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.