A charge-coupled device (CCD) is a two-dimensional grid of semiconductor capacitors that can transfer charge between each other. While the term is widely used for image sensors, it actually describes the process by which the charges are transferred through the capacitors towards the grid edge.

CCD History

The concept of digital photography was first developed in 1961 at the Jet Propulsion Laboratory, but it wasn't until 1969 that the first CCD was invented at Bell Labs. Commercial CCDs started appearing during the early 1970s, and one of the first uses was in camcorders.

How are CCDs Made

CCDs are semiconductors made using photolithography techniques, similar to those used to make transistors and integrated circuits. Chemical layers are deposited on a silicone wafer and then etched away to build up the channels and gates that form the capacitors and diodes. Many layers are required to make one CCD, and many CCDs are made on one wafer at the same time, which are later separated and mounted in cases.

How CCDs Work

Light photons create a small charge each time they strike a capacitor. The more photons that strike a capacitor, the more charge that builds up in it, so that the total charge is proportional to the light intensity at that point. This does not measure color, only intensity, so color CCDs require multiple sensors, or one sensor and a color filtering mask.

When the exposure period is finished, the process of transferring the charges off the grid begins. The CCD is essentially an analogue shift register. In one edge row, the charge in each capacitor is shifted along until it reaches the corner capacitor, where the voltage is measured and transferred to memory by an analogue-to-digital converter. This shifting process is controlled by a gate driver and a clock signal. Once all the capacitor voltages in the row have been measured, the charges in other rows are shifted towards the edge row. Millions of capacitors are sampled in a fraction of a second by this method.

CCD Applications

Their main use is in photography, where CCD image sensors are found in virtually all digital cameras and scanners. Astronomers were early adopters of CCDs because they are up to one hundred times more sensitive than photographic film. However, they need to be cooled to temperatures well below zero to reduce thermally generated charges that cause errors. CCDs are also used in other fields such as electron microscopy, spectroscopy, and fluroscopy. They can also be found in night vision equipment.