Editing Photodiode (section)

==Features==
[[File:Response silicon photodiode.svg|thumb|upright=1.2|Response of a silicon photo diode vs wavelength of the incident light]]
Critical performance parameters of a photodiode include spectral responsivity, dark current, response time and noise-equivalent power.

; [[Responsivity|Spectral responsivity]]: The 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 which is a unitless quantity.
; [[Dark current (physics)|Dark current]]: The dark current is 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.
; [[Response time (technology)|Response time]]: The response time is the time required for the detector to respond to an optical input. 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 [[Elementary charge|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]] (<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.
; [[Noise-equivalent power]]: Noise-equivalent power (NEP) is the minimum input optical power to generate photocurrent, equal to the rms noise current in a 1&nbsp;[[hertz]] bandwidth.  NEP is essentially the minimum detectable power. The related ''characteristic detectivity'' (<math>D</math>) is the inverse of NEP (1/NEP) and the ''[[specific detectivity]]'' (<math>D^\star</math>) 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&nbsp;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>Brooker, Graham (2009) ''Introduction to Sensors for Ranging and Imaging'', ScitTech Publishing. p. 87. {{ISBN|9781891121746}}</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.

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]]''.