| MadSci Network: Physics |
Reply:
>How does a charge Coupled Device (ccd) work? I have been reasearching
>this for a long time and have learned a lot. I have even learned a
>little about potential wells, but I don't really understand what one
>is yet. Can anybody there help me? Please don't tell me that, "A
>CCD works when a photon hits it. The CCD then converts the photon
>into an electrical impulse, depending on how much energy the photon
>had." I know this already. Thank you for your help.
Well, you have the beginning of the story correct! Hopefully I can help you understand the end of it. As you accurately stated, a CCD is composed of an array of potential wells. When a photon strikes the CCD, it knocks an electron loose from the silicon lattice, which then bounces around and falls into one of these potential wells. (Think of a ball rolling around until it falls into a hole.) These potential wells are created by two effects in different directions. In one direction, there are channel stops which divide the CCD into strips of conducting (really semi-conducting) areas, and the electrons are constricted to bounce around inside these channels. Above the channels are gate electrodes. When the electrode is turned on, it creates the potential well you spoke of, and that's where the electrons get trapped.
One thing you said isn't quite right. All one photon can do is stimulate one electron. This is independent of how much energy it has (although it must be above a certain threshold determined by the particulars of the silicon lattice. What makes multiple electrons be stimulated is multiple photons, corresponding to the intensity of the incident light. This one-to-one correspondence is true up to very high intensities of light, when cascading effects start to happen, but we won't worry about those.
To continue the story, during the CCD exposure, photons are knocking electrons into these potential wells, where they sit for a long time. This makes CCDs powerful cameras--they can be exposed for a long, long time without saturating. When the exposure is over, the readout process begins.
This is really the sensitive part of a CCD. The reason is fairly clear. Remember the channels in one direction, separated by channel stops? The way a CCD is read out is to first read the first pixel in the channel (by counting how many electrons have been accumulated in that potential well), then all the channels are shifted up by one pixel, and the next row is read out. As you can imagine, these shifts must be very noiseless, or by the time the end rows are read out, they are extremely contaminated with noise. This is in fact one of the limitations on CCD technology, and CCD manufacturers of good-quality CCDs have to get their charge transfer efficiencies (CTE) up over 0.99995, where 1 means absolutely perfect. That means that only a few electrons in 100, 000 are lost in every shift along the channel!
For further reading, I suggest this link, which points to an introductory document maintained by Scientific Imaging Technologies: Intro to Scientific CCDs
-Greg Billock