Editing Photodiode (section)

===Passive-pixel image sensor===
The [[passive-pixel sensor]] (PPS) is a type of photodiode array. It was the precursor to the [[active-pixel sensor]] (APS).<ref name="Fossum2014">{{cite journal |last1=Fossum |first1=Eric R. |author1-link=Eric Fossum |last2=Hondongwa |first2=D. B. |title=A Review of the Pinned Photodiode for CCD and CMOS Image Sensors |journal=IEEE Journal of the Electron Devices Society |date=2014 |volume=2 |issue=3 |pages=33–43 |doi=10.1109/JEDS.2014.2306412 |doi-access=free }}</ref> A passive-pixel sensor consists of passive pixels which are read out without [[amplifier|amplification]], with each pixel consisting of a photodiode and a [[MOSFET]] switch.<ref name="Kozlowski">{{cite journal |last1=Kozlowski |first1=L. J. |last2=Luo |first2=J. |last3=Kleinhans |first3=W. E. |last4=Liu |first4=T. |editor-first1=Bedabrata |editor-first2=Terrence S. |editor-last1=Pain |editor-last2=Lomheim |title=Comparison of passive and active pixel schemes for CMOS visible imagers |journal=Infrared Readout Electronics IV |date=14 September 1998 |volume=3360 |pages=101–110 |doi=10.1117/12.584474 |url=https://www.researchgate.net/publication/268189518 |publisher=International Society for Optics and Photonics|bibcode=1998SPIE.3360..101K |s2cid=123351913 }}</ref> In a photodiode array, pixels contain a [[p–n junction]], integrated [[capacitor]], and MOSFETs as selection [[transistors]]. A photodiode array was proposed by G. Weckler in 1968, predating the CCD.<ref name=fossum93>{{cite book |last1=Fossum |first1=Eric R. |chapter=Active pixel sensors: Are CCDS dinosaurs? |author1-link=Eric Fossum |title=Charge-Coupled Devices and Solid State Optical Sensors III |journal=SPIE Proceedings Vol. 1900: Charge-Coupled Devices and Solid State Optical Sensors III |volume=1900 |date=12 July 1993 |doi=10.1117/12.148585 |publisher=International Society for Optics and Photonics |pages=2–14 |editor1-last=Blouke |editor1-first=Morley M.|citeseerx=10.1.1.408.6558 |bibcode=1993SPIE.1900....2F |s2cid=10556755 }}</ref> This was the basis for the PPS.<ref name="Fossum2014"/>

The [[noise (electronics)|noise]] of photodiode arrays is sometimes a limitation to performance. It was not possible to [[semiconductor device fabrication|fabricate]] active pixel sensors with a practical pixel size in the 1970s, due to limited [[microlithography]] technology at the time.<ref name=fossum93/>