The Evolution of the LED

The lighting industry has been buzzing with news of solid-state light source advancements for several years. As one light-emitting diode (LED) lighting fixture manufacturer puts it, flat printed circuit boards versus bulbous or tubular linear sources open up a whole new world of design possibilities. Now, the LED and its sister technology, the organic light-emitting diode (OLED) - two light sources that constitute solid-state lighting - have the potential to replace many light sources that either heat a tungsten filament to incandescence or use a pair of filaments within a glass envelope to create an ionized arc stream.

What are LEDs?

Light-emitting diodes are tiny devices made from semiconductor materials that convert electrical energy into visible and near-UV wavelengths - and some heat - when they are assembled in a package and connected to an electrical circuit. Specifically, the semiconductor materials are crystals comprised of combinations of two or three elements, such as gallium phosphide (GaP) or gallium indium nitride (GaInN). These unique combinations of elements have distinctive crystalline structures that can accommodate both electrons (negatively charged) and holes (positively charged electron vacancies), which exist at different energy levels, separated by a "band-gap."

Generally called a die or chip, which can vary in size from tenths of a millimeter to more than a square millimeter, the LED (diode) permits current to flow in only one direction. This diode is formed by bringing together two slightly different semiconductor materials, called layers - an n-type layer that has an excess of negative charge (electrons) and a p-type layer that has an excess of positive charge carriers (holes), which are locations for the electrons to fall into. Electrodes are placed on each end of this assembly, or structure. The junction or interface of the two layers (called the p-n junction) is where electrons and holes are injected into an active region.

When a forward voltage is applied to this structure (negative to the n-layer and positive to the p-layer), electrons move from the n layer toward the p layer, and holes move toward the n area. Near the junction, an electron and a hole radiatively recombine, emitting a photon (essentially, the electrons move across the p-n interface and fill holes on the p side, falling into a...