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Three Si and one Ge (top) photodiodes.
Working principleConverts light into current
Pin configurationanode and cathode
Electronic symbol
Photodiode symbol.svg

Photodiodes are similar to regular semiconductor diodes 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 specially as a photodiode use a PIN junction rather than a p–n junction, to increase the speed of response. A photodiode is designed to operate in reverse bias.[1]


Principle of operationEdit


To first order, for a given spectral distribution, the photocurrent is linearly proportional to the irradiance.[3]

Photovoltaic modeEdit

When used in zero bias or photovoltaic mode, photocurrent flows out of the anode through a short circuit to the cathode. If the circuit is opened or has a load impedance, restricting the photocurrent out of the device, a voltage builds up in the direction that forward biases the diode, that is, anode positive with respect to cathode. If the circuit is open or the impedance is high, a forward current will consume all or some of the photocurrent. This mode exploits the photovoltaic effect, which is the basis for solar cells – a traditional solar cell is just a large area photodiode. For optimum power output, the photovoltaic cell will be operated at a voltage that causes only a small forward current compared to the photocurrent.[3]

Photoconductive modeEdit

In this mode the diode is 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 and increases the region with an electric field that will cause electrons to be quickly collected. The reverse bias also reduces the dark current without much change in the photocurrent.

Although this mode is faster, the photoconductive mode can exhibit more electronic noise due to dark current or avalanche effects.[4] 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.

Other modes of operationEdit

Avalanche photodiodes are photodiodes with structure optimized for operating with high reverse bias, approaching the reverse breakdown voltage. 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.

Electronic symbol for a phototransistor

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 junction. It was invented by Dr. John N. Shive (more famous for his wave machine) at Bell Labs in 1948,[5]:205 but it was not announced until 1950.[6] 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 hfe). If the base and collector leads are used and 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] 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.

A Solaristor is a two-terminal gate-less phototransistor. A compact class of two-terminal phototransistors or solaristors have been demonstrated in 2018 by ICN2 researchers. The novel concept is a two-in-one power source plus transistor device that runs on solar energy by exploiting a memresistive effect in the flow of photogenerated carriers.[7]



Materials commonly used to produce photodiodes include:[8]

Material Electromagnetic spectrum
wavelength range (nm)
Silicon 190–1100
Germanium 400–1700
Indium gallium arsenide 800–2600
Lead(II) sulfide <1000–3500
Mercury cadmium telluride 400–14000


Unwanted photodiode effectsEdit

Any p–n junction, if illuminated, is potentially a photodiode. Semiconductor devices such as diodes, 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;[9][10] 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), diodes, transistors and ICs can malfunction[11] due to induced photo-currents. Background radiation from the packaging is also significant.[12] Radiation hardening mitigates these effects.

In some cases, the effect is actually wanted, for example to use LEDs as light-sensitive devices (see LED as light sensor) or even for energy harvesting, then sometimes called light-emitting and -absorbing diodes (LEADs).[13]


Response of a silicon photo diode vs wavelength of the incident light

Critical performance parameters of a photodiode include:

Dark current
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 current. The finite duration of this current is known as the transit-time spread and can be evaluated by using Ramo's theorem. One can also show with this theorem that the total charge generated in the external circuit is e and not 2e as one might expect 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  . 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.
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 ( ) is the inverse of NEP, 1/NEP. There is also the specific detectivity ( ) which is the detectivity multiplied by the square root of the area ( ) of the photodetector, ( ) 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.[14] Although it is traditional to give ( ) in many catalogues as a measure of the diode's quality, in practice, it is hardly ever the key parameter.




Photodiodes are used in consumer electronics devices such as compact disc players, smoke detectors, medical devices[15] and the receivers for infrared remote control devices used to control equipment from televisions 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.

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 couple two digital or analog circuits while maintaining extremely high electrical isolation between them, often for safety (optocoupler). The combination of LED and photodiode is also used in many sensor systems to characterize different types of products based on their optical absorbance.



PIN diodes are much faster and more sensitive than p–n junction diodes, and hence are often used for optical communications and in lighting regulation.



Comparison with photomultipliersEdit

Advantages compared to photomultipliers:[17]

  3. Low noise
  4. Ruggedized to mechanical stress
  5. Low cost
  6. Compact and light weight
  7. Long lifetime
  8. High quantum efficiency, typically 60–80% [18]
  9. No high voltage required

Disadvantages compared to photomultipliers:

  1. Small area
  2. No internal gain (except avalanche photodiodes, but their gain is typically 102–103 compared to 105-108 for the photomultiplier)
  3. Much lower overall sensitivity
  5. Response time for many designs is slower
  6. latent effect

Photodiode arrayEdit


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.[19] 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 CMOS or CCD sensor.

See alsoEdit



  1. ^ Cox, James F. (2001). Fundamentals of linear electronics: integrated and discrete. Cengage Learning. pp. 91–. ISBN 978-0-7668-3018-9.
  2. ^ Tavernier, Filip and Steyaert, Michiel (2011) High-Speed Optical Receivers with Integrated Photodiode in Nanoscale CMOS. Springer. ISBN 1-4419-9924-8. Chapter 3 From Light to Electric Current – The Photodiode
  3. ^ a b Häberlin, Heinrich (2012). Photovoltaics: System Design and Practice. John Wiley & Sons. pp. SA3–PA11–14. ISBN 9781119978381. Retrieved 19 April 2019.
  4. ^ "Photodiode Application Notes – Excelitas – see note 4" (PDF).
  5. ^ Riordan, Michael; Hoddeson, Lillian (1998). Crystal Fire: The Invention of the Transistor and the Birth of the Information Age. ISBN 9780393318517.
  6. ^ "The phototransistor". Bell Laboratories RECORD. May 1950.
  7. ^ Pérez-Tomás, Amador; Lima, Anderson; Billon, Quentin; Shirley, Ian; Catalan, Gustau; Lira-Cantú, Mónica (2018). "A Solar Transistor and Photoferroelectric Memory". Advanced Functional Materials. 28 (17): 1707099. doi:10.1002/adfm.201707099. ISSN 1616-3028.
  9. ^ Shanfield, Z. et al (1988) Investigation of radiation effects on semiconductor devices and integrated circuits, DNA-TR-88-221
  10. ^ Iniewski, Krzysztof (ed.) (2010), Radiation Effects in Semiconductors, CRC Press, ISBN 978-1-4398-2694-2
  11. ^ Zeller, H.R. (1995). "Cosmic ray induced failures in high power semiconductor devices". Solid-State Electronics. 38 (12): 2041–2046. doi:10.1016/0038-1101(95)00082-5.
  12. ^ May, T.C.; Woods, M.H. (1979). "Alpha-particle-induced soft errors in dynamic memories". IEEE Transactions on Electron Devices. 26: 2–9. doi:10.1109/T-ED.1979.19370. Cited in Baumann, R. C. (2004). "Soft errors in commercial integrated circuits". International Journal of High Speed Electronics and Systems. 14 (2): 299–309. doi:10.1142/S0129156404002363. 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].
  13. ^ Erzberger, Arno (2016-06-21). "Halbleitertechnik Der LED fehlt der Doppelpfeil". Elektronik [de] (in German). Archived from the original on 2017-02-14. Retrieved 2017-02-14.
  14. ^ Brooker, Graham (2009) Introduction to Sensors for Ranging and Imaging, ScitTech Publishing. p. 87. ISBN 9781891121746
  15. ^ E. Aguilar Pelaez et al., "LED power reduction trade-offs for ambulatory pulse oximetry," 2007 29th Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Lyon, 2007, pp. 2296-2299. doi: 10.1109/IEMBS.2007.4352784, URL:
  16. ^ Difference between Buried Photodiode and Pinned Photodiode.
  17. ^ Photodiode Technical Guide on Hamamatsu website
  18. ^ Knoll, F.G. (2010). Radiation detection and measurement, 4th ed. Wiley, Hoboken, NJ. p. 298. ISBN 978-0-470-13148-0
  19. ^ Gao, Wei (2010). Precision Nanometrology: Sensors and Measuring Systems for Nanomanufacturing. Springer. pp. 15–16. ISBN 978-1-84996-253-7.

External linksEdit