A light-emitting diode (LED) is a two-lead semiconductor light source. It is a basic pn-junction diode, which emits light when activated.When a fitting voltage is applied to the leads, electrons are able to recombine with electron holeswithin the device, releasing energy in the form of photons. This effect is called electroluminescence, and the color of the light (corresponding to the energy of the photon) is determined by the energy band gap of the semiconductor.
LEDs are produced in a variety of shapes and sizes. The color of the plastic lens is often the same as the actual color of light emitted,
but not always. For instance, purple plastic is often used for infrared LEDs, and most blue devices have colorless housings. Modern
high-power LEDs such as those used for lighting and backlighting are generally found in surface-mount technology (SMT) packages (not shown).