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

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