Techniques of Observational Astronomy
AST3722C

CCD Basics

CCDs are the culmination of a search by astronomers to find a detector that had the 2-dimensional imaging capability of a photographic emulsion with the quantum efficiency and linearity of a photomultiplier.

The solution is to create an array of cells (called "pixels") in a silicon wafer, isolated by electric field potentials into rows and by channel stops (formed by deposited positive ions) into columns.

Photoelectrons are collected deep in the well beneath the positive potential set up by the gate electrode, isolated from the surface by the n-type silicon buried channel.

Conductive Poly Silicon gates are used to store and move the stored charge by "clocking" the potential on the gates. A single gate is first held to a more positive potential to provide a collecting well for the photoelectrons. An adjacent gate is taken to the same potential, spreading the charge into a shared well. The initial gate is then taken back to a less positive potential, leaving all the charge under the adjacent gate.

This process of clocking is applied to an entire row of pixels, moving it down the columns of the CCD. After one downward shift, the readout row of the CCD is then clocked in a similar fashion to move the charge packets, one by one, out of the CCD to the associated electronics.

Some excellent Tutorials on CCDs and related topics are available on the web

This page was last edited October 14, 2004 1:37 PM