LEDs (light emitting diodes) are solid-state lighting devices that produce light when a forward voltage is applied. An LED consists of a semiconductor diode packaged in a clear epoxy or silicon gel. The diode contains two slightly different materials: a P-type semiconductor and an N-type semiconductor. The P-type semiconductor has "holes" created by a lack of electrons, producing a positive charge. Conversely, the N-type material has an excess of electrons, resulting in a negative charge. The P- and N-type semiconductors are placed in direct contact in the diode and the region where they meet is referred to as the PN junction. When an electric current passes through the device, electrons flow toward the P region and holes flow toward the N region. Near the PN junction, electrons and holes combine and the electrons shed the extra energy they acquired from the electric current. This energy is released in the form of a photon, the basic unit of light. In this way, an LED emits visible light.
The energy of the photons corresponds to the color of the light emitted. In the visible light spectrum, blue and purple light results from the greatest energy emission whereas yellow and red light is a result of the lowest energy emission. By utilizing materials with different band gaps, engineers can alter energy emission and thus the color of light produced by an LED.