TFT LCD Display

Thin film transistor

TFT) is a special kind of field effect transistor made by depositing thin films of a semiconductor active layer as well as the dielectric layer and metallic contacts over a supporting substrate. A common substrate is glass, since the primary application of TFTs is in liquid crystal displays. This differs from the conventional transistor where the semiconductor material typically is the substrate, such as a silicon wafer.

TFTs can be made using a wide variety of semiconductor materials. A common material is silicon. The characteristics of a silicon based TFT depend on the crystalline state. That is, the semiconductor layer can be either amorphous silicon, microcrystalline silicon , or it can be annealed into polysilicon. Other materials which have been used as semiconductors in TFTs include compound semiconductors such as cadmium selenium (CdSe) and metal oxides such as Zinc Oxide. TFT's have also been made using organic materials (referred to as an Organic TFT or OTFT).

By using transparent semiconductors and transparent electrodes, such as indium tin oxide (ITO), some TFT devices can be made completely transparent.

Because the substrate cannot stand for the high annealing temperature, the deposition process has to be completed under relatively low temperature. Chemical vapor deposition, physical vapor deposition (usually sputtering) are applied. Also the first solution processed transparent TFTs (TTFTs), based on zinc oxide were reported in 2003 by researchers at Oregon State University.

A 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. TFT LCD is one type of active matrix LCD, though it is usually synonymous with LCD. It is used in televisions, flat panel displays and projectors.

Construction

Normal liquid crystal displays like those found in calculators 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 which reduce the connection count from millions to thousands. If all the pixels in one row are driven with a positive voltage and all the pixels in one column are driven with a negative voltage, then the pixel at the intersection has the largest applied voltage and is switched. The problem with this solution is that all the pixels in the same column see a fraction of the applied voltage as do all the pixels in the same row, so although they are not switched completely, they do tend to darken. The solution to the problem is to supply each pixel with its own transistor switch which allows each pixel to be individually controlled. The low leakage current of the transistor also means that the voltage applied to the pixel does not leak away between refreshes to the display image. Each pixel is a small capacitor with a transparent ITO layer at the front, a transparent layer at the back, and a layer of insulating liquid crystal between.

The circuit layout of a TFT-LCD is very similar to the one used in a DRAM memory. However, rather than building the transistors out of silicon which has been formed into a crystalline wafer, they are fabricated from a thin film of silicon deposited on a glass panel. Transistors take up only a small fraction of the area of each pixel, and the silicon film is etched away in the remaining areas, allowing light to pass through.

The silicon layer for TFT-LCDs is typically deposited using the PECVD process from a silane gas precursor to produce an amorphous silicon film. Polycrystalline silicon is also used in some displays where higher performance is needed from the TFTs, typically in very high resolution displays or ones where performing some data processing on the display itself is desirable. Both amorphous and polycrystalline silicon TFTs have very poor performance compared with transistors fabricated from single-crystal silicon.

IPS

IPS (in-plane switching) was developed by Hitachi in 1996 to improve on the poor viewing angles and color reproduction of TN panels. Most also support true 8-bit color. These improvements came at a loss of response time, which was initially on the order of 50ms. IPS panels were also extremely expensive.

IPS has since been superseded by S-IPS (Super-IPS, Hitachi in 1998), which has all the benefits of IPS technology with the addition of improved pixel refresh timing. Though color reproduction approaches that of CRTs, the contrast ratio remains relatively weak. S-IPS technology is widely used in panel sizes of 20" and above. LG and Philips remain one of the main manufacturers of S-IPS based panels.

AS-IPS – Advanced Super 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. AS-IPS is also a term used for NEC displays (e.g., NEC LCD20WGX2) based on S-IPS technology, in this case, developed by LG.Philips.

A-TW-IPS – Advanced True White IPS, developed by LG.Philips LCD for NEC, is a custom S-IPS panel with a TW (True White) color filter to make white look more natural and to increase color gamut. This is used in professional/photography LCDs.

H-IPS – Released sometime late 2006, was the H-IPS panel which is an evolution of the IPS panel which improves upon its predecessor, the S-IPS panel. The H-IPS panel can be seen in the NEC LCD2690WUXi, Mitsubishi RDT261W 26″ LCD and Apple's newest Aluminum 24" iMac.

So to sum up, the pros/cons of the H-IPS over the S-IPS:

Pros:

• Much less backlight bleed.
• No purple hue visible at an angle
• Backlight bleed improves looking at an angle
• Less noise or glitter seen on the panel surface (smoother surface)

Cons:

• Still some backlight bleed in areas that are green.
• Viewing angles may have sacrificed in order to improve pros.

Fringe Field Switching is a technique to accomplish wider viewing angle and transmittance on IPS displays.

Safety

The liquid crystals inside the display are poisonous. It must not be ingested, or touched by your skin or clothes. If spills occur due to a cracked display, wash off immediately with soap and water.

Display industry

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

1. LG.Philips
2. AU Optronics
3. S-LCD Corporation (a Samsung/Sony joint venture)
4. Chi Mei Optoelectronics
5. Sharp Corporation
6. Samsung

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. 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 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).

Value TFT screens and most 15 inch (381 mm) sized LCDs usually lack a digital input like DVI connector, so their future proofing may be limited. Most displays larger than 17 inch (432 mm) have both an VGA analog input and a DVI digital input sockets. Almost all professional screens include a DVI socket and some also include a pivot mode for portrait-mode display.