September 2000

LEDs Light the Future

Continued from page 2

By Neil Savage

The White Answer

The first LEDs were built in the early 1960s. The tiny chips of semiconductor material, encased in a clear epoxy, give off a single color of light when electricity runs through them. Negatively charged electrons move to fill positively charged regions in the material, called "holes," where electrons are missing. The combination of an electron and a hole produces a photon of light. The greater the difference in energy between electron and hole-the so-called bandgap-the higher the energy of the photon that emerges. The energy of the photon corresponds in turn to the color of the light; within the visible spectrum, blue and violet photons carry the most energy, orange and red photons the least. Different materials naturally have different bandgaps, so to change the energy level and hence the color of the photons, engineers grow the crystalline semiconductors out of different alloys (see companion article: " They Come in Colors").

High-brightness monochromatic LEDs are already making headway in the marketplace. About 10 percent of the red traffic lights in the United States have been replaced by LEDs. They are more expensive than conventional light bulbs but have other advantages that outweigh the cost issue. One is efficiency: A red LED traffic light uses only 15 watts of electricity instead of the 150 watts consumed by traditional stoplights. Another is longevity-the LEDs are expected to bring traffic to a halt without burning out for a full decade. Single-color LEDs' compactness, low power, intense colors and low heat also have them popping up as car taillights, airplane warning lights on radio towers and runway lights at airports. But the minds of researchers and the eyes of the lighting industry are focused on white.

And that creates a challenge: How do you get white light out of devices that are, by nature, monochromatic? One method involves mixing LEDs of different colors so they appear white. Just as a television set makes all the colors it displays-including white-out of glowing red, green and blue phosphors, the right combination of LEDs can give the appearance of white. The standard way to mix is with three separate red, green and blue diodes, but the right combination of just two-say blue and orange-can also produce white.