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Charge-Coupled Devices, CCDs

These exist in fax machines and scanners ( one dimensional arrays ) and many digital cameras ( 2- D arrays ) although they are being challenged in household cameras by other related n-p detectors called Active Pixel Sensors. They are used in astronomical telescopes and are rapidly replacing film!  The device was invented in 1969.

There are now many types of them, they rely on the photoelectric effect - photons moving electrons in semiconductors - for their operation.

CCDs are dense arrays of photodiodes, n-p junctions sensitive to light, built also as capacitors and combined with electron transfer systems. A single CCD for a camera can consist of millions of these on a chip of about a couple of square centimetres. ( In 2000, 2048x2048 - 4 Mpixels, were quite standard.)

The capacitor effect is managed by charging electrodes placed above the n-type semiconductor, positive. The region of the n-type immediately adjacent to the transition ( diffusion or depletion zone ) is slightly positive as discussed in the earlier pages on the p-n junction. It tends to act as an electron trap, an effect which is enhanced considerably by charging the region near it positive using an electrode. Altering the charge on the  electrodes can move the accumulated electrons from one region to another.

Electron-hole pairs are created in the p-type material by the photoelectric effect when electrons are promoted from the valence band to the conduction band by the photon. The electrons are then attracted to the electron trap under the electrode and accumulate. Holes disperse and eventually are neutralised.

The capacitors are linked  ( coupled ) so the accumulated charge can be transferred from one to another sequentially using control voltages to the electrodes.

The individual pixel outputs are moved along the array out of the image area.

In the animation below, each pixel consists of 3 electrodes and end stops at right angles. When being exposed, the middle electrode of each pixel is charged positive.

The output from each region is measured, "counted", stored on another capacitor and amplified. The result is a measure of the grayscale light falling on the pixel. The total can, in turn then translated back to an image.

To get colour, red, green and blue filters are used. Unlike film which can only detect a few percent of the incident photons, CCDs are about 70% efficient - some 70% of photons are detected! This makes them very efficient in low light situations. Astronomers love them. They are, however, "slow" in their translating the light signals to electrical signals - definitely not liked by photographers!

Factors affecting the efficiency include