Photodetector

Photosensors or photodetectors are sensors of light or other electromagnetic energy.^[1]

Types

• Active-pixel sensors (APSs) are image sensors. Usually made in a CMOS process, and also known as CMOS image sensors, APSs are commonly used in cell phone cameras, web cameras, and some DSLRs.
• Bolometers measure the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. A microbolometer is a specific type of bolometer used as a detector in a thermal camera.
• Cadmium zinc telluride radiation detectors can operate in direct-conversion (or photoconductive) mode at room temperature, unlike some other materials (particularly germanium) which require liquid nitrogen cooling. Their relative advantages include high sensitivity for x-rays and gamma-rays, due to the high atomic numbers of Cd and Te, and better energy resolution than scintillator detectors.
• Charge-coupled devices (CCD), which are used to record images in astronomy, digital photography, and digital cinematography. Before the 1990s, photographic plates were most common in astronomy. The next generation of astronomical instruments, such as the Astro-E2, include cryogenic detectors.
• In experimental particle physics, a particle detector is a device used to track and identify elementary particles.
• Chemical detectors, such as photographic plates, in which a silver halide molecule is split into an atom of metallic silver and a halogen atom. The photographic developer causes adjacent molecules to split similarly.
• Cryogenic detectors are sufficiently sensitive to measure the energy of single x-ray, visible and infrared photons.^[2]
• Gaseous ionization detectors are detectors that can detect photons and particles with sufficient energy to ionize a gas atom or molecule. Electrons and ions generated by ionisation cause a current flow which can be measured.
• HgCdTe infrared detectors. Detection occurs when an infrared photon of sufficient energy kicks an electron from the valence band to the conduction band. Such an electron is collected by a suitable external readout integrated circuits (ROIC) and transformed into an electric signal.
• LEDs which are reverse-biased to act as photodiodes. See LEDs as Photodiode Light Sensors.
• Optical detectors, which are mostly quantum devices in which an individual photon produces a discrete effect.
• Optical detectors that are effectively thermometers, responding purely to the heating effect of the incoming radiation, such as bolometers, pyroelectric detectors, Golay cells, thermocouples and thermistors, but the latter two are much less sensitive.
• Photoresistors or Light Dependent Resistors (LDR) which change resistance according to light intensity. Normally the resistance of LDRs decreases with increasing intensity of light falling on it.^[3]
• Photovoltaic cells or solar cells which produce a voltage and supply an electric current when illuminated.
• Photodiodes which can operate in photovoltaic mode or photoconductive mode.
• Photomultiplier tubes containing a photocathode which emits electrons when illuminated, the electrons are then amplified by a chain of dynodes.
• Phototubes containing a photocathode which emits electrons when illuminated, such that the tube conducts a current proportional to the light intensity.
• Phototransistors, which act like amplifying photodiodes.
• Quantum dot photoconductors or photodiodes, which can handle wavelengths in the visible and infrared spectral regions.
• Semiconductor detectors are employed in gamma and X-ray spectrometry and as particle detectors.
• Silicon drift detectors (SDDs) are X-ray radiation detectors used in x-ray spectrometry (EDS) and electron microscopy (EDX).

Frequency range

In 2014 a technique for extending semiconductor-based photodetector's frequency range to longer, lower-energy wavelengths. Adding a light source to the device effectively "primed" the detector so that in the presence of long wavelengths, it fired on wavelengths that otherwise lacked the energy to do so.^[4]

See also

• List of sensors
• Optoelectronics
• Photosensitivity

References

1. Haugan, H. J.; Elhamri, S.; Szmulowicz, F.; Ullrich, B.; Brown, G. J.; Mitchel, W. C. (2008). "Study of residual background carriers in midinfrared InAs∕GaSb superlattices for uncooled detector operation". Applied Physics Letters 92 (7): 071102. Bibcode:2008ApPhL..92g1102H. doi:10.1063/1.2884264.
2. Enss, Christian (Editor) (2005). Cryogenic Particle Detection. Springer, Topics in applied physics 99. ISBN 3-540-20113-0.
3. "Photo Detector Circuit". oscience.info.
4. Claycombe, Ann (2014-04-14). "Research finds "tunable" semiconductors will allow better detectors, solar cells". Rdmag.com. Retrieved 2014-08-24.

External links

• Fundamentals of Photonics: Module on Optical Detectors and Human Vision