An LED is a type of semiconductor diode made with a chip of semiconducting material impregnated (or doped) with impurities to create
a "p-n junction". An electric current forces charge-carriers (electrons and electron "holes") to flow into the junction. When an electron meets a hole, it falls into a lower
energy level, releasing energy in the form of a photon. The wavelength of the light emitted depends on the materials forming the junction (red ones typically use gallium
arsenide phosphide).
Spectrum of Red LED Bulb