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| [[File:Fotodio.jpg|thumb|Three Si and one Ge (bottom) photodiodes]]
| | Ambulance Officer Weldon from Bolton, enjoys football, swagbucks hack and sailing. Keeps a travel blog and has plenty to write about after visiting Cathedral of St Peter.<br><br>Also visit my weblog: [http://xib.me/sjv5 swagbucks hack download] |
| [[Image:Photodiode symbol.svg|right|thumb|Symbol for photodiode.]]
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| [[File:Photodiode operation.png|thumb|350px|I-V characteristic of a photodiode. The linear [[Load line (electronics)|load lines]] represent the response of the external circuit: I=(Applied bias voltage-Diode voltage)/Total resistance. The points of intersection with the curves represent the actual current and voltage for a given bias, resistance and illumination.]]
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| A '''photodiode''' is a type of [[photodetector]] capable of converting [[light]] into either [[electric current|current]] or [[voltage]], depending upon the mode of operation.<ref>{{GoldBookRef|title=Photodiode|file=P04598}}</ref> If the anode and cathode leads of a photodiode are joined together by a wire, when in the dark, no current will flow. On the other hand, when in the light, current flows from the cathode to the anode.
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| Photodiodes may contain [[optical filter|optical filters]], built-in lenses, and may have large or small surface areas. Photodiodes usually have a slower response time as its surface area increases. The common, traditional [[solar cell]] used to generate electric [[solar power]] is a large area photodiode.
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| Photodiodes are similar to regular [[semiconductor]] [[diode]]s except that they may be either exposed (to detect [[vacuum UV]] or [[X-rays]]) or packaged with a window or [[optical fiber]] connection to allow light to reach the sensitive part of the device. Many diodes designed for use specifically as a photodiode use a [[PIN diode|PIN junction]] rather than a [[p-n junction]], to increase the speed of response. A photodiode is designed to operate in [[reverse bias]].<ref>{{cite book|author=James F. Cox|title=Fundamentals of linear electronics: integrated and discrete|url=http://books.google.com/books?id=FbezraN9tvEC&pg=PA91|accessdate=20 August 2011|date=26 June 2001|publisher=Cengage Learning|isbn=978-0-7668-3018-9|pages=91–}}</ref>
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| ==Principle of operation==
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| A photodiode is a [[p-n junction]] or [[PIN diode|PIN structure]]. When a [[photon]] of sufficient energy strikes the diode, it creates an [[electron]], [[electron hole|hole]] pair. This mechanism is also known as the inner [[photoelectric effect]]. If the absorption occurs in the junction's [[depletion region]], or one diffusion length away from it, these carriers are swept from the junction by the built-in electric field of the depletion region. Thus holes move toward the [[anode]], and electrons toward the [[cathode]], and a [[photocurrent]] is produced. The total current through the photodiode is the sum of the dark current (current that flows with or without light) and the photocurrent, so the dark current must be minimized to maximize the sensitivity of the device.<ref>Filip Tavernier, Michiel Steyaert ''High-Speed Optical Receivers with Integrated Photodiode in Nanoscale CMOS'' Springer, 2011 ISBN 1-4419-9924-8, Chapter 3 ''From Light to Electric Current - The Photodiode''</ref>
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| ===Photovoltaic mode===
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| When used in zero [[bias (electrical engineering)|bias]] or ''photovoltaic mode'', the flow of photocurrent out of the device is restricted and a voltage builds up. This mode exploits the [[photovoltaic effect]], which is the basis for [[solar cell]]s – a traditional solar cell is just a large area photodiode.
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| ===Photoconductive mode===
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| In this mode the diode is often [[p-n junction#Reverse bias|reverse biased]] (with the cathode driven positive with respect to the anode). This reduces the response time because the additional reverse bias increases the width of the depletion layer, which decreases the junction's [[capacitance]]. The reverse bias also increases the [[Dark current (physics)|dark current]] without much change in the photocurrent. For a given spectral distribution, the photocurrent is linearly proportional to the [[illuminance]] (and to the [[irradiance]]).<ref>{{cite web | url = http://hyperphysics.phy-astr.gsu.edu/hbase/Electronic/photdet.html| title = Photodiode slide}}</ref>
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| Although this mode is faster, the photoconductive mode tends to exhibit more electronic noise.{{Citation needed|date=January 2008}} The leakage current of a good PIN diode is so low (<1 nA) that the [[Johnson–Nyquist noise]] of the load resistance in a typical circuit often dominates.
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| ===Other modes of operation===
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| '''[[Avalanche photodiode]]s''' have a similar structure to regular photodiodes, but they are operated with much higher reverse bias. This allows each ''photo-generated'' carrier to be multiplied by [[avalanche breakdown]], resulting in internal gain within the photodiode, which increases the effective ''responsivity'' of the device.
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| [[File:PhototransistorSymbol.png|100px|thumb|Electronic symbol for a phototransistor]]
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| A '''phototransistor''' is a light-sensitive transistor. A common type of phototransistor, called a photobipolar transistor, is in essence a [[bipolar transistor]] encased in a transparent case so that [[light]] can reach the ''base-collector [[p-n junction|junction]]''. It was invented by Dr. [[John N. Shive]] (more famous for his [[Shive wave machine|wave machine]]) at Bell Labs in 1948,<ref name="crystal-fire">{{cite book
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| | title = Crystal Fire: The Invention of the Transistor and the Birth of the Information Age
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| | isbn = 9780393318517
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| | author = Michael Riordan and Lillian Hoddeson
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| }}</ref>{{rp|205}} but it wasn't announced until 1950.<ref>{{cite journal
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| | url = http://www.smecc.org/phototransistor.htm
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| | title = The phototransistor
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| | date = May 1950
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| | journal = Bell Laboratories RECORD
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| }}</ref> The electrons that are generated by photons in the base-collector junction are injected into the base, and this photodiode current is amplified by the transistor's current gain β (or h<sub>fe</sub>). If the emitter is left unconnected, the phototransistor becomes a photodiode. While phototransistors have a higher [[responsivity]] for light they are not able to detect low levels of light any better than photodiodes.{{Citation needed|date=May 2011}} Phototransistors also have significantly longer response times. Field-effect phototransistors, also known as photoFETs, are light-sensitive field-effect transistors. Unlike photobipolar transistors, photoFETs control drain-source current by creating a gate voltage.
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| ==Materials==
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| The material used to make a photodiode is critical to defining its properties, because only [[photon]]s with sufficient energy to excite [[electron]]s across the material's [[bandgap]] will produce significant photocurrents.
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| Materials commonly used to produce photodiodes include:<ref>Held. G, Introduction to Light Emitting Diode Technology and Applications, CRC Press, (Worldwide, 2008). Ch. 5 p. 116. ISBN 1-4200-7662-0</ref>
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| {| class="wikitable"
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| ! Material !! [[Electromagnetic spectrum]]<br />[[wavelength]] range (nm)
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| |-
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| | [[Silicon]] || 190–1100
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| |- wrtgrhrth
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| | [[Germanium]] || 400–1700
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| |-
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| | [[Indium gallium arsenide]] || 800–2600
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| |-
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| | [[Lead(II) sulfide]] || <1000–3500
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| |}
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| Because of their greater bandgap, silicon-based photodiodes generate less noise than germanium-based photodiodes.
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| ===Unwanted photodiode effects===
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| Any p-n junction, if illuminated, is potentially a photodiode. Semiconductor devices such as transistors and ICs contain p-n junctions, and will not function correctly if they are illuminated by unwanted electromagnetic radiation (light) of wavelength suitable to produce a photocurrent;<ref>Z. Shanfield et al, 1988, Investigation of radiation effects on semiconductor devices and integrated circuits, DNA-TR-88-221, www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA210165</ref><ref>Krzysztof Iniewski, ed, 2010, Radiation Effects in Semiconductors, CRC Press, ISBN 978-1-4398-2694-2</ref> this is avoided by encapsulating devices in opaque housings. If these housings are not completely opaque to high-energy radiation (ultraviolet, X-rays, gamma rays), transistors and ICs can malfunction<ref>H.R. Zeller, 1995, Cosmic ray induced failures in high power semiconductor devices, www.sciencedirect.com/science/article/pii/0038110195000825</ref> due to induced photo-currents. Background radiation from the packaging is also significant.<ref>T. May and M. Woods, Alpha-particle-induced soft errors in dynamic memories", IEEE Trans. Elec. Dev., vol. 26, No. 1, pp 2, Jan 1979, cited in R. C. Baumann 2004, Soft errors in commercial integrated circuits, Int. J. High Speed Electronics and Systems, Vol. 14, No 2 (2004) 299-309: "alpha particles emitted from the natural radioactive decay of uranium, thorium, and aughter isotopes present as impurities in packaging materials were found to be the dominant cause of [soft error rate] in [dynamic random-access memories]."</ref> [[Radiation hardening]] mitigates these effects.
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| ==Features==
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| [[File:Response silicon photodiode.svg|thumb|upright=1.2|Response of a silicon photo diode vs wavelength of the incident light]]
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| Critical performance parameters of a photodiode include:
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| ;[[Responsivity]]: The [[responsivity|Spectral responsivity]] is a ratio of the generated photocurrent to incident light power, expressed in [[Ampere|A]]/[[Watt|W]] when used in photoconductive mode. The wavelength-dependence may also be expressed as a ''[[Quantum efficiency]]'', or the ratio of the number of photogenerated carriers to incident photons, a unitless quantity.
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| ;[[Dark current (physics)|Dark current]]: The current through the photodiode in the absence of light, when it is operated in photoconductive mode. The dark current includes photocurrent generated by background radiation and the saturation current of the semiconductor junction. Dark current must be accounted for by [[calibration]] if a photodiode is used to make an accurate optical power measurement, and it is also a source of [[Electronic noise|noise]] when a photodiode is used in an optical communication system.
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| ;[[Response time (technology)|Response time]]: A photon absorbed by the semiconducting material will generate an electron-hole pair which will in turn start moving in the material under the effect of the electric field and thus generate a [[Electric current|current]]. The finite duration of this current is known as the transit-time spread and can be evaluated by using [[Shockley–Ramo theorem|Ramo's theorem]]. One can also show with this theorem that the total charge generated in the external circuit is well [[Elementary charge|e]] and not 2e as might seem by the presence of the two carriers. Indeed the [[integral]] of the current due to both electron and hole over time must be equal to e. The resistance and capacitance of the photodiode and the external circuitry give rise to another response time known as [[RC time constant]] <math>\tau=RC</math>. This combination of R and C integrates the photoresponse over time and thus lengthens the [[impulse response]] of the photodiode. When used in an optical communication system, the response time determines the bandwidth available for signal modulation and thus data transmission.
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| ;[[Noise-equivalent power]]: (NEP) The minimum input optical power to generate photocurrent, equal to the rms noise current in a 1 [[hertz]] bandwidth. NEP is essentially the minimum detectable power. The related characteristic detectivity (<math>D</math>) is the inverse of NEP, 1/NEP. There is also the [[specific detectivity]] (<math>D^\star</math>) which is the detectivity multiplied by the square root of the area (<math>A</math>) of the photodetector, (<math>D^\star=D\sqrt{A}</math>) for a 1 Hz bandwidth. The specific detectivity allows different systems to be compared independent of sensor area and system bandwidth; a higher detectivity value indicates a low-noise device or system.<ref>Graham Brooker, ''Introduction to Sensors for Ranging and Imaging'', ScitTech Publishing, 2009 ISBN 9781891121746 page 87</ref> Although it is traditional to give (<math>D^\star</math>) in many catalogues as a measure of the diode's quality, in practice, it is hardly ever the key parameter.
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| When a photodiode is used in an optical communication system, all these parameters contribute to the ''[[sensitivity (electronics)|sensitivity]]'' of the optical receiver, which is the minimum input power required for the receiver to achieve a specified ''[[bit error rate]]''.
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| ==Applications==
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| P-N photodiodes are used in similar applications to other [[photodetector]]s, such as [[photoconductor]]s, [[charge-coupled device]]s, and [[photomultiplier]] tubes. They may be used to generate an output which is dependent upon the illumination (analog; for measurement and the like), or to change the state of circuitry (digital; either for control and switching, or digital signal processing).
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| Photodiodes are used in [[consumer electronics]] devices such as [[compact disc]] players, [[smoke detector]]s, and the receivers for infrared [[Remote control|remote control devices]] used to control equipment from [[television]]s to air conditioners. For many applications either photodiodes or photoconductors may be used. Either type of photosensor may be used for light measurement, as in [[camera]] light meters, or to respond to light levels, as in switching on street lighting after dark.
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| Photosensors of all types may be used to respond to incident light, or to a source of light which is part of the same circuit or system. A photodiode is often combined into a single component with an emitter of light, usually a [[light-emitting diode]] (LED), either to detect the presence of a mechanical obstruction to the beam ([[slotted optical switch]]), or to [[Coupling (electronics)|couple]] two digital or analog circuits while maintaining extremely high [[electrical isolation]] between them, often for safety ([[optocoupler]]).
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| Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a more linear response than photoconductors.
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| They are also widely used in various medical applications, such as detectors for [[computed tomography]] (coupled with [[scintillator]]s), instruments to analyze samples ([[immunoassay]]), and [[pulse oximeter]]s.
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| [[PIN diode]]s are much faster and more sensitive than p-n junction diodes, and hence are often used for [[optical communication]]s and in lighting regulation.
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| P-N photodiodes are not used to measure extremely low light intensities. Instead, if high sensitivity is needed, [[avalanche photodiode]]s, [[intensified charge-coupled device]]s or [[photomultiplier]] tubes are used for applications such as [[astronomy]], [[spectroscopy]], [[night vision equipment]] and [[laser rangefinder|laser rangefinding]].
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| ===Comparison with photomultipliers===
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| Advantages compared to [[photomultiplier]]s:<ref>[http://sales.hamamatsu.com/assets/html/ssd/si-photodiode/index.htm Photodiode Technical Guide] on Hamamatsu website</ref>
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| # Excellent linearity of output current as a function of incident light
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| # Spectral response from 190 nm to 1100 nm ([[silicon]]), longer [[wavelength]]s with other [[semiconductor materials]]
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| # Low noise
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| # Ruggedized to mechanical stress
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| # Low cost
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| # Compact and light weight
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| # Long lifetime
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| # High [[quantum efficiency]], typically 60-80% <ref>Knoll, F.G. (2010). Radiation detection and measurement -4th ed. p. 298. Wiley, Hoboken, NJ. ISBN 978-0-470-13148-0</ref>
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| # No high voltage required
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| Disadvantages compared to [[photomultiplier]]s:
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| # Small area
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| # No internal gain (except [[avalanche photodiode]]s, but their gain is typically 10<sup>2</sup>–10<sup>3</sup> compared to up to 10<sup>8</sup> for the photomultiplier)
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| # Much lower overall sensitivity
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| # Photon counting only possible with specially designed, usually cooled photodiodes, with special electronic circuits
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| # Response time for many designs is slower
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| #latent effect
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| ==Photodiode array==
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| [[File:Photodiode array chip.jpg|thumb|150px|right|A 2 x 2 cm photodiode array chip with more than 200 diodes]]
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| A one-dimensional array of hundreds or thousands of photodiodes can be used as a [[position sensor]], for example as part of an [[angle sensor]].<ref>
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| {{cite book
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| | title = Precision Nanometrology: Sensors and Measuring Systems for Nanomanufacturing
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| | edition =
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| | author = Wei Gao
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| | publisher = Springer
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| | year = 2010
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| | isbn = 978-1-84996-253-7
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| | pages = 15–16
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| | url = http://books.google.com/books?id=N0ys_sSxD60C&pg=PA15
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| }}</ref>
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| One advantage of photodiode arrays (PDAs) is that they allow for high speed parallel read out since the driving electronics may not be built in like a traditional [[Active pixel sensor|CMOS]] or [[Charge-coupled device|CCD]] sensor.
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| ==See also==
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| {{colbegin|3}}
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| * [[Electronics]]
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| * [[Band gap]]
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| * [[Infrared]]
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| * [[Optoelectronics]]
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| * [[Optical interconnect]]
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| * [[Light Peak]]
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| * [[Interconnect bottleneck]]
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| * [[Optical fiber cable]]
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| * [[Optical communication]]
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| * [[Parallel optical interface]]
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| * [[Opto-isolator]]
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| * [[Semiconductor device]]
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| * [[Solar cell]]
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| * [[Avalanche photodiode]]
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| * [[Transducer]]
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| * [[LEDs as Photodiode Light Sensors]]
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| * [[Light meter]]
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| * [[Image sensor]]
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| * [[Transimpedance amplifier]]
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| {{colend}}
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| ==References==
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| {{FS1037C}}
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| {{Reflist}}
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| {{refbegin}}
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| *Gowar, John, ''Optical Communication Systems'', 2 ed., Prentice-Hall, Hempstead UK, 1993 (ISBN 0-13-638727-6)
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| {{refend}}
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| ==External links==
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| {{Commons category|Photodiodes}}
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| *[http://sales.hamamatsu.com/assets/html/ssd/si-photodiode/index.htm Technical Information Hamamatsu Photonics]
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| *[http://www.emant.com/324003.page Using the Photodiode to convert the PC to a Light Intensity Logger]
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| *[http://www.fairchildsemi.com/an/AN/AN-3005.pdf Design Fundamentals for Phototransistor Circuits]
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| *[http://ece-www.colorado.edu/~bart/book/book/chapter4/ch4_7.htm Working principles of photodiodes]
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| {{Electronic components}}
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| [[Category:Optical devices]]
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| [[Category:Optoelectronics]]
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| [[Category:Optical diodes]]
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| [[Category:Photonics]]
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