Photodetector

The development of photodetectors began with the discovery of the photoelectric effect by Heinrich Hertz in 1887, later explained by Albert Einstein in 1905.{{cite book |last=Einstein |first=Albert |title=On a Heuristic Point of View Concerning the Production and Transformation of Light |publisher=Annalen der Physik |year=1905 |doi=10.1002/andp.19053220607}} Early photodetectors, such as selenium cells invented in the late 19th century, were used in light meters and telegraph systems.{{cite book |last=Smith |first=Willoughby |title=Selenium Cells |publisher=Ernest Benn Limited |year=1913}} The 1930s saw the invention of photomultiplier tubes, enabling the detection of faint light signals, which revolutionized fields like nuclear physics and astronomy. The mid-20th century brought semiconductor-based photodetectors, such as photodiodes and phototransistors, which transformed industries like telecommunications and computing.{{cite book |last=Donati |first=Silvano |title=Photodetectors: Devices, Circuits and Applications |publisher=Prentice Hall |year=2000 |isbn=978-0130203373}} Today, advancements continue with high-speed detectors and quantum technologies.

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

Image:USB-photodetector.png

Photodetectors may be classified by their mechanism for detection:{{cite web|last1=Donati|first1=S.|title=Photodetectors|url=http://www-3.unipv.it/donati/private/Photodetectors/introd.pdf|website=unipv.it|publisher=Prentice Hall|access-date=1 June 2016}}{{Unreliable source?|date=March 2017}}{{cite journal|last1=Yotter|first1=R.A.|last2=Wilson|first2=D.M.|title=A review of photodetectors for sensing light-emitting reporters in biological systems|journal=IEEE Sensors Journal|date=June 2003|volume=3|issue=3|pages=288–303|doi=10.1109/JSEN.2003.814651|bibcode=2003ISenJ...3..288Y}}{{cite journal|last1=Stöckmann|first1=F.|title=Photodetectors, their performance and their limitations|journal=Applied Physics|date=May 1975|volume=7|issue=1|pages=1–5|doi=10.1007/BF00900511|bibcode=1975ApPhy...7....1S|s2cid=121425624 }}

• 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.{{Cite journal |last1=Singh |first1=Yogesh |last2=Kumar |first2=Manoj |last3=Yadav |first3=Reena |last4=Kumar |first4=Ashish |last5=Rani |first5=Sanju |last6=Shashi |last7=Singh |first7=Preetam |last8=Husale |first8=Sudhir |last9=Singh |first9=V. N. |date=2022-08-15 |title=Enhanced photoconductivity performance of microrod-based Sb2Se3 device |url=https://www.sciencedirect.com/science/article/pii/S0927024822001854 |journal=Solar Energy Materials and Solar Cells |language=en |volume=243 |pages=111765 |doi=10.1016/j.solmat.2022.111765 |issn=0927-0248|url-access=subscription }}

• 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{{cite journal|last1=A. Grinberg|first1=Anatoly|last2=Luryi|first2=Serge|title=Theory of the photon-drag effect in a two-dimensional electron gas|journal=Physical Review B|date=1 July 1988|volume=38|issue=1|pages=87–96|doi=10.1103/PhysRevB.38.87|pmid=9945167 |bibcode=1988PhRvB..38...87G}}{{cite journal|last1=Bishop|first1=P.|last2=Gibson|first2=A.|last3=Kimmitt|first3=M.|title=The performance of photon-drag detectors at high laser intensities|journal=IEEE Journal of Quantum Electronics|date=October 1973|volume=9|issue=10|pages=1007–1011|doi=10.1109/JQE.1973.1077407|bibcode=1973IJQE....9.1007B}} 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, 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). Various methods are employed together to improve its characteristics, such as manipulating the vertical structure, etching, changing the substrate, and utilizing plasmonics.{{Cite journal |last1=Singh |first1=Yogesh |last2=Parmar |first2=Rahul |last3=Srivastava |first3=Avritti |last4=Yadav |first4=Reena |last5=Kumar |first5=Kapil |last6=Rani |first6=Sanju |last7=Shashi |last8=Srivastava |first8=Sanjay K. |last9=Husale |first9=Sudhir |last10=Sharma |first10=Mahesh |last11=Kushvaha |first11=Sunil Singh |last12=Singh |first12=Vidya Nand |date=2023-06-16 |title=Highly Responsive Near-Infrared Si/Sb 2 Se 3 Photodetector via Surface Engineering of Silicon |url=https://pubs.acs.org/doi/10.1021/acsami.3c04043 |journal=ACS Applied Materials & Interfaces |volume=15 |issue=25 |pages=30443–30454 |language=en |doi=10.1021/acsami.3c04043 |issn=1944-8244|url-access=subscription }} 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.{{Citation |last1=Stillman |first1=G. E. |title=Chapter 5 Avalanche Photodiodes**This work was sponsored by the Defense Advanced Research Projects Agency and by the Department of the Air Force. |date=1977-01-01 |url=https://www.sciencedirect.com/science/article/pii/S0080878408601507 |volume=12 |pages=291–393 |editor-last=Willardson |editor-first=R. K. |access-date=2023-05-11 |series=Semiconductors and Semimetals |publisher=Elsevier |language=en |last2=Wolfe |first2=C. M. |editor2-last=Beer |editor2-first=Albert C.}}
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

• 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{{cite journal|last1=Hu|first1=Yue|title=Modeling sources of nonlinearity in a simple pin photodetector|journal=Journal of Lightwave Technology|date=1 October 2014|volume=32|issue=20|pages=3710–3720|url=https://www.osapublishing.org/jlt/abstract.cfm?uri=jlt-32-20-3710|bibcode=2014JLwT...32.3710H|doi=10.1109/JLT.2014.2315740|citeseerx=10.1.1.670.2359|s2cid=9882873 }}
• Spectral response: The response of a photodetector as a function of photon frequency.

Grouped by mechanism, photodetectors include the following devices:

• Gaseous ionization detectors are used in experimental particle physics to detect photons and particles with sufficient energy to ionize gas atoms or molecules. Electrons and ions generated by ionization cause a current flow which can be measured.
• 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.
• Microchannel plate detectors use a porous glass substrate as a mechanism for multiplying electrons. They can be used in combination with a photocathode like the photomultiplier described above, with the porous glass substrate acting as a dynode stage

• Active-pixel sensors (APSs) are image sensors. Usually made in a complementary metal–oxide–semiconductor (CMOS) process, and also known as CMOS image sensors, APSs are commonly used in cell phone cameras, web cameras, and some DSLRs.
• 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) are image sensors 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.{{Update inline|reason=The Astro-E2 was launched in 2005, and shut down in 2015 after it had exceeded its intended mission time by 8 years, although the cryogenic X-Ray detector vented all Helium after 19 days. That's not exactly next generation|date=August 2023}}
• 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.
• 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.{{cite web|url=http://oscience.info/infos/photo-detector-circuit/|title=Photo Detector Circuit|work=oscience.info}}
• Photodiodes which can operate in photovoltaic mode or photoconductive mode.{{cite book|last1=Pearsall|first1=Thomas|title=Photonics Essentials, 2nd edition|publisher=McGraw-Hill|date=2010|url=https://www.mheducation.com/highered/product/photonics-essentials-second-edition-pearsall/9780071629355.html|isbn=978-0-07-162935-5|access-date=2021-02-24|archive-date=2021-08-17|archive-url=https://web.archive.org/web/20210817005021/https://www.mheducation.com/highered/product/photonics-essentials-second-edition-pearsall/9780071629355.html|url-status=dead}}{{Cite web|url=https://www.rp-photonics.com/photodetectors.html|title=Encyclopedia of Laser Physics and Technology - photodetectors, photodiodes, phototransistors, pyroelectric photodetectors, array, powermeter, noise|last=Paschotta|first=Dr. Rüdiger|website=www.rp-photonics.com|access-date=2016-05-31}} Photodiodes are often combined with low-noise analog electronics to convert the photocurrent into a voltage that can be digitized.{{cite web| url = https://www.thorlabs.com/drawings/f6d76d5893edbf38-CD161A84-96F3-D89C-CC3AA1878E7976E1/PDA10A-Manual.pdf| title = PDA10A(-EC) Si Amplified Fixed Gain Detector User Manual| access-date = 24 April 2018| publisher = Thorlabs}}{{cite web| url = https://resolvedinstruments.com/DPD80-760nm-photodetector-datasheet| title = DPD80 760nm Datasheet| access-date = 24 April 2018| publisher = Resolved Instruments}}
• Phototransistors, which act like amplifying photodiodes.
• Pinned photodiodes, a photodetector structure with low lag, low noise, high quantum efficiency, and low dark current, widely used in most CCD and CMOS image sensors.{{cite journal |last1=Fossum |first1=E. R. |last2=Hondongwa |first2=D. B. |title=A Review of the Pinned Photodiode for CCD and CMOS Image Sensors |journal=IEEE Journal of the Electron Devices Society |date=2014 |volume=2 |issue=3 |pages=33–43 |doi=10.1109/JEDS.2014.2306412 |doi-access=free }}
• 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.{{Citation needed|date=December 2019|reason=removed citation to predatory publisher content}}
• Silicon drift detectors (SDDs) are X-ray radiation detectors used in x-ray spectrometry (EDS) and electron microscopy (EDX).{{Cite web|url=https://tools.thermofisher.com/content/sfs/brochures/TN52342_E_0512M_SiliconDrift_H.pdf|title=Silicon Drift Detectors|website=tools.thermofisher.com|publisher=Thermo Scientific}}

• Photovoltaic cells or solar cells which produce a voltage and supply an electric current when sunlight or certain kinds of light shines on them.

• 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.{{cite book|editor= Enss, Christian|title=Cryogenic Particle Detection|publisher=Springer, Topics in applied physics 99|year=2005|isbn=978-3-540-20113-7}}
• 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.

• Photoreceptor cells in the retina detect light through, for instance, a rhodopsin photon-induced chemical cascade.
• 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.

• The photorefractive effect is used in holographic data storage.
• Polarization-sensitive photodetectors use optically anisotropic materials to detect photons of a desired linear polarization.{{cite journal|last1=Yuan|first1=Hongtao|last2=Liu|first2=Xiaoge|last3=Afshinmanesh|first3=Farzaneh|last4=Li|first4=Wei|last5=Xu|first5=Gang|last6=Sun|first6=Jie|last7=Lian|first7=Biao|last8=Curto|first8=Alberto G.|last9=Ye|first9=Guojun|last10=Hikita|first10=Yasuyuki|last11=Shen|first11=Zhixun|last12=Zhang|first12=Shou-Cheng|last13=Chen|first13=Xianhui|last14=Brongersma|first14=Mark|last15=Hwang|first15=Harold Y.|last16=Cui|first16=Yi|title=Polarization-sensitive broadband photodetector using a black phosphorus vertical p–n junction|journal=Nature Nanotechnology|date=1 June 2015|volume=10|issue=8|pages=707–713|doi=10.1038/nnano.2015.112|pmid=26030655|bibcode=2015NatNa..10..707Y|arxiv=1409.4729}}

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.{{cite journal|last1= Yu|first1=Ting|last2=Wang|first2=Feng|last3=Xu|first3=Yang|last4=Ma|first4=Lingling|last5=Pi|first5=Xiaodong|last6=Yang|first6=Deren|title=Graphene Coupled with Silicon Quantum Dots for High-Performance Bulk-Silicon-Based Schottky-Junction Photodetectors|journal=Advanced Materials|volume=28|issue=24|pages=4912–4919|date=2016|doi=10.1002/adma.201506140|pmid=27061073|s2cid=205267070 }}

Photodetectors are integral to numerous fields:

• Consumer electronics: CCD and CMOS sensors in cameras, optical storage devices.
• Telecommunications: Fiber optic communication for high-speed data transmission.
• Scientific research: Spectroscopy, particle detection, and astronomy.
• Industrial automation: Barcode scanners, quality control systems.
• Medical devices: Pulse oximeters, endoscopes.
• Environmental monitoring: Air and water quality sensors, weather stations.

Emerging applications include autonomous vehicles and quantum computing.{{cite journal |last=Hadfield |first=Robert H. |title=Single-photon detectors for optical quantum information applications |journal=Nature Photonics |volume=3 |issue=12 |pages=696–705 |year=2009 |doi=10.1038/nphoton.2009.230}}

Recent developments in photodetector technology include:

• High-speed detectors: For faster optical communication.
• Quantum photodetectors: For quantum computing and cryptography.
• Novel materials: Organic and perovskite detectors for flexible electronics.
• Integration with AI: For advanced image processing in autonomous systems.

Future research focuses on improving sensitivity, reducing noise, and expanding wavelength detection ranges.{{cite journal |last=Konstantatos |first=Gerasimos |title=Sensitive solution-processed visible-wavelength photodetectors |journal=Nature Photonics |volume=1 |issue=9 |pages=531–534 |year=2007 |doi=10.1038/nphoton.2007.147}}

• Lighting control system
• List of sensors
• Optoelectronics
• Photoelectric sensor
• Photosensitivity
• Readout integrated circuit
• Resonant-cavity-enhanced photo detector
• Photodetection

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