Photodetectors, also called photosensors, are sensors of light or other electromagnetic radiation.[1] There are a wide variety of photodetectors which may be classified by mechanism of detection, such as photoelectric or photochemical effects, or by various performance metrics, such as spectral response. Semiconductor-based photodetectors typically use a p–n junction that converts photons into charge. 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.

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



Photodetectors can be classified based on their mechanism of operation and device structure. Here are the common classifications:

Based on mechanism of operation

A commercial amplified photodetector for use in optics research

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

  • Photoconductive effect: These detectors work by changing their electrical conductivity when exposed to light. The incident light generates electron-hole pairs in the material, altering its conductivity. Photoconductive detectors are typically made of semiconductors.[5]
  • Photoemission or photoelectric effect: Photons cause electrons to transition from the conduction band of a material to free electrons in a vacuum or gas.
  • 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[6][7] 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.

A photodetector or array is typically covered by an illumination window, sometimes having an anti-reflective coating.

Based on device structure


Based on device structure, photodetectors can be classified into the following categories:

  1. MSM Photodetector: A metal-semiconductor-metal (MSM) photodetector consists of a semiconductor layer sandwiched between two metal electrodes. The metal electrodes are interdigitated, forming a series of alternating fingers or grids. The semiconductor layer is typically made of materials such as silicon (Si), gallium arsenide (GaAs), indium phosphide (InP) or antimony selenide (Sb2Se3).[5] Various methods are employed together to improve its characteristics, such as manipulating the vertical structure, etching, changing the substrate, and utilizing plasmonics.[8] The best achievable efficiency is shown by Antimony Selenide photodetectors.
  2. Photodiodes: Photodiodes are the most common type of photodetectors. They are semiconductor devices with a PN junction. Incident light generates electron-hole pairs in the depletion region of the junction, producing a photocurrent. Photodiodes can be further categorized into: a. PIN Photodiodes: These photodiodes have an additional intrinsic (I) region between the P and N regions, which extends the depletion region and improves the device's performance. b. Schottky Photodiodes: In Schottky photodiodes, a metal-semiconductor junction is used instead of a PN junction. They offer high-speed response and are commonly used in high-frequency applications.
  3. Avalanche Photodiodes (APDs): APDs are specialized photodiodes that incorporate avalanche multiplication. They have a high electric field region near the PN junction, which causes impact ionization and produces additional electron-hole pairs. This internal amplification improves the detection sensitivity. APDs are widely used in applications requiring high sensitivity, such as low-light imaging and long-distance optical communication.[9]
  4. Phototransistors: Phototransistors are transistors with a light-sensitive base region. Incident light causes a change in the base current, which controls the transistor's collector current. Phototransistors offer amplification and can be used in applications that require both detection and signal amplification.
  5. Charge-Coupled Devices (CCDs): CCDs are imaging sensors composed of an array of tiny capacitors. Incident light generates charge in the capacitors, which is sequentially read and processed to form an image. CCDs are commonly used in digital cameras and scientific imaging applications.
  6. CMOS Image Sensors (CIS): CMOS image sensors are based on complementary metal-oxide-semiconductor (CMOS) technology. They integrate photodetectors and signal processing circuitry on a single chip. CMOS image sensors have gained popularity due to their low power consumption, high integration, and compatibility with standard CMOS fabrication processes.
  7. Photomultiplier Tubes (PMTs): PMTs are vacuum tube-based photodetectors. They consist of a photocathode that emits electrons when illuminated, followed by a series of dynodes that multiply the electron current through secondary emission. PMTs offer high sensitivity and are used in applications that require low-light detection, such as particle physics experiments and scintillation detectors.

These are some of the common photodetectors based on device structure. Each type has its own characteristics, advantages, and applications in various fields, including imaging, communication, sensing, and scientific research.



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

  • 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[10]
  • Spectral response: The response of a photodetector as a function of photon frequency.



Grouped by mechanism, photodetectors include the following devices:

Photoemission or photoelectric







  • 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.[18]
  • Pyroelectric detectors detect photons through the heat they generate and the subsequent voltage generated in pyroelectric materials.
  • Thermopiles detect electromagnetic radiation through heat, then generating 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


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.[20]

See also



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  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. Bibcode:2003ISenJ...3..288Y. doi:10.1109/JSEN.2003.814651.
  4. ^ Stöckmann, F. (May 1975). "Photodetectors, their performance and their limitations". Applied Physics. 7 (1): 1–5. Bibcode:1975ApPhy...7....1S. doi:10.1007/BF00900511. S2CID 121425624.
  5. ^ a b Singh, Yogesh; Kumar, Manoj; Yadav, Reena; Kumar, Ashish; Rani, Sanju; Shashi; Singh, Preetam; Husale, Sudhir; Singh, V. N. (2022-08-15). "Enhanced photoconductivity performance of microrod-based Sb2Se3 device". Solar Energy Materials and Solar Cells. 243: 111765. doi:10.1016/j.solmat.2022.111765. ISSN 0927-0248.
  6. ^ 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. Bibcode:1988PhRvB..38...87G. doi:10.1103/PhysRevB.38.87. PMID 9945167.
  7. ^ 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. Bibcode:1973IJQE....9.1007B. doi:10.1109/JQE.1973.1077407.
  8. ^ Singh, Yogesh; Parmar, Rahul; Srivastava, Avritti; Yadav, Reena; Kumar, Kapil; Rani, Sanju; Shashi; Srivastava, Sanjay K.; Husale, Sudhir; Sharma, Mahesh; Kushvaha, Sunil Singh; Singh, Vidya Nand (2023-06-16). "Highly Responsive Near-Infrared Si/Sb 2 Se 3 Photodetector via Surface Engineering of Silicon". ACS Applied Materials & Interfaces. 15 (25): 30443–30454. doi:10.1021/acsami.3c04043. ISSN 1944-8244.
  9. ^ Stillman, G. E.; Wolfe, C. M. (1977-01-01), Willardson, R. K.; Beer, Albert C. (eds.), Chapter 5 Avalanche Photodiodes**This work was sponsored by the Defense Advanced Research Projects Agency and by the Department of the Air Force., Semiconductors and Semimetals, vol. 12, Elsevier, pp. 291–393, retrieved 2023-05-11
  10. ^ Hu, Yue (1 October 2014). "Modeling sources of nonlinearity in a simple pin photodetector". Journal of Lightwave Technology. 32 (20): 3710–3720. Bibcode:2014JLwT...32.3710H. CiteSeerX doi:10.1109/JLT.2014.2315740. S2CID 9882873.
  11. ^ "Photo Detector Circuit".
  12. ^ Pearsall, Thomas (2010). Photonics Essentials, 2nd edition. McGraw-Hill. ISBN 978-0-07-162935-5. Archived from the original on 2021-08-17. Retrieved 2021-02-24.
  13. ^ Paschotta, Dr. Rüdiger. "Encyclopedia of Laser Physics and Technology - photodetectors, photodiodes, phototransistors, pyroelectric photodetectors, array, powermeter, noise". Retrieved 2016-05-31.
  14. ^ "PDA10A(-EC) Si Amplified Fixed Gain Detector User Manual" (PDF). Thorlabs. Retrieved 24 April 2018.
  15. ^ "DPD80 760nm Datasheet". Resolved Instruments. Retrieved 24 April 2018.
  16. ^ Fossum, E. R.; Hondongwa, D. B. (2014). "A Review of the Pinned Photodiode for CCD and CMOS Image Sensors". IEEE Journal of the Electron Devices Society. 2 (3): 33–43. doi:10.1109/JEDS.2014.2306412.
  17. ^ "Silicon Drift Detectors" (PDF). Thermo Scientific.
  18. ^ Enss, Christian, ed. (2005). Cryogenic Particle Detection. Springer, Topics in applied physics 99. ISBN 978-3-540-20113-7.
  19. ^ 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. arXiv:1409.4729. Bibcode:2015NatNa..10..707Y. doi:10.1038/nnano.2015.112. PMID 26030655.
  20. ^ 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. 28 (24): 4912–4919. doi:10.1002/adma.201506140. PMID 27061073. S2CID 205267070.
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