Dark Current

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.