Types of Photodetectors

Photodetectors are devices used for the detection of light – in most cases of optical powers. As the requirements for applications vary considerably, there are many types of photodetectors which may be appropriate in a particular case:

• 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. Such devices can be very compact, fast, highly linear, and exhibit a high quantum efficiency (i.e., generate nearly one electron per incident photon) and a high dynamic range, provided that they are operated in combination with suitable electronics. A particularly sensitive type is that of avalanche photodiodes, which are sometimes used even for photon counting.
• 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.
• Phototransistors are similar to photodiodes, but exploit internal amplification of the photocurrent. They are less frequently used than photodiodes.

FWDM Photo Detector

• Photoresistors are also based on certain semiconductors, e.g. cadmium sulfide (CdS). They are cheaper than photodiodes, but they are fairly slow, are not very sensitive, and exhibit a strongly nonlinear response.
• 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.
• Thermal detectors (power meters) measure a temperature rise caused by the absorption of light. Such detectors can be very robust and be used for the measurement of very high laser powers, but exhibit a low sensitivity, moderate linearity, and relatively small dynamic range.
• Research is performed on novel photodetectors based on carbon nanotubes (CNT) and graphene, which can offer a very broad wavelength range and a very fast response. Ways for integrating such devices into optoelectronic chips are explored.

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