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Current Developments of LED

To overcome these difficult issues new technology was needed. LED designers turned to laser diode technology for solutions. In parallel with the rapid developments in LED technology, laser diode technology had also been making progress. In the late 1980’s laser diodes with output in the visible spectrum began to be commercially produced for applications such as bar bode readers, measurement and alignment systems and next generation storage systems. LED designers looked to using similar techniques to produce high brightness and high reliability LEDs. This led to the development of InGaAlP (Indium Gallium Aluminum Phosphide) visible LEDs. The use of InGaAlP as the luminescent material allowed flexibility in the design of LED output color simply by adjusting the size of the energy band gap. Thus, green, yellow, orange and red LEDs all could be produced using the same basic technology. Additionally, light output degradation of InGaAlP material is significantly improved even at elevated temperature and humidity.

As a result of these developments, much of the growth for LEDs in the 1990’s was concentrated in three main areas: The first was in traffic control devices such as stop lights, pedestrian signals, barricade lights and road hazard signs. The second was in variable message signs such as the one located in Times Square New York which displays commodities, news and other information. The third concentration was in automotive applications.

The visible LED has come a long way since its introduction more than 30 years ago and has yet to show any signs of slowing down. Blue LEDs, which were introduced in the early to mid 1990’s, have become the cornerstone to an entire generation of new applications. Blue LEDs because of their high photon energies (>2.5eV) and relatively low eye sensitivity (465nm typical wavelength) have always been difficult to manufacture. In addition the technology necessary to fabricate these LEDs is very different and far less advanced than standard LED materials. The blue LEDs available today consist of GaN (gallium nitride) and SiC (silicon carbide) construction with brightness levels in excess of 10000mcd @ 20mA. Since blue is one of the primary colors, (the other two being red and green), full color solid state LED signs, TV’s etc. are becoming commercially available. The first decade of the 21st century will see a large growth in RGB (full color) LED applications. Other applications for blue LEDs include medical diagnostic equipment and photolithography.

It is also possible to produce other colors using the same basic GaN technology and growth processes. For example, a high brightness green (approximately 500nm – 530nm) LED has been developed that is currently being used as a replacement to the green bulb in traffic lights. Other colors including purple are also possible. With the introduction of blue LEDs, it became possible to produce white light probably the most exciting new development in LED technology to date. White light is currently made in one of 2 ways. The first is by selectively combining the proper combination of red, green and blue light. This process however, requires sophisticated software and hardware design to implement. In addition, the brightness level is low and the overall light output of each RGB die being used degrades at a different rate resulting in an eventual color unbalance. The 2nd and most dominant method of achieving white light output is to use a phosphor coating (typically - Yttrium Aluminum Garnet or YAG) on the surface of a blue LED. The blue die excites the phosphor causing it to glow white.

In summary, LED’s have gone from infancy to adolescence and are experiencing some of the most rapid market growth of their lifetime. By using InGaAlP material with MOCVD as the growth process, combined with efficient delivery of generated light and efficient use of injected current, some of the brightest, most efficient and most reliable LEDs are now available. This technology together with other novel LED structures will ensure wide application of LEDs. Further developments on white light output will also guarantee the continued increase in applications of these economical light sources and may eventually replace standard incandescent and fluorescent lighting.