What is an LED and What Causes the LED to Emit Light?
LED is short for light emitting diode. When first introduced, do to the lack of intensity, LED’s were commonly only used as indicators in electronic appliances to show whether the circuit is closed or not. LED’s have moved out of the on off switch to more custom applications that used incandescent and neon bulbs do to brighter and larger chips. LED’s have many advantages over incandescent and neon: extreme long life (5years or more if left on constant without any change in efficiency), Specific colors so that an external covering is not needed, and the number 1 advantage is power consumption (watts replaced by milliwatts of consumption.) A clear (or sometimes colored) epoxy case encloses the heart of an LED, the semi-conductor chip. Your typical thru-hole LED has two wires extending below the LED epoxy enclosure commonly named leads. There are two ways to indicate the negative lead of an LED: 1) if the enclosure has a small flange at its base, a pronounced flat spot will be on the negative and 2) by the shorter of the two leads extending from the LED. Most LED's operate at relative low voltages between about 1.5 to 4 volts, and draw currents between about 10 and 40 milliamperes, which why they work well with small coin cell battery like a CR2032(Panasonic). Voltages and currents substantially above these values can melt an LED chip, however with the introduction of larger semi-conductor chips, multiple chip combos and mounting on heat sinks 350mA and 1 watt of power has become a common occurrence. The most important part of an LED is the semi-conductor chip located in the center of the bulb as shown at the right. The chip has two regions separated by a junction, the p region (positive electric ions or charges) and the n region (negative electric ions or charges). The junction acts as a barrier to the flow of electrons between the p and the n regions. Only sufficient applied voltage to the semi-conductor chip allows current flow and the electrons to cross the junction into the p region. In the absence of a large enough electric potential difference (voltage) across the LED leads, the junction presents an electric potential barrier to the flow of electrons. A reversed LED put in to a circuit des not emit light and in fact acts as a rectifier not allowing any or very little current flow. Two leaded Bi-color LED’s take advantage of this allowing indication of current flow in a current switching circuit.

What Determines the Color of the Light and Brightness?
When a voltage is applied and 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 and the positive charges, due to the mutual coulomb forces of attraction between opposite electric charges, "re-combine". Each time an electron recombines with a positive charge; electric potential energy converts into electromagnetic energy. For each recombination of a negative and a positive charge, a quantum of electromagnetic energy emits in the form of a photon of light with a frequency characteristic of the semi-conductor material. Light emitted from a LED is in very narrow spectrum of light. LED’s emit different colors depending on the different semi-conductor materials, and may require different energies to light them. A combination of the chemical elements gallium, arsenic and phosphorus (GaAsP) emit colors ranging from a yellow to a reddish orange, at a low intensity of under 100mcd’s (millicandela) and will operate at 1.9V and 10mA. A combination of Indium, gallium, nitrogen (InGaN) will emit either pure green or blue in brightness is well over 1 to 2 Candelas and requires 3.2V and 20mA for best performance. The shape, color, thickness and amount of diffusion of the epoxy lens can affect brightness. A clear & tall lens will emit a narrow and very intense beam, while a short and diffused lens found on SMD’s (Surface Mount LED) has a very wide and less bright pattern.