Thin-film transistor
A thin film transistor (TFT) is made by depositing thin films for the metallic contacts, semiconductor active layer, and dielectric layer.
Its main application is in design of flat-panel displays, a kind of LCD (liquid crystal display). TFTs are the better variant of the LCD technology and more flexible one, since with them each pixel is controlled individually. This is achieved by means of several transistors. As of 2004, this kind of techology has been providing the highest resolution of LCDs.
Due to its design TFT displays are often called Active matrix LCDs.
Most TFTs are not transparent themselves, but its electrodes and interconnects can be. The first transparent TFTs, based on zinc oxide were reported in 2003 by the researchers of the Oregon State University.
Screens
Computer and TV TFT screen are more and more usual ( with 17 '' under 300$). Modern TFT screens are suitable for almost all personal computer tasks, including office work, CAD, home movie, gameplay. The notable exception is top-notch graphics design and studiowork, where the superiorty of CRT monitors' accurate color representation stands unchallenged.
The LCD screens for personal desktop computers usually come in sizes 15, 17, 19 and 23 inches. Sizes 18 and 20 inches are less common. The smaller 15 inches screens are getting less popular with vendors and customers, due to their relatively high price per pixel. Sizes 17 and 19 inches are considered mainstream as of end 2004. Such a TFT screen consists of app. 5 million individual light switching transistors and increasingly consumers expect TFT screens to be free of pixel defects totally!
TFT LCD screens come in several variations, each of which introduce different compromises between speed, image quality and pricetag.
Basic consumer displays are of the type "TN", which is the lowest cost kind, yet it offers fast enough (12, 14 or 16ms) framerates to allow playing games or watch DVD movies without shadow-trail artefacts. The TN displays suffer from limited vertical field of view and are unable to display the full 16.7 million colors available from modern VGA graphics cards. They can display up to 16.2 million colors using a dither method which quickly cycles pixels through a 6-bit pattern out of the available 256 colors each to simulate a given shade. The pattern generating algorithms are proprietary goods and good ones are quite hard to compile. About 10 to 15% of all people are able to see the dither cycling on TN screens and many of them are disturbed by the resulting checkered artefacts. They may not be able to use TN screens for sore eye. TN screens are realtively resistant to abuse, but the addition of a hard translucent coversheet is getting more and more popular with TFT monitor vendors, due to the large number of cracked screen complaints.
The newer and slightly more expensive "MVA" (Multi Vertical Alignment) type screens offer wider viewing angles and the dithered check-board problem is almost completely gone. Better MVA screens may even be able to display the full 16.7 million colors. The price for these innovations is a moderate loss in screen brightness, due to a more complex orientation of liquid crystal particles. MVA panels have fast enough speed to play FSP games or watch multimedia content on LCD screen.
The PVA type panels are of a more advanced version of MVA technology as touted by some manufacturers.
The more expensive range of "IPS" (In-Plane Switching) technology panels are top notch LCD displays which work with true 16.7 million colors and offer wide viewing angles to match CRT displays. Consequently IPS displays are well-suited for office work and presentation purposes, where up to three sitting and three standing persons can confortably watch a single screen. Their brightness is good, their contrast and color fidelity is excellent and most IPS screens are compatible with the advanced s-RGB colour management standard. Only the hardness of TFT color shades and grades prohibits their use at top-notch computer graphics studios. The main drawback of IPS screens is their slow pace with 20 to 25ms typical refresh rates. This makes FPS action gaming impossible and fast-paced DVD movies leave a disorienting shadow-trail. The IPS screens are sensitive to abuse, never touch their display surface!
The newest and quite expensive S-IPS (Super-IPS) screens have all the benefits of IPS technology and offer much faster speeds, certainly enough for multimedia purposes and sometimes even action-packed gaming is made possible.
Due to the immense cost of building TFT producing factories, the number of OEM panel vendors probably does not exceed four or five major players. The raw LCD TFT panels are usually factory-sorted into three categories, with regards to the number of dead pixels, backlight even-ness and general product quality. Additionally, there may be up to +/- 2ms maximum refresh rate differences between between individual panel that came off the same assembly line the same day! The poorest performing screens are then sold to no-name vendors or used in "value" TFT monitors (often marked with letter V behind the type number), the medium performers are incorporated in gamer-oriented or home office bound TFT displays (sometimes marked with the capital letter S) and the best screens are usually reserved for use in "professional" grade TFT monitors (usually marked with letter P after their type number).
Value edition TFT screens (the Vs) and most 15" sized LCDs usually fail to include a digital signal compatible DVI socket, thus their future-proofing may be limited. The upper end of 17" or 19" gamer and office TFT screens have dual analog-VGA and DVI sockets and almost all "P" professional screens have DVI and pivot mode for a 100% sized, full, vertical A4/Letter page display. However, the use of DVI video signal does not automatically guarantee better image quality. A video card with good RAMDAC, properly shielded analogoue VGA cable may offer the same on-screen experience. Indeed, sometimes vendors offer better panels in their non-DVI panels, due to overlapping model generation changes.
When stunning colours and very large screensizes are required, TFT LCD screens cannot yet compete with plasma TV technology.
Applications
Active layers used
Organic Semiconductors:
- Pentacene
- Poly-3-Hexylthiophene
Types of processes
Bottom-gate
Top-gate
Co-planar
Staggered
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References
Detailed but popular article about colour LCD screen technology : http://www.avdeals.com/classroom/what_is_tft_lcd.htm