LEDs poised to replace light bulbs

A recent advance in light-emitting diodes may illuminate the path to replacing light bulbs with LEDs within the next five years, according to researchers.

Fred Schubert, a professor at Rensselaer Polytechnic Institute, claims to have invented a 99 percent efficient reflector that promises to speed the replacement of light bulbs with LEDs.

"Until now, all lighting systems, especially incandescent bulbs, generated more heat than light. But our 99 percent efficient reflectors for LEDs makes them the first candidate for light-bulb replacement that generates more light than heat," said Schubert.

By tipping the scales towards more light than heat, Schubert said, there's also light at the end of the tunnel. He envisions white LEDs beginning to replace incandescent light bulbs within five years. Schubert has so far only demonstrated his omnidirectional reflector LEDs in red and blue, achieving more than twice the brightness of earlier LEDs. The next step is to demonstrate white LEDs for light-bulb replacement.

The National Science Foundation recently award Schubert's team a $210,000 grant to create in three years a commercial version of his patented omnidirectional reflector. Schubert previously received a two-year, $250,000 contract from the Defense Advanced Research Projects Agency to develop the current prototype.

"In an LED, light emits from inside the semiconductor in every direction, but our omnidirectional mirror reflects light equally well no matter what the angle of incidence. Other types of reflectors are only efficient when the angle is normal, or 90 degrees perpendicular to the surface," said Schubert.

The omnidirectional reflector was made by integrating a silver mirror onto the same LED chip. A layer of dielectric is lithographically patterned with arrays of micron-sized holes and deposited on the bottom of the LED semiconductor layer. The mirrored reflector was then deposited as a thin film of silver on the dielectric. Where it penetrated through the patterned holes in the dielectric, it made arrays of microcontacts with the semiconductor beneath.

Current then flows through the backside silver microcontacts to activate the semiconductor layer, causing it to emit omnidirectional light that is reflected off the silver back and through the top of the LED with over 99 percent efficiency.

"The LED market has already been growing about 10 percent a year. We think the market will continue growing at 10 percent or more per year for the forseeable future," said Schubert.

White LEDs could be used to light homes, businesses, museums, airports and streets. Schubert claimed that lighting accounts for 25 percent of U.S. electrical energy consumption. Since white LEDs emit more light per dollar and generate less unwanted heat, they are potentially a major energy saver.

- R. Colin Johnson

EE Times