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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 has a p–n junction that converts light photons into current. The junction is covered by an illumination window, usually having an anti-reflective coating. The absorbed photons make electron-hole pairs in the depletion region. Photodiodes and photo transistors are a few examples of photo detectors. Solar cells convert some of the light energy absorbed into electrical energy.


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

  • Photoemission or photoelectric effect: Photons cause electrons to transition from the conduction band of a material to free electrons in a vacuum or gas.
  • Photoelectric[dubious ]: 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.
  • Nonlinearity: The RF-output is limited by the nonlinearity of the photodetector[7]


Grouped by mechanism, photodetectors include the following devices:

Photoemission or photoelectric[edit]




  • 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.[12]
  • Pyroelectric detectors detect photons through the heat they generate and the subsequent voltage generated in pyroelectric materials.
  • Thermopiles detect electromagnetic radiation through heat, then genetating a voltage in thermocouples.
  • 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.[14]

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.[15]

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. ^ Hu, Yue (1 October 2014). "Modeling sources of nonlinearity in a simple pin photodetector". Journal of Lightwave Technology. 32 (20): 3710–3720. 
  8. ^ "Photo Detector Circuit". 
  9. ^ Paschotta, Dr. Rüdiger. "Encyclopedia of Laser Physics and Technology - photodetectors, photodiodes, phototransistors, pyroelectric photodetectors, array, powermeter, noise". Retrieved 2016-05-31. 
  10. ^ 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. 
  11. ^ "Silicon Drift Detectors" (PDF). Thermo Scientific. 
  12. ^ Enss, Christian (Editor) (2005). Cryogenic Particle Detection. Springer, Topics in applied physics 99. ISBN 3-540-20113-0. 
  13. ^ 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. 
  14. ^ 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. 
  15. ^ Claycombe, Ann (2014-04-14). "Research finds "tunable" semiconductors will allow better detectors, solar cells". Retrieved 2014-08-24. 

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