Dark count or Dark Current is one of the most important CCD specifications,

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Dark count or Dark Current is one of the most important CCD specifications,

along with resolution, quantum efficiency and noise figure.

Dark count or Dark Current is one of the most important CCD

specifications, along with resolution, quantum efficiency and noise

figure. Dark current causes an output to occur without the previous

introduction of an input. This is caused by the thermal generation and

then diffusion of charge. This Dark Current is generated at different

locations in the CCD, but the cause of this charge generation is

basically the same in all locations. This is because all cases are to

do with irregularities in the fundamental crystal structure of the

silicon present in CCDs. Examples of these irregularities include

metal impurities such as gold, copper or iron, and crystal defects

such as dislocations and stacking faults, all of which are known to be

thermal generation sites of charge carriers in silicon. THE BEST

EXAMPLE OF WHERE THIS DARK Current can be located is at the Silicon

Oxide to silicon interface of the MOS capacitor (which is the main

building block of a CCD) . This is due to the fact that this is where

the largest irregularity in the silicon crystal structure occurs.

However, it can also be located in the in the bulk of the silicon both

inside and outside the depletion layer, which is defined by the CCD

potential wells.

To give an example of the amount of Dark Current, which is induced A

Kodak KAF-0400 and KAF-1600 sensor is specified to have a Dark Current

of 50 electrons being induced per second at 25 degrees c. Taking a

one-minute exposure at this temperature would generate 3000 electrons

of thermally induced charge, which is calculated by 60secs times 50

electrons.

In theory the average Dark Current value can be subtracted from the

output value to provide the signal due to photoelectrons. However in

reality there are fluctuations in the Dark Current called Dark noise,

and this cannot be accounted for in the same way. Noise occurs in

different forms such as shot noise and trapping noise which both

appear in the CCD channel. It also occurs at the input and output

stages as KTC noise, but my main focus shall be on shot noise as it is

closely associated with Dark Current.

We know that the generation of Dark Current is a totally random

process, due to the fact that the generation sites are randomly

distributed and perhaps more importantly, the Dark Current generation

process is random as a function of time. The number of electrons

generated and then collected in a charge packet travelling through the

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