LED Development
The first commercially usable LEDs were developed in the 1960’s by combining three primary elements: gallium, arsenic and phosphorus (GaAsP) to obtain a 655nm red light source. Although the luminous intensity was very low with brightness levels of approximately 1-10mcd @ 20mA, they still found use in a variety of applications, primarily as indicators. Following GaAsP, GaP, or gallium phosphide, red LEDs were developed. These devices were found to exhibit very high quantum efficiencies, however, they played only a minor role in the growth of new applications for LEDs.
As LED technology progressed through the 1970’s, additional colors and wavelengths became available. The most common materials were GaP green and red, GaAsP orange or high efficiency red and GaAsP yellow, all of which are still used today. The trend towards more practical applications was also beginning to develop. LEDs were found in such products as calculators, digital watches and test equipment.
It wasn’t until the 1980’s when a new material, GaAlAs (gallium aluminum arsenide) was developed, that a rapid growth in the use of LEDs began to occur. The brightness was over 10 times greater than standard LEDs. The voltage required for operation was lower resulting in a total power savings. The LEDs could also be easily pulsed or multiplexed. This allowed their use in variable message and outdoor signs. LEDs were also designed into such applications as bar code scanners, fiber optic data transmission systems, and medical equipment. Although this was a major breakthrough in LED technology, there were still significant drawbacks to GaAlAs material.
To overcome these difficult issues, LED designers turned to laser diode technology for solutions. 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.
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. 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. 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. 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. 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. 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.