Editing Charge-coupled device (section)

=== Charge generation ===
Before the MOS capacitors are exposed to light, they are [[biasing|biased]] into the depletion region; in n-channel CCDs, the silicon under the bias gate is slightly ''p''-doped or intrinsic. The gate is then biased at a positive potential, above the threshold for strong inversion, which will eventually result in the creation of an ''n'' channel below the gate as in a [[MOSFET]]. However, it takes time to reach this thermal equilibrium: up to hours in high-end scientific cameras cooled at low temperature.<ref>For instance, the specsheet of PI/Acton's [http://www.princetoninstruments.com/Uploads/Princeton/Documents/Datasheets/Princeton_Instruments_SPEC-10_2K_eXcelon_rev_N3_9.22.2011.pdf SPEC-10 camera] specifies a dark current of 0.3 electron per pixel per hour at {{convert|-110|°C|°F|abbr=on}}.</ref> Initially after biasing, the holes are pushed far into the substrate, and no mobile electrons are at or near the surface; the CCD thus operates in a non-equilibrium state called deep depletion.<ref name=sze>{{cite book
 | last1 = Sze
 | first1 = S. M.
 | last2 = Ng
 | first2 = Kwok K.
 | author-link = Simon Sze
 | title = Physics of semiconductor devices
 | publisher = [[John Wiley and Sons]]
 | edition = 3
 | year = 2007
 | isbn = 978-0-471-14323-9
}} Chapter 13.6.</ref>
Then, when [[electron–hole pair]]s are generated in the depletion region, they are separated by the electric field, the electrons move toward the surface, and the holes move toward the substrate. Four pair-generation processes can be identified:
* photo-generation (up to 95% of [[quantum efficiency]]),
* generation in the depletion region,
* generation at the surface, and
* generation in the neutral bulk.

The last three processes are known as dark-current generation, and add noise to the image; they can limit the total usable integration time. The accumulation of electrons at or near the surface can proceed either until image integration is over and charge begins to be transferred, or thermal equilibrium is reached. In this case, the well is said to be full. The maximum capacity of each well is known as the well depth,<ref>{{Cite web |date=March 29, 2001 |title=Pixel Binning |url=http://www.ccd.com/ccd103.html |url-status=dead |archive-url=https://web.archive.org/web/20020605105409/http://www.ccd.com/ccd103.html |archive-date=Jun 5, 2002 |website=Apogee Instruments}}</ref> typically about 10<sup>5</sup> electrons per pixel.<ref name=sze /> CCDs are normally susceptible to ionizing radiation and energetic particles which causes noise in the output of the CCD, and this must be taken into consideration in satellites using CCDs.<ref>{{cite book |doi-access=free | doi=10.1117/12.2309026 | chapter=Radiation effects on image sensors | title=International Conference on Space Optics — ICSO 2012 | date=2017 | editor-last1=Cugny | editor-last2=Karafolas | editor-last3=Armandillo | editor-first1=Bruno | editor-first2=Nikos | editor-first3=Errico | last1=Auvergne | first1=Michel | last2=Ecoffet | first2=Robert | last3=Bardoux | first3=Alain | last4=Gilard | first4=Olivier | last5=Penquer | first5=Antoine | page=12 | isbn=978-1-5106-1617-2 |publisher=SPIE Digital Library }}</ref><ref>{{Cite web |last1=Marshall |first1=Cheryl J. |last2=Marshall |first2=Paul W. |date=6 October 2003 |title=CCD Radiation Effects and Test Issues for Satellite Designers (Review Draft 1.0) |url=https://radhome.gsfc.nasa.gov/radhome/papers/CCD_Lessons_Learned.pdf |url-status=live |archive-url=https://web.archive.org/web/20240214005433/https://radhome.gsfc.nasa.gov/radhome/papers/CCD_Lessons_Learned.pdf |archive-date=Feb 14, 2024 |website=NASA/GSFC Radiation Effects & Analysis}}</ref>