thin films

Thin-film transistor

A thin-film transistor (TFT) is a special kind of field-effect transistor made by depositing thin films of a semiconductor active layer as well as the dielectric layer and metallic contacts over a supporting substrate. A common substrate is glass, since the primary application of TFTs is in liquid crystal displays. This differs from the conventional 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 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 selenium (CdSe) and metal oxides such as Zinc Oxide. TFT's have also been made using organic materials (referred to as an Organic TFT or OTFT).

By using transparent semiconductors and transparent electrodes, such as indium tin oxide (ITO), some TFT devices can be made completely transparent.

Because the substrate cannot withstand the high annealing temperature, the deposition process has to be completed under relatively low temperature. Chemical vapor deposition, physical vapor deposition (usually sputtering) are applied. Also the first solution processed transparent TFTs (TTFTs), based on zinc oxide were reported in 2003 by researchers at Oregon State University.

Meanwhile, Portuguese laboratory CENIMAT, Universidade Nova de Lisboa, discovered a way of producing TFT at room temperature, having 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, a variant of LCD technology. Transistors are embedded within the panel itself, reducing crosstalk between pixels and improving image stability.

As of 2008, all color LCD TVs and monitors use this technology. TFT panels are heavily used in digital radiography applications in General Radiography. It is used in both Direct and Indirect capture as a base for the Image Receptor in Radiography.

The new AMOLED (Active Matrix OLED) screens also contain a TFT layer.

See also


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