Thin film transistor

Various types of TFT constructions.

Thin-film transistor (TFT) or thin-film transistor is a special type of transistor field effect fabricated by depositing thin films of an active semiconductor as well as a layer of dielectric material and metal contacts onto a support substrate. A very common substrate is glass.

One of the main applications of TFTs are liquid crystal displays. This differentiates it from a conventional transistor where the semiconductor material is usually the substrate, such as a silicon wafer.

Structure

1. Glass plates 2 and 3. Horizontal and vertical polarizers 4. RGB color mask 5 and 6. Horizontal and vertical command line 7. Resistant polymer layer 8. Separators 9. Thin film transistors 10. Front electrode 11. Ass electrodes

Manufacturing

Process of manufacture of a panel thin-film transistor.

TFTs can be made from a wide variety of semiconductor materials. The most common is silicon. The characteristics of the silicon-based TFT depend on its crystalline state. That is, the semiconductor layer may be amorphous silicon, microcrystalline silicon, or it may have been annealed on polysilicon.

Other materials that can be used as semiconductors in TFTs are cadmium selenide (CdSe) and metal oxides such as zinc oxide. TFTs can also be fabricated using organic materials (Organic Transistors or OTFTs).

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

Because conventional substrates cannot withstand high temperature annealing, the deposition process has to be performed at relatively low temperatures. Chemical vapor deposition and physical vapor deposition (usually sputtering) are used. In addition, the first transparent TFT processing solution (TTFTs), based on zinc oxide, was announced in 2003 by researchers at Oregon State University.

The Portuguese laboratory CENIMAT at the Universidade Nova de Lisboa has produced the first TFT completely transparent at room temperature. CENIMAT also developed the first paper transistor, which can lead to applications such as magazines and magazine pages with moving images.

Applications

The best-known application of thin-film transistors is in TFT LCDs, an implementation of liquid crystal display technology. The transistors are integrated into the panel itself, reducing crosstalk between pixels and improving image stability.

Since 2008, many color LCD monitors and televisions use this technology. TFT displays are widely used in digital radiography and general radiography applications. A TFT is used in both direct and indirect capture as the basis for the image receptor in medical radiology.

The new AMOLED ('Active Matrix Organic Light Emitting Diode') screens also contain a TFT layer.

The most beneficial aspect of TFT technology is a transistor for each pixel on the screen. As each transistor decreases, so does the amount of charge required for control. This allows a very fast redraw of the screen.

Prior to TFT, passive matrix LCD displays could not smoothly display fast-moving images. A pointer dragged across the screen, for example, from point A to point B, appears to disappear between the two points. Pointer tracking can be done on a TFT monitor, resulting in a display that can be used for gaming and other forms of multimedia.