LIGHTING:LIGHT-EMITTING DIODES.

LIGHT-EMITTING DIODES

Light-emitting diodes, commonly called LEDs, function like tiny light bulbs and are found virtually everywhere. A diode is a simple semiconductor device that has a varying ability to conduct electrical current. In a LED, the conductor material is typically aluminum-gallium-arsenide (AlGaAs). In pure AlGaAs, all of the atoms bond perfectly, leaving no mobile electrons that could potentially conduct electricity. A “doped” semiconductor has additional atoms that either add mobile electrons or create holes where electrons can go. This doping process increases the conductivity of the material. Semiconductors doped with extra electrons are know as N-type materials, since they have extra negatively charged particles that tend to migrate toward positively charged areas. Semiconductors with holes are known as P-type materials, since they effectively have extra positively charged particles. Electrons jump from hole to hole, moving from a negatively charged area to a positively charged area. A diode is simply a section of N-type semiconductor bonded to a section of P-type material with electrodes on each end. When no voltage is present, the N-type material fills the holes from the P-type material at the junction between the two layers. In this arrangement, all of the holes are filled and the material behaves as an insulator. To get electrons moving, the N-type section is attached to the negative end of a circuit and the P- type section to the positive end of the circuit. The free electrons in the N-type section are repelled by the negative electrode and attracted to the positive electrode. The holes in the P-type material move in the opposite direction, migrating toward the negative end of the circuit. As such, this arrangement only conducts electricity in one direction. The interaction between electrons and holes has an interesting effect in that it produces light. As the electrons move across a diode, they can fall into empty holes in the P-type layer. As these electrons drop from their mobile state to a lower orbital, the electrons release energy in the form of photons. For example, the atoms in a standard silicon diode are arranged so that the electrons release a relatively small amount of energy, producing infrared light. These diodes are ideal for use in remote controls, among other products. The gap between the conduction band and the lower orbitals determines the frequency of the photons emitted; in other words, it con- trols the color of light produced. By adjusting the properties of the materials used for the P-type and N-type semiconductors, virtually any color of light can be produced from LEDs. The main advantage of LEDs is their efficiency compared with normal incandescent lights. These devices are tailored to produce a specific wavelength of visible light. As such, they produce very little heat because they do not produce infrared light unless they are intended to do so.