Who would have imagined that a single semiconductor device could be part of almost every aspect of our lives? Although LEDs have been used in device backlighting and indicator applications for decades, they are gaining popularity in many applications where incandescent and fluorescent lighting are typically used. This push seems to have been spurred by the development of the white LED-or, more specifically, the blue LED that made it possible.
Gallium nitride (GaN) and indium gallium nitride (InGaN) blue LEDs enabled white LEDs by adding a combination of red and green LEDs or by covering the blue LED with a yellowish Ce3+:YAG (cerium-doped yttrium aluminum garnet) phosphor, the most common technique today1. Cheap backlit color displays were finally possible for cellular phones and other portable devices with the advent of the white LED, which has become the default device for backlighting in portable applications.
LEDs are also becoming attractive for lighting in automotive and domestic applications. Solid-state lighting (SSL), where white LEDs are grouped together as an alternative to incandescent and fluorescent applications, are not yet viable, because of their relatively high cost, special power requirements and moderate light output. Fluorescent lighting is still more efficient in terms of cost and light output.
However, new technology such as scattered photon extraction (SPE)2, being developed at Rensselaer Polytechnic Institute's Lighting Research Center, promises to increase luminous efficacy (light output per wat, or lm/W) by more than 60 percent. In the typical Ce3+:YAG phosphor-based white LED, a major portion of the light is reflected back by the phosphor layer into the LED and absorbed, greatly reducing light output. SPE places the phosphor layer further from the blue die than typical white LEDs, and reshapes the lens to extract the maximum amount of reflected light.
By increasing the luminous efficacy of white LEDs, SPE will enable them to maintain the typical required light output while being driven at much lower currents and, in turn, much lower forward voltages. This will enable white-LED drivers to increase their conversion efficiency and do so over larger portion of the typical Li-ion battery range.
New methods of creating white light from LEDs use no phosphors, eliminating the issue with light reflection and inefficiency due to the phosphor layer. One method uses a homoepitaxial-grown zinc selenide (ZnSe) on a zinc selenide substrate to produce a blue light from the active region and a yellow light from the substrate at the same time3.
A recent LED development came from Michael Bowers, a grad student at Vanderbilt University4, while he was trying to create very small quantum dots. (Quantum dots are small crystals, only a few nanometers in diameter, that produce a unique color when a light is shone on them.) During his experiments with a laser, he found that blue light caused his quantum dots to emit a white light similar to an incandescent bulb. Commercialization of this discovery would make it possible to illuminate any object on which quantum dots can be applied, further opening the realm of possible applications for LEDs.
Advancements in LED power and light efficiency, size, and technology will make LEDs an even greater part of our lives than they already are.
1. Nichia Corp, Japan, (www.nichia.co.jp)
2. Narendran, Nadarajah (2005), "Improved Performance White LED," Fifth International Conference on Solid State Lighting, Proceedings of SPIE 5941, 45-50. Bellingham, WA: International Society of Optical Engineers.
3. Takebe, T, (Sumitomo Electr. Ind. Ltd., Hyogo, Japan), "ZnSe-based white LED," The 5th Pacific Rim Conference on Lasers and Electro-Optics (CLEO/
Pacific Rim 2003), 22 Vol 1, DOI: 10.1109/CLEOPR.2003. 1274502, 15-19 Dec., 2003.
4. Carey, Bjorn, "Accidental Invention Points to End of Light Bulbs" (www. livescience.com/technology/051021_nano_light.html), 21 October 2005.
About the author
Karl L. Schlossstein is an applications engineer for the Power Core Products group at National Semiconductor's Grass Valley Design Center in California.