What are Photo and Laser Diodes?
All laser diodes are based on light-emitting diodes (LEDs). As with the photodiode, the underlying structure is a p-n junction, the only difference is the direction of the applied voltage or bias. The p-n junctions consist of a semiconductor layer (silicon in most photodiodes) doped with atoms carrying extra valence electrons (n-type semiconductors) under a layer doped with atoms carrying one valence electron less than silicon (p-type semiconductor). Charge migration creates a depletion region with an electric field directed toward the p region, allowing current to flow in only one direction. In a photodiode, a reverse bias potential is applied across the diode, preventing current from flowing in the absence of light. With exposure to light, electron-hole pairs are created, generating a current. In a laser diode or a LED, the process is exactly reverse. A positive bias is applied, causing current to flow. As the electrons form the n-type semiconductor flow into the p-type semiconductor
