TFT LCD

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Thin film transistor liquid crystal display (TFT-LCD) is a variant of liquid crystal display (LCD) which uses thin-film transistor (TFT) technology to improve image quality (e.g., addressability, contrast). TFT LCD is one type of active matrix LCD, though all LCD-screens are based on TFT active matrix addressing. TFT LCDs are used in television sets, computer monitors, mobile phones, handheld video game systems, personal digital assistants,navigation systems, projectors, etc.^[1]

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[edit]Construction

A diagram of the pixel layout

Small liquid crystal displays as used in calculators and other devices have direct driven image elements – a voltage can be applied across one segment without interfering with other segments of the display. This is impractical for a large display with a large number of picture elements (pixels), since it would require millions of connections – top and bottom connections 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 to thousands. The column and row wires attach totransistor switches, one for each pixel. The one-way current passing characteristic of the transistor prevents the charge applied to the pixel from draining between refreshes to the display image. Each pixel is a small capacitor with a layer of insulating liquid crystal sandwiched between transparent conductive ITO layers.

The circuit layout process of a TFT-LCD is very similar to that of semiconductor products. However, rather than fabricating the transistors from siliconformed into a crystalline silicon wafer, they are made from a thin film of amorphous silicon deposited on a glass panel. The silicon layer for TFT-LCDs is typically deposited using the PECVD process^[2]. Transistors take up only a small fraction of the area of each pixel; the rest of the silicon film is etched away to allow light to pass through.

Polycrystalline silicon is sometimes used in displays requiring higher TFT performance. Examples include small high-resolution displays such as those found in projectors or view finders. Amorphous silicon-based TFTs are by far the most common due to their lower production cost, whereas polycrystalline silicon TFTs are more costly and difficult to produce^[3].

[edit]Types

[edit]Twisted nematic (TN)

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

The inexpensive twisted nematic display is the most common consumer display type. The pixel response time on modern TN panels is sufficiently fast to avoid the shadow-trail and ghosting artifacts of earlier production. The fast response time has been emphasised in advertising TN displays, although in most cases this number does not reflect performance across the entire range of possible color transitions.^{[citation needed]} More recent use of RTC (Response Time Compensation / Overdrive) technologies has allowed manufacturers to significantly reduce grey-to-grey (G2G) transitions, without significantly improving the ISO response time. Response times are now quoted in G2G figures, with 4ms and 2ms now being commonplace for TN-based models. The good response time and low cost has led to the dominance of TN in the consumer market.^{[citation needed]}

TN displays suffer from limited viewing angles, especially in the vertical direction. Colors will shift when viewed off-perpendicular. In the vertical direction, colors will shift so much that they will invert past a certain angle.

Also, most TN panels represent colors using only 6 bits per RGB color, or 18 bit in total, and are unable to display the 16.7 million color shades (24-bittruecolor) that are available from graphics cards. Instead, these panels display interpolated 24-bit color using a dithering method that combines adjacent pixels to simulate the desired shade. They can also use a form of temporal dithering called Frame Rate Control (FRC), which cycles between different shades with each new frame to 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.^[4] 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 color gamut(often referred to as a percentage of the NTSC 1953 color gamut) are also due to backlighting technology. It is not uncommon for displays with CCFL (Cold Cathode Fluorescent Lamps)-based lighting to range from 10% to 26% of the NTSC color gamut, whereas other kind of displays, utilizing RGB LED backlights, may extend past 100% of the NTSC color gamut—a difference quite perceivable by the human eye.

The transmittance of a pixel of an LCD panel typically does not change linearly with the applied voltage,^[5] and the sRGB standard for computer monitors requires a specific nonlinear dependence of the amount of emitted light as a function of the RGB value.

[edit]In-plane switching (IPS)

In-plane switching was developed by Hitachi Ltd. in 1996 to improve on the poor viewing angle and the poor color reproduction of TN panels at that time.^[6] 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.^[7]

Initial iterations of IPS technology were plagued with slow response time and a low contrast ratio but later evolutions 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.

Hitachi IPS evolving technology^[8]
Name	Nickname	Year	Advantage	Transmittance/ contrast ratio	Remarks
Super TFT	IPS	1996	Wide viewing angle	100/100 Base level	Most panels also support true 8-bit per channel color. These improvements came at the cost of a slower 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 by S-IPS (Super-IPS, Hitachi Ltd. in 1998), which has all the benefits of IPS technology with the addition of improved pixel refresh timing.
Advanced Super-IPS	AS-IPS	2002	High transmittance	130/250	AS-IPS, also developed by Hitachi Ltd. in 2002, improves substantially on the contrast ratio of traditional S-IPS panels to the point where they are second only to some S-PVAs.
IPS-Provectus	IPS-Pro	2004	High contrast ratio	137/313	The latest panel from IPS Alpha Technology with a wider color gamut and contrast ratio matching PVA and ASV displays without off-angle glowing.

LG IPS evolving technology
Name	Nickname	Year	Remarks
Super-IPS	S-IPS	2001	LG Display remains as one of the main manufacturers of panels based on Hitachi Super-IPS.
Advanced Super-IPS	AS-IPS	2005	Increased contrast ratio with better color gamut.
Horizontal IPS	H-IPS	2007	Improves contrast ratio by twisting electrode plane layout. Also introduces an optional Advanced True White polarizing film from NEC, to make white look more natural. This is used in professional/photography LCDs.
Enhanced IPS	E-IPS	2009	Wider aperture for light transmission, enabling the use of lower-power, cheaper backlights. Improves diagonal viewing angle and further reduce response time to 5ms.
Professional IPS	P-IPS	2010	Offer 1.07 billion colours (30-bit colour depth). More possible orientations per sub-pixel (1024 as opposed to 256) and produces a better true colour depth.

[edit]Advanced fringe field switching (AFFS)

This is an LCD technology derived from the IPS by Boe-Hydis of Korea. Known as fringe field switching (FFS) until 2003,^[9] advanced fringe field switching is a technology similar to IPS or S-IPS offering superior performance and colour gamut with high luminosity. AFFS is developed by HYDIS TECHNOLOGIES CO.,LTD, Korea (formally Hyundai Electronics, LCD Task Force).^[10]

AFFS-applied notebook applications minimize colour distortion while maintaining its superior wide viewing angle for a professional display. Colour shift and deviation caused by light leakage is corrected by optimizing the white gamut which also enhances white/grey reproduction.

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.

[edit]Multi-domain vertical alignment (MVA)

Multi-domain vertical alignment was originally developed in 1998 by Fujitsu as a compromise between TN and IPS.^{[citation needed]} It achieved pixel response which was fast for its time, wide viewing angles, and high contrast at the cost of brightness and color reproduction. 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. When MVA panels are viewed off-perpendicular, colors will shift, but much less than for TN panels.

There are several "next-generation" technologies based on MVA, including AU Optronics' P-MVA and A-MVA, as well as Chi Mei Optoelectronics' S-MVA. Analysts^[who?] predicted that MVA would dominate the mainstream market, but the less expensive and slightly faster TN overtook it. The pixel response times of MVAs rise dramatically with small changes in brightness. Less expensive MVA panels can use dithering and FRC (Frame Rate Control).

[edit]Patterned vertical alignment (PVA)

Patterned vertical alignment and super patterned vertical alignment (S-PVA) are alternative versions of MVA technology offered by Samsung's and Sony's joint venture S-LCD. Developed independently, they offer similar features to MVA, but with higher contrast ratios of up to 3000:1. Less expensive PVA panels often use dithering and FRC, while S-PVA panels all use at least 8 bits per color component and do not use color simulation methods. S-PVA also largely eliminated off angle glowing of solid blacks and reduced the off angle gamma shift. Some newer S-PVA panels offered by Eizooffer 16-bit color internally, which enables gamma and other corrections with reduced color banding. Some high end Sony BRAVIA LCD-TVs offer 10bit and xvYCC color support.^{[citation needed]} PVA and S-PVA offer the best black depth of any LCD type along with wide viewing angles.^{[citation needed]} S-PVA also offers fast response times using modern RTC technologies.

[edit]Advanced super view (ASV)

Advanced super view, also called axially symmetric vertical alignment was developed by Sharp. 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.^[11]

[edit]Display industry

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

LCD glass panel suppliers
Panel type	Company	Remarks
IPS-Pro	Panasonic	Solely for LCD TV markets and known as IPS Alpha Technology Ltd.^[12]
H-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.
S-IPS	Hannstar
S-IPS	Chuangwa Picture Tubes, Ltd.
A-MVA	AU Optronics
S-MVA	Chi Mei Optoelectronics
S-PVA	S-LCD (Samsung/Sony joint venture)
AFFS	Samsung	For small and medium size special project.
ASV	Sharp Corporation	Solely for LCD TV markets

Raw LCD TFT panels are usually factory-sorted into three categories, with regard to the number of dead pixels, backlight evenness and general product quality.^{[citation needed]} Additionally, there may be up to +/- 2ms maximum response time differences between individual panels that came off the same assembly line on the same day. The poorest-performing screens are then sold to no-name vendors or used in "value" TFT monitors (often^{[citation needed]} 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 (often marked with letter P or S after their type number).

[edit]Electrical interface

External consumer display devices like a TFT LCD feature one or more analog VGA, DVI, HDMI, or DisplayPort interface, with many featuring a selection of these interfaces. Inside external display devices there is a controller board that will convert CVBS, VGA, DVI, HDMI etc. into digital RGB at 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 the backlight is usually included on the same controller board.

The low level interface of STN, DSTN, or TFT display panels use either single ended TTL 5V signal for older displays or TTL 3.3V for slightly newer displays that transmits Pixel clock, Horizontal sync,Vertical sync, Digital red, Digital green, Digital blue in parallel. Some models also feature input/display enable, horizontal scan direction and vertical scan direction signals.

New and large (>15 in) TFT displays often use LVDS or TMDS signaling that transmits the same contents as the parallel interface (Hsync, Vsync, RGB) but will put control and RGB bits into a number of serial transmission lines synchronized to a clock at 1/3 of the data bitrate. Usually with 3 data signals and one clock line. Transmitting 3x7 bits for one clock cycle giving 18-bpp. An optional 4th signal enables 24-bpp.

Backlight intensity is usually controlled by varying a few volts DC, or generating a PWM signal, or adjusting a potentiometer or simply fixed. This in turn controls a high-voltage (1.3 kV) DC-AC inverteror a matrix of LEDs.

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 color LSB bits to present a consistent interface (8bit->6bit/color).

The reason why laptop displays can't be reused directly with an ordinary computer graphics card or as a television, is mainly because it lacks a hardware rescaler (often using some discrete cosine transform) that can resize the image to fit the native resolution of the display panel.^{[citation needed]} With analogue signals like VGA the display controller also needs to perform a highspeed analog to digital conversion. With digital input signals like DVI or HDMI some simple bit stuffing is needed before feeding it to the rescaler if input resolution doesn't match the display panel resolution. For CVBS (TV) usage a tuner and color decode from a quadrature amplitude modulation (QAM) to Luminance (Y), Blue-Y (U), Red-Y (V) representation which in turn is transformed into Red, Green Blue is needed.^{[citation needed]}