Photodetectors – Definition and Applications:

Photodetectors are the semiconductor devices that can detect optical signals through electronic processes. The extension of coherent and incoherent light sources into the far-infrared region on one hand and the ultraviolet (UV) region on the other has enhanced the requirement for high-speed, sensitive photodetectors.

A general photodetector has basically three processes:

  1. carrier generation by incident light,
  2. carrier transport and/or multiplication by whatever current-gain mechanism may be present, and
  3. interaction of current with external circuit to give the output signal.

Photodetectors play vital role in optical-fiber communication systems operated in the near-infrared region (0.8 to 1.6 μm). They demodulate optical signals (convert the optical variations into electrical variations), that are subsequently amplified and further processed.

For these applications the stringent requirements that are to be met by the photodetectors include high speed response, high sensitivity at operating wavelengths and minimum noise. The photodetector must also be compact in size, use small biasing voltages/current, and be reliable under operating conditions.

The best known devices of these types are the light dependent resistor (LDR), the photodiode and phototransistor.