Light Emitting Diodes or LEDs are now considered the greatest invention in lighting since Edison’s light bulb in 1879. LEDs are rapidly replacing light bulbs in a variety of high reliability applications including traffic lights, brake lights, and emergency exit signs.
LEDs are constructed from tiny solid-state chips similar to those used in computers. These chips directly convert electricity to light without the use of a filament or glass bulb. Instead, the chips are encapsulated in solid plastic that can be made into a variety of shapes and sizes.
LEDs have no filament, which means there is nothing to burn out and no need for a breakable glass bulb, and because there is no filament very little heat is produced.
An LED is a special type of diode, and is similar to a transistor. Diodes and transistors are solid state devices that are made from semiconductors such as silicon. The semiconductor is made to contain two types of special impurities. The first type of impurity, called “N” for negative, is a material with an excess of electrons. The other type of impurity, called “P” for positive, has a deficiency of electrons in holes. These two kinds of impurities are dispersed into the semiconductor at different regions, so that a P-N junction, or active layer, is created at the border.
Semiconductors do not freely conduct electricity like materials such as copper. Instead, they can be made to conduct electricity under certain conditions. For diodes and transistors, these amount to connecting electric power across the junction to make the “P” side more positive and the “N” side more negative. When this happens, electricity flows freely across the P-N junction and the electrons fill up the holes. The laws of physics describe the energy between electrons and holes, and show that this energy is a fixed amount depending on the materials. LEDs differ from ordinary diodes in that the P and N materials take this energy and convert it to photons, or light. Since the energy involved is a fixed amount, the light that results has only a single wavelength, or color. Using different materials for the P and N impurities enables the light to be manufactured in different colors.
General Electric introduced the first LEDs in 1962. These LEDs came only in red, the lowest energy of all visible light colors. In the 1970s, yellow and green were introduced, along with intermediate colors such as orange. However, the materials then in use were very inefficient and very little light was produced for each Watt of electric power used. Conventional light bulbs are also very inefficient yielding at best only about 6% of their electric power used in the form of light. The rest of this power turns into heat, as one finds out quickly when touching a lighted bulb. Over the years, LED efficiency, brightness, and lowered cost have exploded in parallel with computer components. Today, LED power efficiency surpasses that of a fluorescent and is increasing with 75% efficiency on the horizon.