In its broadest sense, information technology is not just about processing, conveying and storing information, but displaying it. The most immediate and perhaps the most important interface in desktop computer systems is that between the machine and the eye, mediated by the computer screen. Display technology-the conversion of electronic data to a visual display-is a vast industrial concern, of which computer screens are but one aspect. Television screens are of course basically the same devices; but traffic signals are a very different kind of visual display, and increasingly electronic media are replacing ink and paper as the vehicle for all kinds of written information.
Standard-sized television screens today employ the same technological principles as the earliest of cathode-ray tubes, dating from before the discovery of the electron. A beam of electrons is scanned rapidly over a pixellated array of phosphor dots, which glow in a particular color when irradiated. Separate red, green and blue phosphors at each pixel suffice to generate the colors of most of the visible spectrum.
This is a cumbersome system, since it requires an electron gun placed some distance behind the screen. The TV terminal is therefore the most bulky part of many personal computers. Laptop computers use a different display medium, which is more expensive to produce, but less wasteful of space and has a lower power consumption. These flat screens make use of liquid-crystal light shutters, which can be switched electronically between a transparent and an opaque black state. In the transparent state, a liquid-crystal pixel element lets through light from a background source, which also passes through a color filter to generate the three primaries of each pixel. Some of the challenges for liquid-crystal display technology include faster switching speeds and development of display systems that permit a wide viewing angle, so that the picture does not vanish or become bizarrely colored when not seen face-on.
But still better than this shutter-and-filter method, would be a display in which each element is an intrinsic light emitter that is electronically switchable, robust, flat and cheap to produce. Most efforts in this direction are focused on the fabrication of banks of light-emitting diodes-the challenge is to make them cheap, reliable and bright enough that the flat screen becomes an economically viable product. Light-emitting diodes based on the same kind of inorganic semiconducting materials used in laser diodes have long been in use: gallium arsenide doped with phosphorus, for example, offers light emission in the visible range. But whereas such materials cover the spectrum from red to green, efficient blue LEDs were not available until the advent of gallium nitride. LEDs made from this material were in fact marketed several years before the corresponding blue-light lasers, bringing full-color bright LED displays and TV screens within reach for the first time. An indium-doped version of gallium nitride is an efficient emitter of green light, and is used in LED-based traffic lights, which are not only brighter than those that use incandescent bulbs but also have much lower power consumption and longer lifetimes. Applications like these seem set to secure gallium nitride as a major technological material in the next several decades.