Semiconductors have 2 energy bands -- valence band and conduction band. The valence band is filled with electrons; the conduction band has practically none. The electrical conductivity of a material is due to the flow of electrons in the conduction band; in semiconductors, this flow can be manipulated by incorporating appropriate impurities. Some impurities donate electrons to the conduction band, rendering the semiconductors negative, or n-type, while others siphon off electrons from the valence band, creating a positive hole or p-type semiconductors.

LEDs are produced by growing n-type material on p-type base or vice versa. When electricity flows through the LED, the electrons and holes move towards each other, emitting light when they recombine at the p-n junction. The colour of the emitted light is determined by the energy difference between the bands and impurity levels, and can be optimised by choosing the right materials.