TechArchive

This article was originally written in the period 1995-2000

Flat screens can use a variety of technologies, including the traditional cathode ray tube (CRT), as well LCDs and others. Alan Quinn looks at what’s happening.

It’s reckoned that the CRT still holds market dominance for full colour TV and is likely to do for some time, especially for large screen sizes (above 12in). However, even the CRT manufacturers themselves are contributing a large part of the R&D work carried out on alternative display technologies. The LCD has already become a clear market leader in small TV and computer application flat panel displays. The key questions seem to be: which LCD technology to use; and how fast will larger sizes become possible?.

Most of the market in terms of volume is in TV and consumer applications, having a major impact on portable entertainment electronics, and this is served primarily by colour liquid crystal active matrix displays in sizes up to approximately 8 to 10 in diagonal. In the short term, most will probably be based on amorphous silicon, but many companies are actively pursuing polysilicon-based active matrices because of the potential cost and performances advantages offered for integrating additional drive/interface circuitry on the display surface.

For the market up to approximately 14in diagonal, monochrome displays are no longer acceptable and active matrix LCDs are the major technology likely to oust CRTs.

The market for plasma and electroluminescent displays will probably overlap that for the active matrix at about the 12in size (14-15in TFT LCD displays are being produced now by Hitachi, but only on a semi-custom basis, they are not in mass production). If basic technological problems are resolved, they will cover the colour display market up to the 30-35in region. In this size, the use of an emissive technology appears to offer advantage over the flat panel liquid crystal because of the backlighting requirement in terms of size and power consumption.

Various companies are looking into full colour EL and plasma displays and they are addressing all aspects of the technology, from fundamental work on phosphor excitation mechanism and alternative phosphors, through to investigations of drive electronics and integration.

Above 30-35in diagonal, there is a move towards liquid crystal projection type displays and a number of companies are looking at liquid crystal light valve displays. These could be based on, for example, polysilicon TFT liquid crystal light valves with 320 x 220 elements. Such displays will also find application in a number of office environment applications.

Generally, LCD has the lowest power consumption display device of any flat panel display – 0.01 to 1% of the power excluding the drive circuit). It can also be driven easily by CMOS LSI, which is a low power semiconductor device. Together with a thin profile and low weight, these features make it best suited to personal/portable applications, such as watches, portable TVs and notebook PCs.

Liquid crystal is the intermediate condition between being liquid and solid (crystal). Liquid crystal has the fluidity of liquid and the ordered alignment of molecules in a crystal.

There are three types of liquid crystal – smectic, nematic and cholesteric. The smectic types show the greatest level of ordering and their molecules tend to form into layers within the liquid. Molecules of all types of LC tend to orientate themselves into specific directions and it is this property, combined with a twist in the molecular alignment, that gives the difference between ON and OFF states. Alignment of the liquid crystal molecules can be changed by applying an electric field and this change induces the optical characteristics used in displays. Most conventional LCDs use twisted nematic (TN) crystals, with a molecular twist of 90o. Although these do not offer switching speeds compatible with TV line scans, they do have good contrast, provided they are not multiplexed above about 100:1.

Developed in 1971, the simple principle of TN is that, as voltage is applied, the twisted molecular construction is destroyed and no light passes through. Production increased to supply the market for watches and calculators. From the late 1970s, larger display sizes became available, with improvements in materials and processes, and applications expanded to the computer/word processor area.

With supertwisted (STN) technology, molecular chains are highly twisted, causing light waves to bend and product darker characters, increasing contrast and providing a wider viewing angle.

Smectic LCs have been proposed to remove the multiplexing problem A one that will support at least 500 lines and ideally 1050 to fit in with proposals for high definition television. These only require a pulse to change state and will remember the information carried on the last pulse indefinitely. Smectic LCs can switch very rapidly at speeds compatible with television scans. However, TFT active matrix types are another, now established solution to this problem.

Active Matrix LCD

Production of active matrix LCD started in 1983 for portable TV. The start of production of large size TFT (thin film transistor) LCD for PC application in 1991 promises to be the start of the next expansion of the LCD market. Active matrix LCDs offer a higher speed response, high contrast and less crosstalk. As with simple matrix types, there are several types. Toshiba uses one of the TFT variants.

Nearly all of the TV displays are using the active matrix principle using an array of TFTs, generally using amorphous silicon technology. The development of TFTs in polysilicon would allow the drivers to be located on the same glass substrate. This seems to be happening first in smaller screen sizes.

Immense quality control difficulties had to be overcome to produce full colour display panels using TFTs. Each pixel has to be associated with a switching element which must be produced using integrated circuit production techniques. Even early displays were made up of 330,000 pixels and all TFTs must function properly if the display is to be saleable.

TFT LCDs can be produced to be normally black or normally white, this describing the appearance of the display in the non-voltage applied mode. Normally white is said to provide higher contrast than the normally black. Colour is achieved using the LCs in reverse video mode with back illumination and a series of fine filters on the front panel.

Toshiba is primarily a colour TFT LCD manufacturer producing TFT products suitable for use in laptop PCs, notebooks and sub- notebooks. together with special modules used in the avionics and automotive markets. Hitachi also uses colour TFT, but also colour STN for lower cost options.

According to Adrian Anor of Hawke Components (distributor for Kyocera), STN (Super-Twisted Nematic) types will always be subordinate to TFT on response time and hence speed, viewing angle and contrast ratio. But newer dual scan colour STN displays bring much enhanced properties and the gap is being closed, so that STN now offers a realistic, low cost alternative. Although real time video still requires TFT, STN is now acceptable for real-time related areas, such as mouse applications.

Ferroelectric LCDs

Ferroelectric LCDs are still in their infancy for TV, though it is generally agreed that TV is possible using this technique. Research work and prototyping activity has been reported with work being done by the University of Hull, STC, Thorn EMI, RSRE. The advantages are speed, high contrast and wide viewing angle. The disadvantages are thin cells (2aem), mechanical stability and no analogue grey scale. Special arrangements are needed for colour – colour filters or sequential colour backlight.

High speed materials, faster than those available at present. It is believed that many Japanese are active in this area, but then the materials design and synthesis capability in the UK is also reported to be very strong.

Vacuum Fluorescent Displays

Some significant advances have been made with this display technology for information display applications (instrumentation, automotive products and consumer products, such as VCRs). There are some multi-colour graphics panels available, aimed primarily at the computer display market with performance rapidly advancing. For full colour TV applications, VFDs have some way to go, especially in comparison to the advances in the LCD technologies.

Electroluminescence

Electro luminescent displays are thin, bright and attractive. In addition, it is possible to superimpose key spaces over the surface of the display. But this technology has had a mixed reputation. Some users, particularly in the membrane keyboard industry, have felt that EL displays are of limited value. Irreparable damage to individual pixels has been the primary problem, though use of powder phosphor and DC supply ensures that minor imperfections in manufacture do not lead to catastrophic pixel failure.

A benefit is the possibility of superimposing capacitive switches at any point within the pixel matrix, without affecting the quality of the display. Additional screen-printed information may be superimposed on another layer of glass. They have a large market in panels for information display.

A key feature that marks EL as a potential alternative to the CRT is the refresh rate A ideally 300 frames per second but adequate at 50 or 60, the EL display can cope easily with video picture rates at a supply voltage of 110Vdc.

The display hardware is based around a sheet of glass coated with a conductive, transparent oxide film, which forms the positive electrode. A layer of powder phosphor, typically 30aem thick, is deposited onto the oxide and is covered by a vacuum evaporated aluminium layer to form the positive electrode.

The phosphor layer, typically made of ZnS doped with manganese to give a green colour, is encapsulated in a controlled atmosphere by a glass, metal or ceramic back cap, bonded to the substrate. Electrical contacts are brought out around the edge of the display.

Full colour TV applications are promised, but the technology is still a long way behind CRTs and LCDs.

Gas Discharge Plasma Displays

Plasma panels utilise an inert gas sandwiched between glass panels. Light is produced by single intensity electrical excitation of gas. They are said to provide a rather low resolution and additional control hardware is needed. At one time (this may have changed because the information is not up to date) only light and dark pictures are possible, with no grey scale.

However, they do give the flattest screen and the picture has high dimensional accuracy. Their major advantage over some other technologies is that the screen size is not necessarily technology restricted – screen sizes up to 1.5m have been reported. Other features are low power consumption, lightness and compactness. Selective reading and writing is possible. The transparent screen allows related information to be superimposed from the rear of the displayed image.

Prototype multi-colour displays have been developed, but again these are primarily computer-driven and are said to be some way behind LCDs in the full colour TV application field. Problem areas are in full-colour working ability, limited non-continuous grey scale, complex drive requirements and relatively low efficiency.

Acknowledgements:

• Toshiba Electronics
• Anders Electronics
• Eiger Technologies (for Hitachi)
• Hawke Components (for Kyocera)
• Microtouch Systems