Photodetectors are devices used for the detection of light. Photosensors or Photodetectors are sensors of light or other electromagnetic energy. Photodetectors are used primarily as an optical receiver to convert light into electricity. Photodetector operates by converting light signals that hit the junction to a voltage or current. The junction uses an illumination window with an anti-reflect coating to absorb the light photons. The result of the absorption of photons is the creation of electron-hole pairs in the depletion region.

Various Types of Photodetectors

  • Photodiodes are semiconductor devices with a p–n junction or p–i–n structure (i = intrinsic material) (p–i–n photodiodes), where light is absorbed in a depletion region and generates a photocurrent.
  • Phototransistors are similar to photodiodes, but exploit internal amplification of the photocurrent.
  • Photoresistors or Light Dependent Resistors (LDR) which change resistance according to light intensity. They are cheaper than photodiodes, but they are fairly slow, are not very sensitive, and exhibit a strongly nonlinear response.
  • Metal–semiconductor–metal (MSM) Photodetectors contain two Schottky contacts instead of a p–n junction. They are potentially faster than photodiodes, with bandwidths up to hundreds of gigahertz.
  • Photomultipliers are based on vacuum tubes. They can exhibit the combination of an extremely high sensitivity (even for photon counting) with a high speed. However, they are expensive, bulky, and need a high operating voltage.
  • Pyroelectric Photodetectors exploit a pyroelectric voltage pulse generated in a nonlinear crystal (e.g. LiTaO3) when heated by absorption of a light pulse on an absorbing coating on the crystal. They are often used for measurement of microjoule pulse energies from Q-switched lasers.
  • Cryogenic Detectors are sufficiently sensitive to measure the energy of single x-ray, visible and near infra-red photons.
  • Thermal Photodetectors – Optical detectors that are effectively thermometers, responding purely to the heating effect of the incoming radiation, such as pyroelectric detectors, Golay cells, thermocouples and thermistors. Thermal photodetectors (powermeters) measure a temperature rise caused by the absorption of light.
  • Charge-coupled Devices (CCD), which are used as image sensors to record images in astronomy, digital photography, and digital cinematography.
  • LED Photodetectors – Reverse-biased LED to act as photodiodes.