An LED, or light-emitting diode, is a special type of diode that emits light when connected in a circuit. A diode is an electronic device that has two electrodes arranged in such a manner as to allow electrical current to flow in one direction only. With its ability to control the flow of electrons, a diode is often used as a rectifier, which is a device that converts alternating current into direct current. Alternating current is an electric current that flows first in one direction and then in the other. Since alternating current fed into a diode can move in one direction only, the diode converts the current to a one-way flow known as a direct current.
An LED converts electrical energy to light by means of a semiconductor, a substance that conducts an electric current, but only very poorly. A semiconductor for an LED is made from an extremely thin slice of crystal. A clear (or often colored) epoxy case encloses the semiconductor. Two leads extend down below the epoxy enclosure.
The semiconductor has two regions— p and n —separated by a junction. The p region is dominated by positive electric charges; the n region is dominated by negative electric charges (electrons). The junction acts as a barrier to the flow of electrons between the p and the n regions. Only
Diode: Electronic device that allows current to flow in one direction only.
Rectifier: Device that converts alternating current (AC) to direct current (DC).
Semiconductor: Substance, such as silicon or germanium, whose ability to carry electrical current is lower than that of a conductor (like metal) and higher than that of insulators (like rubber).
when sufficient voltage is applied to the semiconductor can the current flow, and the electrons cross the junction into the p region.
When electric current is applied to the semiconductor across the leads of the LED, electrons can move easily in only one direction across the junction between the p and n regions. Once the current starts to flow, electrons in the n region have sufficient energy to move across the junction into the p region. Once in the p region, the electrons are immediately attracted to the positive charges. When an electron moves sufficiently close to a positive charge in the p region, the two charges "recombine." In order to return to its low-energy position after recombining with a positive charge, an electron must shed the energy it picked up from the current
in the form of photons, which emit visible light. This is the light produced by an LED.
The color produced by an LED is dependent on the material composing its semiconductor. Early LEDs emitted only red light. Green and amber LEDs were introduced next. By the mid 1990s, blue and white LEDs had been developed.
LEDs are commonly used as indicator lights. They are found in electronic toys, computers, calculators, telephones, and many other household devices. They are immune to electromagnetic interference, power surge hazards, and changes in temperature. Because they are a completely cool light source, LEDs are much more efficient than standard incandescent lightbulbs that give off a lot of energy in the form of infrared radiation (heat). In comparison, LEDs are often up to ten times more efficient.
Many cities in the United States are replacing their incandescent traffic lights with LED units (hundreds of LEDs in an array) because the LED units are brighter (they have equal brightness across their entire surface), they last longer (years versus months), and they are much more energy efficient.
[ See also Diode ]