Charge Coupled Device (CCD)

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Physical description of a CCD

The CCD is a Silicon plate organized to form a matrix of photosensitive elements in which to accumulate charges (produced by the incident radiation) and read the contents later.

The structure of a CCD basic element can be schematized as seen in fgure below. It is an element called M.O.S. (Metal, Oxide, Silicon). By applying a potential to the electrode, this particular structure allows to create a region where accumulate charges generated by photoelectric effect.

 

 

 CCD main functions
By schematizing, four fundamental moments can be identified:

1. charge generation by photoelectric effect;
2. charge collection through potential well generation;
3. charge transfer, by varying the electrode potentials appropriately;
4. charge extraction by meansof the output circuit.

Typical sructure of a three phase CCD
The figure on the right shows the scheme of a three phase CCD. In this case the pixel is made by 3 MOS elements. Along the vertical columns the separation is obtained with the stop channels (dashed lines).

In this scheme 4 fundamental areas can be identified:

1. The image area, sensitive to radiation and in which charges accumulate.
2. The storage area, cwhich is covered so as not to be sensitive to radiation, in which the charges generated in the image area are moved to allow a reading free from further radiation contamination (in practice the use of the shutter is eliminated).
3. The read-ou register, where an entire row of pixels is transferred, and where the transfer of each pixel to the output circuit begins. Also in this register the pixel is made with 3 MOS elements.
4. The output circuit that allows the charge extraction.

Lessons on CCDs and other photon detectors are available in PDF (Italian language)

Characteristics of some CCDs used in astronomical instruments