Light Emitting Diodes (LEDs) are specially designed p-n junctions made from compound semiconductors (like GaAs or GaP).

• When forward biased, electrons are injected into the p-region and holes into the n-region.
• Charge carriers recombine near the junction.
• In direct band gap materials, this recombination releases energy in the form of photons rather than heat.
• The energy of the emitted photon is approximately equal to the band gap energy $E_g$ of the material: $h\nu \approx E_g$.
• The color of light depends on the band gap.