Editing Photodiode (section)

==Principle of operation==

A photodiode is a [[PIN diode|PIN structure]] or [[p–n junction]]. 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 is passed in the absence of light) and the photocurrent, so the dark current must be minimized to maximize the sensitivity of the device.<ref>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''</ref> Therefore, photodiodes operate most ideally in [[reverse bias]].

To first order, for a given spectral distribution, the photocurrent is linearly proportional to the [[irradiance]].<ref name=haberlin>{{cite book |last1=Häberlin |first1=Heinrich |title=Photovoltaics: System Design and Practice |date=2012 |publisher=John Wiley & Sons |isbn=9781119978381 |pages=SA3–PA11–14 |url=https://books.google.com/books?id=w8k3aXBnDP4C&q=photocurrent+proportional-to-irradiance&pg=SA3-PA11 |access-date=19 April 2019}}</ref>

===Photovoltaic mode===

[[File:Photodiode operation-en.svg|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.]]

In photovoltaic mode (zero [[bias (electrical engineering)|bias]]), photocurrent flows into 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 shorted or the impedance is low, a forward current will consume all or some of the photocurrent. This mode exploits the [[photovoltaic effect]], which is the basis for [[solar cell]]s – 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.<ref name=haberlin/>

===Photoconductive mode===

In photoconductive mode the diode is [[p–n junction#Reverse bias|reverse biased]], that is, 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 creates [[Dark current (physics)|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.<ref>{{cite web | url =http://www.pacer.co.uk/Assets/Pacer/User/Photodiodes.pdf | title =Photodiode Application Notes – Excelitas – see note 4 | access-date =2014-11-13 | archive-url =https://web.archive.org/web/20141113175955/http://www.pacer.co.uk/Assets/Pacer/User/Photodiodes.pdf | archive-date =2014-11-13 | url-status =dead }}</ref> The leakage current of a good PIN diode is so low (&lt;1 nA) that the [[Johnson–Nyquist noise]] of the load resistance in a typical circuit often dominates.