Up: Properties of CCD's
Thermal fluctuations in the CCD cause problems with our data readout
because photoelectrons may be introduced in the process when the
temperature is high enough so that a few electrons may be kicked off
of the valence band within the silicon semiconductor and is collected
in the pixel well. Since we cannot distinguish between these
electrons and photoelectrons generated from incoming photons, these
thermally generated electrons will be read out in the data as an
increased intensity at that pixel.
The dark current is actually a function of both temperature and
exposure time. If the temperature is greater, the thermal energy is
greater and there is a greater chance that an electron will be kicked
off and collected in the pixel well. Furthermore, if the exposure time is
increased, there is a greater amount of time for the thermal energy to
kick off more electrons.