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TFT LCD

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Athin-film-transistor liquid-crystal display(TFT LCD) is a type ofliquid-crystal displaythat usesthin-film-transistortechnology to improve image qualities such as addressability and contrast.[1]A TFT LCD is anactive matrixLCD, in contrast topassive matrixLCDs or simple, direct-driven (i.e. with segments directly connected to electronics outside the LCD) LCDs with a few segments.

TFT LCDs are used intelevision sets,computer monitors,mobile phones,video gamesystems,personal digital assistants,navigation systems,projectors,[2]anddashboardsin someautomobilesand in medium to high endmotorcycles.

History[edit]

Further information:History of display technologyandThin-film transistor

In February 1957,John WallmarkofRCAfiled a patent for a thin filmMOSFET.Paul K. Weimer,also ofRCA,implemented Wallmark's ideas and developed thethin-filmtransistor(TFT) in 1962, a type of MOSFET distinct from the standard bulk MOSFET. It was made with thin films ofcadmium selenideandcadmium sulfide.

The idea of a TFT-basedliquid-crystal display(LCD) was conceived byBernard LechnerofRCA Laboratoriesin 1968. In 1971, Lechner, F. J. Marlowe, E. O. Nester and J. Tults demonstrated a 2-by-18 matrix display driven by a hybrid circuit using thedynamic scatteringmode of LCDs.[3]In 1973,T. Peter Brody,J. A. Asars and G. D. Dixon atWestinghouse Research Laboratoriesdeveloped aCdSe(cadmium selenide) TFT, which they used to demonstrate the first CdSe thin-film-transistor liquid-crystal display (TFT LCD).[4][5]Brody and Fang-Chen Luo demonstrated the first flatactive-matrix liquid-crystal display(AM LCD) using CdSe TFTs in 1974, and then Brody coined the term "active matrix" in 1975.[3]

By 2013, most modernhigh-resolutionand high-qualityelectronic visual displaydevices used TFT-based active matrix displays.[6][7][4][8][9][10]

As of 2024, TFT LCD displays are still dominant, but compete withOLEDfor high brightness and high resolution displays, and compete withelectronic paperfor low power displays.

Construction[edit]

A diagram of thepixellayout

The liquid crystal displays used in calculators and other devices with similarly simple displays have direct-driven image elements, and therefore avoltagecan be easily applied across just one segment of these types of displays without interfering with the other segments. This would be impractical for a largedisplay,because it would have a large number of (color) picture elements (pixels), and thus it would require millions of connections, both top and bottom for each one of the three colors (red, green and blue) of every pixel. To avoid this issue, the pixels are addressed in rows and columns, reducing the connection count from millions down to thousands. The column and row wires attach totransistorswitches, one for each pixel. The one-way current passing characteristic of the transistor prevents the charge that is being applied to each pixel from being drained between refreshes to a display's image. Each pixel is a smallcapacitorwith a layer ofinsulatingliquid crystal sandwiched between transparent conductive layers ofindium tin oxide (ITO).

The circuit layout process of a TFT-LCD is very similar to that of semiconductor products. However, rather than fabricating the transistors fromsilicon,that is formed into acrystalline siliconwafer, they are made from athin filmofamorphous siliconthat is deposited on aglasspanel. The silicon layer for TFT-LCDs is typically deposited using thePECVDprocess.[11]Transistors take up only a small fraction of the area of each pixel and the rest of the silicon film is etched away to allow light to easily pass through it.

Polycrystalline siliconis sometimes used in displays that require higher TFT performance. Examples include small high-resolution displays such as those found in projectors or viewfinders. Amorphous silicon-based TFTs are by far the most common, due to their lower production cost, whereas polycrystalline silicon TFTs are more costly and much more difficult to produce.[12]

Types[edit]

Twisted nematic (TN)[edit]

TN display under a microscope, with the transistors visible at the bottom

Thetwisted nematic(TN) display is one of the oldest and frequently cheapest kind of liquid crystal display technologies. TN displays have fast pixel response times and less smearing than other types of LCDs likeIPS displays,but suffer from poor color reproduction and limited viewing angles, especially in the vertical direction. When viewed at an angle that is not perpendicular to the display, colors will shift, sometimes to the point of completely inverting. Modern, high end consumer products have developed methods to overcome the technology's shortcomings, such asRTC (Response Time Compensation / Overdrive) technologies.Modern TN displays can look significantly better than older TN displays from decades earlier, but overall TN has inferior viewing angles and poor color in comparison to other technology like IPS.

Most TN panels can represent colors using only sixbitsper RGB channel, or 18 bit in total, and are unable to display the 16.7 million color shades (24-bittruecolor) that are available using 24-bit color. Instead, these panels display interpolated 24-bit color using aditheringmethod that combines adjacent pixels to simulate the desired shade. They can also use a form of temporal dithering calledFrame Rate Control(FRC), which cycles between different shades with eachnew frameto simulate an intermediate shade. Such 18 bit panels with dithering are sometimes advertised as having "16.2 million colors". These color simulation methods are noticeable to many people and highly bothersome to some.[13]FRC tends to be most noticeable in darker tones, while dithering appears to make the individual pixels of the LCD visible. Overall, color reproduction and linearity on TN panels is poor. Shortcomings in display colorgamut(often referred to as a percentage of theNTSC 1953 color gamut) are also due to backlighting technology. It is common for older displays to range from 10% to 26% of the NTSC color gamut, whereas other kind of displays, utilizing more complicated CCFL or LEDphosphorformulations or RGB LED backlights, may extend past 100% of the NTSC color gamut, a difference that is easily seen by the human eye.

Thetransmittanceof a pixel of an LCD panel typically does not change linearly with the applied voltage,[14]and thesRGBstandard for computer monitors requires a specific nonlinear dependence of the amount of emitted light as a function of theRGBvalue.

In-plane switching (IPS)[edit]

Main article:IPS panel

In-plane switching(IPS) was developed byHitachi Ltd.in 1996 to improve on the poor viewing angle and the poor color reproduction of TN panels at that time.[15][16]Its name comes from the main difference from TN panels, that the crystal molecules move parallel to the panel plane instead of perpendicular to it. This change reduces the amount of light scattering in the matrix, which gives IPS its characteristic wide viewing angles and good color reproduction.[17]

Initial iterations of IPS technology were characterised by slow response time and a low contrast ratio but later revisions have made marked improvements to these shortcomings. Because of its wide viewing angle and accurate color reproduction (with almost no off-angle color shift), IPS is widely employed in high-end monitors aimed at professional graphic artists, although with the recent fall in price it has been seen in the mainstream market as well. IPS technology was sold to Panasonic by Hitachi.

Hitachi IPS technology development[18][19]
Name Nickname Year Advantage Transmittance/
contrast ratio 		Remarks
Super TFT IPS 1996 Wide viewing angle 100/100
Base level		 Most panels also support true8-bit per channel color.These improvements came at the cost of a higher response time, initially about 50 ms. IPS panels were also extremely expensive.
Super-IPS S-IPS 1998 Color shift free 100/137 IPS has since been superseded byS-IPS(Super-IPS,Hitachi Ltd.in 1998), which has all the benefits of IPS technology with the addition of improved pixel refresh timing.[quantify]
Advanced Super-IPS AS-IPS 2002 High transmittance 130/250 AS-IPS, also developed byHitachi Ltd.in 2002, improves substantially[quantify]on the contrast ratio of traditional S-IPS panels to the point where they are second only to some S-PVAs.[citation needed]
IPS-Provectus IPS-Pro 2004 High contrast ratio 137/313 The latest panel from IPS Alpha Technology with a wider color gamut[quantify]and contrast ratio[quantify]matching PVA and ASV displays without off-angle glowing.[citation needed]
IPS alpha IPS-Pro 2008 High contrast ratio Next generation of IPS-Pro
IPS alpha next gen IPS-Pro 2010 High contrast ratio
LG IPS technology development
Name Nickname Year Remarks
Horizontal IPS H-IPS 2007 Improves[quantify]contrast ratio by twisting electrode plane layout. Also introduces an optional Advanced True White polarizing film from NEC, to make white look more natural[quantify].This is used in professional/photography LCDs.[citation needed]
Enhanced IPS E-IPS 2009 Wider[quantify]aperture for light transmission, enabling the use of lower-power, cheaper backlights. Improves[quantify]diagonal viewing angle and further reduce response time to 5ms.[citation needed]
Professional IPS P-IPS 2010 Offer 1.07 billion colors (10-bit color depth).[citation needed]More possible orientations per sub-pixel (1024 as opposed to 256) and produces a better[quantify]true color depth.
Advanced High Performance IPS AH-IPS 2011 Improved color accuracy, increased resolution and PPI, and greater light transmission for lower power consumption.[20]

Advanced fringe field switching (AFFS)[edit]

This is an LCD technology derived from the IPS by Boe-Hydis of Korea. Known as fringe field switching (FFS) until 2003,[21]advanced fringe field switching is a technology similar to IPS or S-IPS offering superior performance and color gamut with high luminosity. Color shift and deviation caused by light leakage is corrected by optimizing the white gamut, which also enhances white/grey reproduction. AFFS is developed by Hydis Technologies Co., Ltd, Korea (formally Hyundai Electronics, LCD Task Force).[22]

In 2004, Hydis Technologies Co., Ltd licensed its AFFS patent to Japan's Hitachi Displays. Hitachi is using AFFS to manufacture high end panels in their product line. In 2006, Hydis also licensed its AFFS to Sanyo Epson Imaging Devices Corporation.

Hydis introduced AFFS+ which improved outdoor readability in 2007.[citation needed]

Multi-domain vertical alignment (MVA)[edit]

It achieved pixel response which was fast for its time, wide viewing angles, and high contrast at the cost of brightness and color reproduction.[citation needed]Modern MVA panels can offer wide viewing angles (second only to S-IPS technology), good black depth, good color reproduction and depth, and fast response times due to the use of RTC (Response Time Compensation) technologies.[citation needed]When MVA panels are viewed off-perpendicular, colors will shift, but much less than for TN panels.[citation needed]

There are several "next-generation" technologies based on MVA, including AU Optronics'P-MVAandAMVA,as well as Chi Mei Optoelectronics'S-MVA.

Patterned vertical alignment (PVA)[edit]

Less expensive PVA panels often use dithering andFRC,whereas super-PVA (S-PVA) panels all use at least 8 bits per color component and do not use color simulation methods.[citation needed]S-PVA also largely eliminated off-angle glowing of solid blacks and reduced the off-angle gamma shift. Some high-end SonyBRAVIALCD TVs offer 10-bit and xvYCC color support, for example, the Bravia X4500 series. S-PVA also offers fast response times using modern RTC technologies.[citation needed]

Advanced super view (ASV)[edit]

Advanced super view, also calledaxially symmetric vertical alignmentwas developed bySharp.[23]It is a VA mode where liquid crystal molecules orient perpendicular to the substrates in the off state. The bottom sub-pixel has continuously covered electrodes, while the upper one has a smaller area electrode in the center of the subpixel.

When the field is on, the liquid crystal molecules start to tilt towards the center of the sub-pixels because of the electric field; as a result, a continuous pinwheel alignment (CPA) is formed; the azimuthal angle rotates 360 degrees continuously resulting in an excellent viewing angle. The ASV mode is also called CPA mode.[24]

Plane line switching (PLS)[edit]

See also:IPS panel § PLS

A technology developed bySamsungis Super PLS, which bears similarities to IPS panels, has wider viewing angles, better image quality, increased brightness, and lower production costs. PLS technology debuted in the PC display market with the release of the Samsung S27A850 and S24A850 monitors in September 2011.[25]

TFT dual-transistor pixel (DTP) or cell technology[edit]

Patent TFT Store Electronic Systems

TFT dual-transistor pixel or cell technology is a reflective-display technology for use in very-low-power-consumption applications such as electronic shelf labels (ESL), digital watches, or metering. DTP involves adding a secondary transistor gate in the single TFT cell to maintain the display of a pixel during a period of 1s without loss of image or without degrading the TFT transistors over time. By slowing the refresh rate of the standard frequency from 60 Hz to 1 Hz, DTP claims to increase the power efficiency by multiple orders of magnitude.

Display industry[edit]

Due to the very high cost of building TFT factories, there are few majorOEMpanel vendors for large display panels. The glass panel suppliers are as follows:

LCD glass panel suppliers
Panel type Company Remarks major TV makers
IPS-Pro Panasonic Solely for LCD TV markets and known as IPS Alpha Technology Ltd.[26] Panasonic, Hitachi, Toshiba
H-IPS & P-IPS LG Display They also produce other type of TFT panels such as TN for OEM markets such as mobile, monitor, automotive, portable AV and industrial panels. LG, Philips, BenQ
S-IPS Hannstar
Chunghwa Picture Tubes, Ltd.
A-MVA AU Optronics
A-HVA AU Optronics
S-MVA Chi MeiOptoelectronics
AAS InnoLux Corporation
S-PVA Samsung, Sony
AFFS For small and medium size special projects.
ASV Sharp Corporation LCD TV and mobile markets Sharp, Sony
MVA Sharp Corporation Solely for LED LCD TV markets Sharp
HVA China Star Optoelectionics Technology HVA and AMOLED TCL[27]

Electrical interface[edit]

External consumer display devices like a TFT LCD feature one or moreanalogVGA,DVI,HDMI,orDisplayPortinterface, with many featuring a selection of these interfaces. Inside external display devices there is a controller board that will convert the video signal usingcolor mappingandimage scalingusually employing thediscrete cosine transform(DCT) in order to convert any video source likeCVBS,VGA,DVI,HDMI,etc. into digitalRGBat the native resolution of the display panel. In a laptop the graphics chip will directly produce a signal suitable for connection to the built-in TFT display. A control mechanism for thebacklightis usually included on the same controller board.

The low level interface ofSTN,DSTN,or TFT display panels use eithersingle endedTTL5 V signal for older displays or TTL 3.3 V for slightly newer displays that transmits the pixel clock,horizontal sync,vertical sync,digital red, digital green, digital bluein parallel. Some models (for example the AT070TN92) also featureinput/display enable,horizontal scan direction and vertical scan direction signals.

New and large (>15 ") TFT displays often useLVDSsignaling that transmits the same contents as the parallel interface (Hsync, Vsync, RGB) but will put control andRGBbits into a number of serial transmission linessynchronizedto a clock whose rate is equal to the pixel rate. LVDS transmits seven bits per clock per data line, with six bits being data and one bit used to signal if the other six bits need to be inverted in order to maintain DC balance. Low-cost TFT displays often have three data lines and therefore only directly support 18bits per pixel.Upscale displays have four or five data lines to support 24 bits per pixel (truecolor) or 30 bits per pixel respectively. Panel manufacturers are slowly replacing LVDS with Internal DisplayPort and Embedded DisplayPort, which allow sixfold reduction of the number of differential pairs.[citation needed]

Backlightintensity is usually controlled by varying a few volts DC, or generating aPWMsignal, or adjusting apotentiometeror simply fixed. This in turn controls a high-voltage (1.3 kV)DC-AC inverteror a matrix ofLEDs.The method to control the intensity of LED is to pulse them with PWM which can be source of harmonic flicker.[citation needed]

The bare display panel will only accept a digital video signal at the resolution determined by the panel pixel matrix designed at manufacture. Some screen panels will ignore theLSBbits of the color information to present a consistent interface (8 bit -> 6 bit/color x3).[citation needed]

With analogue signals like VGA, the display controller also needs to perform a high speedanalog to digitalconversion. With digital input signals like DVI or HDMI some simple reordering of the bits is needed before feeding it to the rescaler if the input resolution does not match the display panel resolution.

Safety[edit]

Liquid crystals are constantly subjected to toxicity and eco-toxicity testing for any hazard potential. The result is that:

  • wastewater from manufacturing is acutely toxic to aquatic life,[28]
  • but may have an irritant, corrosive or sensitizing effect in rare cases. Any effects can be avoided by using a limited concentration in mixtures,
  • are not mutagenic – neither in bacteria (Ames test) nor in mammalian cells (mouse lymphoma assay or chromosome aberration test),
  • are not suspected of being carcinogenic,[29]
  • are hazardous to aquatic organisms (bacteria, algae, daphnia, fish),[28]
  • do not possess any significant bioaccumulation potential,
  • are not easily biodegradable.[29]

The statements are applicable to Merck KGaA as well as its competitors JNC Corporation (formerlyChissoCorporation) and DIC (formerly Dainippon Ink & Chemicals). All three manufacturers have agreed not to introduce any acutely toxic or mutagenic liquid crystals to the market. They cover more than 90 percent of the global liquid crystal market. The remaining market share of liquid crystals, produced primarily in China, consists of older, patent-free substances from the three leading world producers and have already been tested for toxicity by them. As a result, they can also be considered non-toxic.

The complete report is available from Merck KGaA online.[29]

TheCCFLbacklightsused in many LCD monitors containmercury,which is toxic.

See also[edit]

References[edit]

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  2. ^"LCD Panel Technology Explained".Pchardwarehelp.com.Retrieved2013-07-21.
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  4. ^abKuo, Yue (1 January 2013)."Thin Film Transistor Technology—Past, Present, and Future"(PDF).The Electrochemical Society Interface.22(1): 55–61.Bibcode:2013ECSIn..22a..55K.doi:10.1149/2.F06131if.ISSN1064-8208.
  5. ^Brody, T. Peter;Asars, J. A.; Dixon, G. D. (November 1973). "A 6 × 6 inch 20 lines-per-inch liquid-crystal display panel".IEEE Transactions on Electron Devices.20(11): 995–1001.Bibcode:1973ITED...20..995B.doi:10.1109/T-ED.1973.17780.ISSN0018-9383.
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  8. ^Kimizuka, Noboru; Yamazaki, Shunpei (2016).Physics and Technology of Crystalline Oxide Semiconductor CAAC-IGZO: Fundamentals.John Wiley & Sons. p. 217.ISBN9781119247401.
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  11. ^"TFT LCD - Fabricating TFT LCD".Plasma.com. Archived fromthe originalon 2013-05-02.Retrieved2013-07-21.
  12. ^"TFT LCD - Electronic Aspects of LCD TVs and LCD Monitors".Plasma.com. Archived fromthe originalon 2013-08-23.Retrieved2013-07-21.
  13. ^Oleg Artamonov (2004-10-26)."X-bit's Guide: Contemporary LCD Monitor Parameters and Characteristics (page 11)".Xbitlabs.com. Archived fromthe originalon 2009-05-19.Retrieved2009-08-05.
  14. ^Marek Matuszczyk,Liquid crystals in displaysArchived2004-12-23 at theWayback Machine.Chalmers University Sweden, c. 2000.
  15. ^"TN Film, MVA, PVA and IPS - Panel Technologies".TFT Central.Retrieved9 September2009.
  16. ^"IPS or TN panel?".eSport Source.Retrieved23 May2016.
  17. ^"Enhanced Super IPS - Next Generation Image Quality"(PDF).LG Display.Retrieved9 September2009.
  18. ^IPS-Pro (Evolving IPS technology)Archived2010-03-29 at theWayback Machine
  19. ^"Archived copy"(PDF).Archived fromthe original(PDF)on 2012-11-15.Retrieved2013-11-24.{{cite web}}:CS1 maint: archived copy as title (link)
  20. ^tech2 News Staff."LG Announces Super High Resolution AH-IPS Displays".Tech2.in.com. Archived fromthe originalon 2013-06-06.Retrieved2013-07-21.{{cite web}}:CS1 maint: numeric names: authors list (link)
  21. ^"AFFS & AFFS+".Technology.Vertex LCD. Archived fromthe originalon 2016-05-18.Retrieved2010-08-12.
  22. ^K. H. Lee; H. Y. Kim; K. H. Park; S. J. Jang; I. C. Park & J. Y. Lee (June 2006). "A Novel Outdoor Readability of Portable TFT-LCD with AFFS Technology".SID Symposium Digest of Technical Papers.37(1). AIP: 1079–82.doi:10.1889/1.2433159.S2CID129569963.
  23. ^"Sharp Advanced Super View (ASV) - Sharp".www.sharpsma.com.Retrieved2019-06-12.
  24. ^The World of Liquid Crystal Displaysfrom personal.kent.edu/%7Emgu
  25. ^"Samsung SyncMaster SA850: World's First Monitor on PLS Matrix".X-bit labs. 2011-05-30.Retrieved2013-07-21.
  26. ^IPS Alpha Technology LtdArchived2007-12-24 atarchive.today
  27. ^"About Us".www.szcsot.com.Retrieved2019-06-05.
  28. ^abKim, Sae-Bom; Kim, Woong-Ki; Chounlamany, Vanseng; Seo, Jaehwan; Yoo, Jisu; Jo, Hun-Je; Jung, Jinho (15 August 2012). "Identification of multi-level toxicity of liquid crystal display wastewater toward Daphnia magna and Moina macrocopa".Journal of Hazardous Materials.227–228. Seoul, Korea; Laos, Lao: 327–333.doi:10.1016/j.jhazmat.2012.05.059.PMID22677053.
  29. ^abc"Display solutions | Merck KGaA, Darmstadt, Germany".www.merck-performance-materials.com.Retrieved2018-02-17.

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