LED Christmas decorations use light-emitting diodes (called LEDs), which provide a more reliable and more energy efficient alternative to the traditional incandescent light bulbs. Unlike incandescent bulbs, LEDs do not have filaments that generate heat and eventually burn out. Because of their better energy efficiency (up to 90% better than incandescent bulbs), most of the energy is devoted to created light, rather than heat. Therefore, LEDs stay cool to the touch.
LEDs are a special type of diode, a semiconductor device related to the transistor. Most LEDs are made from the semiconductor material aluminum-gallium-arsenide. Semiconductors are a special class of materials which can be made to conduct electricity under certain circumstances. This property makes semiconductors useful for a variety of electronics applications.
Usually, impurities are added to semiconductors to manipulate their electrical properties. For example, adding an impurity with extra electrons will create a negatively-charged, or N-type, semiconductor. Adding an impurity missing some electrons (these missing electrons are called "holes") will create a positively-charged, or P-type, semiconductor. Diodes are created by joining together N-type and P-type semiconductors. The junction where they are joined together is called the PN junction. With no voltage applied, the electrons will fill the holes along this junction, creating what is called a "depletion zone." After the depletion zone is created, no more current flows unless a voltage is applied. The depletion zone acts as an insulator.
To conduct current through the diode, the depletion zone must be eliminated. Applying a voltage to the diode (positive to the P-type side and negative to the N-type side) will cause the free electrons to flow toward the P-type material, and the positively-charged holes to flow toward the N-type material. As the free electrons move into the P-zone, they fall into the empty electron orbits, or holes, in the atoms. The key to understanding how light is created is to consider the energy states of these electrons. Free electrons have a higher energy state than those orbiting atoms, and inside the atoms, electrons in the outer orbits have a higher energy state than those in the lower orbits. The free electrons must release energy when they drop into orbit around an atomic nucleus to get to the required lower energy state. In LEDs, this excess energy in released in the form of a photon, the basic particle of light. The amount of energy released determines the frequency and wavelength of the photon, which in turn determines the color. The energy gap between free electrons and the orbiting electrons precisely designed to give the desired color. LEDs are typically designed to maximize the amount of light produced. Both the diode itself and the plastic bulb around it are designed to direct the light outward.
Until a few years ago, even with all the advantages of efficiency and long life, LEDs were still too expensive to replace incandescent bulbs. LEDs have become feasible for Christmas lights and many other applications in recent years because the price of semiconductor material has dropped. In addition to Christmas ornaments, LEDs are now also being used for such applications as traffic signals and brake lights.
