LED Basics

LED is the acronym for light-emitting diode. A diode is the simplest type of semiconductor. Rather than control the flow of electrons, as a transistor does, a diode just conducts electricity in one direction and blocks it in the other. It made from a semiconducting material such as gallium arsenide or indium gallium nitride, combined with another substance that changes its electrical properties to suit the task at hand.

About a century ago, scientists discovered that diodes emit infrared light as an electrical current passes through them. In the 1960s, several companies developed diodes that produced visible light, and the LED was born. Early LEDs were dim and mostly limited in color to red, green, and amber. Despite these restraints, LEDs quickly replaced incandescent light bulbs for use as indicator lights, primarily because they last so much longer. Almost all LEDs have lifetimes specified in tens of thousands of hours, and some are even rated to last 100,000 hours or longer. In the 1980s and 1990s, brighter LEDs emerged, along with LEDs in white, blue, and other colors. These breakthroughs caught the attention of video engineers. As display technologies evolved away from light-emitting cathode-ray tubes toward light valve?technologies such as LCD and DLP, engineers needed a cool-running, efficient, reliable light source. LED delivers on all three.

Interestingly, most white LEDs are actually blue LEDs coated with a yellow phosphor. Some of the photons emerging from the blue LED excite the yellow phosphor, thus producing yellow photons, which combine with the blue photons to produce white light. By fine-tuning the underlying blue LEDs?color and the phosphor formulation, LED makers are able to deliver white light pure enough to drive high-quality video displays. In fact, white LED light is even broader in spectrum than the light from the coldcathode fluorescent lamps (CCFLs) used as backlights in most LCD TVs. LEDs can therefore produce a wider range of colors.