TFT LCD

A thin-film-transistor liquid-crystal display is a type of liquid-crystal display that uses thin-film transistor technology to improve image qualities such as addressability and contrast. A TFT LCD is an active matrix LCD, in contrast to passive matrix LCDs or simple, direct-driven LCDs with a few segments.
TFT LCDs are used in television sets, computer monitors, mobile phones, video game systems, personal digital assistants, navigation systems, projectors, and dashboards in some automobiles and in medium to high end motorcycles.

History

In February 1957, John Wallmark of RCA filed a patent application for a thin film MOSFET. Paul K. Weimer, also of RCA, implemented Wallmark's ideas and developed the thin-film transistor in 1962, a type of MOSFET distinct from the standard bulk MOSFET. It was made with thin films of cadmium selenide and cadmium sulfide.
The idea of a TFT-based liquid-crystal display was conceived by Bernard Lechner of RCA Laboratories in 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 the dynamic scattering mode of LCDs. In 1973, T. Peter Brody, J. A. Asars and G. D. Dixon at Westinghouse Research Laboratories developed a CdSe TFT, which they used to demonstrate the first CdSe thin-film-transistor liquid-crystal display. Brody and Fang-Chen Luo demonstrated the first flat active-matrix liquid-crystal display using CdSe TFTs in 1974, and then Brody coined the term "active matrix" in 1975.
By 2013, most modern high-resolution and high-quality electronic visual display devices used TFT-based active matrix displays.
As of 2024, TFT LCDs are still dominant, but compete with OLED for high brightness and high resolution displays, and compete with electronic paper for low power displays.

Construction

The liquid crystal displays used in calculators and other devices with similarly simple displays have direct-driven image elements, and therefore a voltage can be easily applied across just one segment of these types of displays without interfering with the other segments. This would be impractical for a large display, because it would have a large number of picture elements, and thus it would require millions of connections, both top and bottom for each one of the three colors 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 to transistor switches, 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 small capacitor with a layer of insulating liquid crystal sandwiched between transparent conductive layers of indium tin oxide.
The circuit layout process of a TFT-LCD is very similar to that of semiconductor products. However, rather than fabricating the transistors from silicon, that is formed into a crystalline silicon wafer, they are made from a thin film of amorphous silicon that is deposited on a glass panel. The silicon layer for TFT-LCDs is typically deposited using the PECVD process. 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 silicon is 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.

Types

Twisted nematic (TN)

The twisted nematic display is one of the oldest and cheapest kind of liquid crystal display technologies. TN displays have fast pixel response times and less smearing than other types of LCDs like IPS 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 as RTC 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 six bits per RGB channel, or 18 bit in total, and are unable to display the 16.7 million color shades that are available using 24-bit color. 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, 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. 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 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 LED phosphor formulations or RGB LED backlights, may extend past 100% of the NTSC color gamut, a difference that is easily seen by the human eye.
The transmittance of a pixel of an LCD panel typically does not change linearly with the applied voltage, 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.

In-plane switching (IPS)

was developed by Hitachi in 1996 to improve on the poor viewing angle and the poor color reproduction of TN panels at that time. 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.
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, 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.

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 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 by S-IPS, 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 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.
IPS alpha	IPS-Pro	2008	High contrast ratio		Next generation of IPS-Pro
IPS alpha next gen	IPS-Pro	2010	High contrast ratio

Name	Nickname	Year	Remarks
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 colors. More possible orientations per sub-pixel and produces a better 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.

Advanced fringe field switching (AFFS)

This is an LCD technology derived from the IPS by Boe-Hydis of Korea. Known as fringe field switching until 2003, 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.
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.