solution,[12] via techniques such as printing[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.

Universidade 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.

Many AMOLED displays use LTPO 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]

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

medical radiography
.

As of 2013, all modern

high-resolution and high-quality electronic visual display devices use TFT-based active matrix displays.[27]

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

white LEDs
), just the TFT-display matrix.

History

In February 1957,

Westinghouse Electric fabricated indium arsenide (InAs) MOS TFTs in both depletion and enhancement modes.[28][29][30][31][32][33]

The idea of a TFT-based

thin-film-transistor liquid-crystal display (TFT LCD).[31][36] The Westinghouse group also reported on operational TFT electroluminescence (EL) in 1973, using CdSe.[37] 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.[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 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.[31][38] The a-Si TFT was soon recognized as being more suitable for a large-area AM LCD.[31] This led to commercial research and development (R&D) of AM LCD panels based on a-Si TFTs in Japan.[39]

By 1982,

Sanritsu 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-inch

SVGA panel for the first commercial color laptop by IBM.[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 a polyimide substrate.[51]

See also

References

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