The pupose of a LED is to create light when a bias is applied. LED is actually a diode; it contains an n-doped region and a p-doped region put in contact through an active layer (or quantum well) where light is produced. The electrons and holes at the interface recombine and a depletion area is created in the n and p type regions close to the active layer. When a bias is applied (pozitive at the p type and negative at the n type), then electrons and holes will be attracted and will pass into the active area, recombining with each other. Through recombination of an electron and a hole, energy is created and a photon is emitted. III-V materials are used for such devices because these semiconductors have a direct bandgap (in comparison with silicon which has indirect bandgap). The light emitting process has a higher probability, thus a bigger efficiency in direct semiconductors (in comparison with silicon, where also a phonon is required for this process to take place). Thus the structure of a LED is : a n-type semiconductor, a p-type semiconductor, an n-contact, a p-contact, and the substrate.