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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.
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