These electrons are generated even when there is no light on a pixel. Thermally generated electrons collected in this manner are indistinguishable from photoelectrons. ![]() Thermally generated electrons: Dark CurrentĮlectrons may also be thermally excited into the conduction band within the substrate, and if this event occurs near or within the depletion region, the electrons created in this process may also be collected in the depletion region along with the photoelectrons. A single frame or a temporal sequence of such frames at a suitably high frame-rate can be displayed on a monitor to show a two dimensional image and/or to show movement or other changes in the field of view. Since the number of photoelectrons is proportional to the number of photons that were incident on the pixel, the matrix of values representing the photoelectrons in each pixel forms a one-dimensional or two-dimensional image frame. Image sensors form an image by collecting and measuring the number of photoelectrons generated in the depletion region of a pixel, the smallest spatial unit on the imaging surface of an image sensor.Īn image sensor may be a one-dimensional or two-dimensional array, constituting a matrix of pixels. Such an electric field is set up in the imager using gates at differing voltages so as to make a potential well a.k.a depletion region. Photoelectrons that are generated by the Photoelectric Effect (described above) can be collected by establishing an electric field within the substrate of the imager. ![]() Hondongwa, Dartmouth College Image Formation Ref: A Review of the Pinned Photodiode for CCD and CMOS Image Sensors” by Eric R. A cross section schematic view of a pixel, showing the absorption of photons, and the resulting accumulation of Photoelectrons in the Depletion Region. Holes are repelled away from the depletion region, thus preventing their recombination with the photoelectrons. Photoelectrons are collected in the static depletion area within each pixel. This produces a positively charged region in each pixel which attracts photoelectrons while repelling holes. These electrons are referred to as photoelectrons since they are generated by photons.ĭepletion regions are created in a pixel by selectively doping silicon with trace amounts of materials that cause a deficiency of electrons. This makes the electron mobile, leaving behind a “hole”, a term used to describe an atom that lacks an electron. ![]() When an incident photon strikes a Silicon atom, an electron in the outer valence band of the Silicon atom may absorb the energy of the photon and transition to a higher energy level. All image sensors rely on the Photoelectric Effect, which describes the interaction between light (photons) and materials (atoms).
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