Editing Photodiode (section)

==Applications==
P–n photodiodes are used in similar applications to other [[photodetector]]s, such as [[photoconductor]]s, [[charge-coupled device]]s (CCD), and [[photomultiplier]] tubes. They may be used to generate an output which is dependent upon the illumination (analog for measurement), or to change the state of circuitry (digital, either for control and switching or for digital signal processing).

Photodiodes are used in [[consumer electronics]] devices such as [[compact disc]] players, [[smoke detector]]s, medical devices<ref name=Aguilar>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: https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=4352784&isnumber=4352185</ref> and the receivers for infrared [[Remote control|remote control devices]] used to control equipment from [[Television set|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 [[Coupling (electronics)|couple]] two digital or analog circuits while maintaining extremely high electrical [[Galvanic isolation|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 [[absorbance|optical absorbance]].

Photodiodes are often used for accurate measurement of light intensity in science and industry. They generally have a more linear response than photoconductors.

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.

[[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.

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]].

===Comparison with photomultipliers===

Advantages compared to [[photomultiplier]]s:<ref>[http://sales.hamamatsu.com/assets/html/ssd/si-photodiode/index.htm Photodiode Technical Guide] {{Webarchive|url=https://web.archive.org/web/20070104122603/http://sales.hamamatsu.com/assets/html/ssd/si-photodiode/index.htm |date=2007-01-04 }} on Hamamatsu website</ref>

# Excellent linearity of output current as a function of incident light
# Spectral response from 190&nbsp;nm to 1100&nbsp;nm ([[silicon]]), longer [[wavelength]]s with other [[semiconductor materials]]
# Low noise
# Ruggedized to mechanical stress
# Low cost
# Compact and light weight
# Long lifetime
# High [[quantum efficiency]], typically 60–80%<ref>Knoll, F.G. (2010). ''Radiation detection and measurement'', 4th ed. Wiley, Hoboken, NJ. p. 298. {{ISBN|978-0-470-13148-0}}</ref>
# No high voltage required

Disadvantages compared to [[photomultiplier]]s:

# Small area
# No internal gain (except [[avalanche photodiode]]s, but their gain is typically 10<sup>2</sup>–10<sup>3</sup> compared to 10<sup>5</sup>-10<sup>8</sup> for the photomultiplier)
# Much lower overall sensitivity
# Photon counting only possible with specially designed, usually cooled photodiodes, with special electronic circuits
# Response time for many designs is slower
# Latent effect

===Pinned photodiode===
{{distinguish|PIN photodiode}}
The pinned photodiode (PPD) has a shallow implant (P+ or N+) in N-type or P-type diffusion layer, respectively, over a P-type or N-type (respectively) substrate layer, such that the intermediate diffusion layer can be fully depleted of majority carriers, like the base region of a [[bipolar junction transistor]].  The PPD (usually PNP) is used in [[CMOS]] [[active-pixel sensor]]s; a precursor NPNP triple junction variant with the MOS buffer capacitor and the back-light illumination scheme with complete charge transfer and no image lag was invented by Sony in 1975. This scheme was widely used in many applications of charge transfer devices.

Early charge-coupled device [[image sensor]]s suffered from [[shutter lag]]. This was largely explained  with the re-invention of the pinned photodiode.<ref name="Fossum2014"/> It was developed by [[Nobukazu Teranishi]], Hiromitsu Shiraki and Yasuo Ishihara at [[NEC]] in 1980.<ref name="Fossum2014"/><ref>{{US patent|4484210|U.S. Patent 4,484,210, which was a floating-surface type buried photodioe with the similar structure of the 1975 Philips invention. Solid-state imaging device having a reduced image lag}}</ref> Sony in 1975 recognized that lag can be eliminated if the signal carriers could be transferred from the photodiode to the CCD. This led to their invention of the pinned photodiode, a photodetector structure with low lag, low [[noise (electronics)|noise]], high [[quantum efficiency]] and low [[dark current (physics)|dark current]].<ref name="Fossum2014"/> It was first publicly reported by Teranishi and Ishihara with A. Kohono, E. Oda and K. Arai in 1982, with the addition of an anti-blooming structure.<ref name="Fossum2014"/><ref>{{cite book |last1=Teranishi |first1=Nobuzaku |author1-link=Nobukazu Teranishi |last2=Kohono |first2=A. |last3=Ishihara |first3=Yasuo |last4=Oda |first4=E. |last5=Arai |first5=K. |title=1982 International Electron Devices Meeting |chapter=No image lag photodiode structure in the interline CCD image sensor |date=December 1982 |pages=324–327 |doi=10.1109/IEDM.1982.190285|s2cid=44669969 }}</ref> The new photodetector structure invented by Sony in 1975, developed by NEC in 1982 by Kodak in 1984 was given the name "pinned photodiode" (PPD) by B.C. Burkey at Kodak in 1984. In 1987, the PPD began to be incorporated into most CCD sensors, becoming a fixture in [[consumer electronic]] [[video cameras]] and then [[digital still camera]]s.<ref name="Fossum2014"/>

A CMOS image sensor with a low-voltage-PPD technology was first fabricated in 1995 by a joint JPL and [[Kodak]] team. The CMOS sensor with PPD technology was further advanced and refined by R.M. Guidash in 1997, K. Yonemoto and H. Sumi in 2000, and I. Inoue in 2003. This led to CMOS sensors achieve imaging performance on par with CCD sensors, and later exceeding CCD sensors.