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. The traditional application of TFTs is in 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 and the large device-to-device variations found in polycrystalline silicon, other materials have been studied for use in TFTs. These include cadmium selenide, metal oxides such as indium gallium zinc oxide (IGZO) or zinc oxide, organic semiconductors, carbon nanotubes, or metal halide perovskites.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, via techniques such as printing or spray coating. Solution-based techniques are hoped to lead to low-cost, mechanically flexible electronics. 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. Some wide band gap semiconductors, most notable metal oxides, are optically transparent. 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. In 2004, Nomura et al. reported room-temperature fabrication of transparent flexible TFTs using amorphous oxide semiconductors. 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. The Portuguese laboratory CENIMAT at the Universidade Nova de Lisboa has 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. 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. Large OLED displays usually use AOS (amporphous oxide semiconductor) TFT transistors instead, also called oxide TFTs and these are usually based on IGZO.
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 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. 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.
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