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A photodetector salvaged from a CD-ROM. The photodetector contains three photodiodes, visible in the photo (in center).

Photosensors or photodetectors are sensors of light or other electromagnetic energy.[1] A photo detector converts light signals that hit the junction into voltage or current. The connection uses an illumination window with an anti-reflect coating to absorb the light photons. This results in creation of electron-hole pairs in the depletion region. Photodiodes and photo transistors are few examples of photo detectors. Solar cells are also similar to photo detectors as they absorb light and turn it into energy.


Photodetectors may be classified by their mechanism for detection:[2][3][4]

  • Photoemission: Photons cause electrons to transition from the conduction band of a material to free electrons in a vacuum or gas.
  • Photoelectric: Photons cause electrons to transition from the valence band to the conduction band of a semiconductor.
  • Photovoltaic: Photons cause a voltage to develop across a depletion region of a photovoltaic cell.
  • Thermal: Photons cause electrons to transition to mid-gap states then decay back to lower bands, inducing phonon generation and thus heat.
  • Polarization: Photons induce changes in polarization states of suitable materials, which may lead to change in index of refraction or other polarization effects.
  • Photochemical: Photons induce a chemical change in a material.
  • Weak interaction effects: photons induce secondary effects such as in photon drag[5][6] detectors or gas pressure changes in Golay cells.

Photodetectors may be used in different configurations. Single sensors may detect overall light levels. A 1-D array of photodetectors, as in a spectrophotometer or a Line scanner, may be used to measure the distribution of light along a line. A 2-D array of photodetectors may be used as an image sensor to form images from the pattern of light before it.


There are a number of performance metrics, also called figures of merit, by which photodetectors are characterized and compared[2][3]

  • Spectral response: The response of a photodetector as a function of photon frequency.
  • Quantum efficiency: The number of carriers (electrons or holes) generated per photon.
  • Responsivity: The output current divided by total light power falling upon the photodetector.
  • Noise-equivalent power: The amount of light power needed to generate a signal comparable in size to the noise of the device.
  • Detectivity: The square root of the detector area divided by the noise equivalent power.
  • Gain: The output current of a photodetector divided by the current directly produced by the photons incident on the detectors, i.e., the built-in current gain.
  • Dark current: The current flowing through a photodetector even in the absence of light.
  • Response time: The time needed for a photodetector to go from 10% to 90% of final output.
  • Noise spectrum: The intrinsic noise voltage or current as a function of frequency. This can be represented in the form of a noise spectral density.


Grouped by mechanism, photodetectors include the following devices:





  • 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.
  • Cryogenic detectors are sufficiently sensitive to measure the energy of single x-ray, visible and infrared photons.[11]
  • Pyroelectric detectors detect photons through the heat they generate and the subsequent voltage generated in pyroelectric materials.
  • Golay cells detect photons by the heat they generate in a gas-filled chamber, causing the gas to expand and deform a flexible membrane whose deflection is measured.



Graphene/silicon photodetectors[edit]

A graphene/n-type silicon heterojunction has been demonstrated to exhibit strong rectifying behavior and high photoresponsivity. Graphene is coupled with silicon quantum dots (Si QDs) on top of bulk Si to form a hybrid photodetector. Si QDs cause an increase of the built-in potential of the graphene/Si Schottky junction while reducing the optical reflection of the photodetector. Both the electrical and optical contributions of Si QDs enable a superior performance of the photodetector..[13]

Frequency range[edit]

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.[14]

See also[edit]


  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. ^ a b Donati, S. "Photodetectors" (PDF). Prentice Hall. Retrieved 1 June 2016. 
  3. ^ a b Yotter, R.A.; Wilson, D.M. (June 2003). "A review of photodetectors for sensing light-emitting reporters in biological systems". IEEE Sensors Journal. 3 (3): 288–303. doi:10.1109/JSEN.2003.814651. 
  4. ^ Stöckmann, F. (May 1975). "Photodetectors, their performance and their limitations". Applied Physics. 7 (1): 1–5. doi:10.1007/BF00900511. 
  5. ^ A. Grinberg, Anatoly; Luryi, Serge (1 July 1988). "Theory of the photon-drag effect in a two-dimensional electron gas". Physical Review B. 38 (1): 87–96. doi:10.1103/PhysRevB.38.87. 
  6. ^ Bishop, P.; Gibson, A.; Kimmitt, M. (October 1973). "The performance of photon-drag detectors at high laser intensities". IEEE Journal of Quantum Electronics. 9 (10): 1007–1011. doi:10.1109/JQE.1973.1077407. 
  7. ^ "Photo Detector Circuit". 
  8. ^ Paschotta, Dr. Rüdiger. "Encyclopedia of Laser Physics and Technology - photodetectors, photodiodes, phototransistors, pyroelectric photodetectors, array, powermeter, noise". Retrieved 2016-05-31. 
  9. ^ Rizzi, M.; D`Aloia, M.; Castagnolo, B. "Semiconductor Detectors and Principles of Radiation-matter Interaction". Journal of Applied Sciences. 10 (23): 3141–3155. doi:10.3923/jas.2010.3141.3155. 
  10. ^ "Silicon Drift Detectors" (PDF). Thermo Scientific. 
  11. ^ Enss, Christian (Editor) (2005). Cryogenic Particle Detection. Springer, Topics in applied physics 99. ISBN 3-540-20113-0. 
  12. ^ Yuan, Hongtao; Liu, Xiaoge; Afshinmanesh, Farzaneh; Li, Wei; Xu, Gang; Sun, Jie; Lian, Biao; Curto, Alberto G.; Ye, Guojun; Hikita, Yasuyuki; Shen, Zhixun; Zhang, Shou-Cheng; Chen, Xianhui; Brongersma, Mark; Hwang, Harold Y.; Cui, Yi (1 June 2015). "Polarization-sensitive broadband photodetector using a black phosphorus vertical p–n junction". Nature Nanotechnology. 10 (8): 707–713. doi:10.1038/nnano.2015.112. 
  13. ^ Yu, Ting; Wang, Feng; Xu, Yang; Ma, Lingling; Pi, Xiaodong; Yang, Deren (2016). "Graphene Coupled with Silicon Quantum Dots for High-Performance Bulk-Silicon-Based Schottky-Junction Photodetectors". Advanced Materials. doi:10.1002/adma.201506140. 
  14. ^ Claycombe, Ann (2014-04-14). "Research finds "tunable" semiconductors will allow better detectors, solar cells". Retrieved 2014-08-24. 

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