Reducing Dark Current
Cooling The CCD
Cooling the CCD reduces dark current to negligible levels, allowing exposure times of up to hours in duration. To achieve the highest possible sensitivity, astronomers cool the CCD with liquid nitrogen, eliminating the dark current produced by thermal generation at room temperature. High energy physicists, on the other hand, use CCDs in ultra high speed cameras to observe transient phenomena where dark current is not relevant.
MPP Operation
Multi-pinned-phase (MPP) or inverted operation reduces the rate of dark current generation by a factor of 20 or more and thus relaxes CCD cooling requirements to the level where a thermoelectric cooler is sufficient for most applications. Most of the dark current in a CCD is generated by interface states at the silicon-silicon dioxide interface just below the parallel gate structure. In MPP mode, this dark current component is significantly reduced by biasing all of the parallel register gates to the same voltage. However, this causes the potential wells essential for operation to disappear, allowing charge to spread up and down columns. Efficient CCD action can be ensured by processing CCDs with a built in potential step that restores the potential wells when the parallel gates are biased at the same voltage. Only CCDs thus processed can be operated in inverted mode.