The Charged Coupled Device

It was just a few years back when an average Joe used to buy tonnes of rolls of films from the nearest store to replenish the stomach of his ever hungry camera so that he could at least capture his most priced moments. The next session comprised of dissecting the camera to retrieve the film and even after such a hassle, our average Joe ended up with two to five slots in the film roll badly developed due to inappropriate synchronization of camera functions.

But everything has changed now. Recent developments in the imaging and photo sensing industry have made the camera more efficient and functional than it was any time before. State of the art technology and relentless research has led to the evolution of amazing products starting from the meagerly sized cameras in spy-bots and mobile phones to the gigantically sized cameras incorporated into modern telescopes. With the digitization of cameras and electromagnetic radiation sensors, optical gadgets have become fast, sensitive and accurate to the pin point. Now our average Joe is happier than ever. He now knows that whatever he snaps is perfectly stored in his camera with superfluous clarity. Almost all the overwhelming features of a modern camera are the result of a minuscule yet stupendously important invention called the Charged Coupled Device (CCD).

The design of the first CCDs was probably made by William Boyle and George E. Smith in 1969 at the famous AT&T Bell Laboratories. The essence of their design was the ability to transfer charge along the surface of a semiconductor. Although initially it was made as a memory device, but later it turned out to be a good imaging device owing to its sensitivity to light. In January 2006, Boyle and Smith were awarded the National Academy of Engineering Charles Stark Draper Prize for their work on CCD.

A CCD for UV Detection

Before understanding the structure of a CCD it is important to know how semiconductors work in the microscopic level. A semiconductor is a material made up of atoms with some electrons which are more tightly held than those in metals, but less tightly held than those in non metals. Raising the temperature of a semiconductor causes the electrons to shake loose due to increased internal energy and thus the conductivity of a semiconductor increases with increase in temperature.

Another way of increasing the conductivity of a semiconductor (like Silicon) is to add minute controlled amounts of impurities. If, for example, elements which have three valence electrons like Aluminum or Indium are added to Silicon, as in figure A, the gap or ‘hole’ is filled by an electron(to make the outer shell structure of Aluminum stable) which might be moving in the silicon lattice. However, the electrons move from atom to atom and an electron which moves into the ‘hole’ must leave a ‘hole’ in the atom it has come from. Therefore the positive ‘hole’ appears to move through the material in the opposite direction of the electron movement. The addition of the impurity increases mobility of charge carriers in the semiconductor, thereby increasing conductivity. This is what a “P-TYPE” semiconductor is:


“N-TYPE” semiconductors contain impurities like Phosphorus and Arsenic which have five valence electrons. These atoms form covalent bonds with four surrounding atoms of silicon. One of the valence electrons of the impurity atom is held very loosely by the protons and detaches very easily by thermal agitation. These electrons from several atoms of the impurity contribute to increased conductivity of the semiconductor as a whole.


When a “N-TYPE” and a “P-TYPE” semiconductor are joined, a barrier potential is developed at the junction preventing majority of the electrons in the “N-TYPE” semiconductor from diffusing into the “P-TYPE” semiconductor.(Further explanations of the mechanisms involved with barrier potential in joined semiconductors is out of the scope of this writing).

In contemporary cameras (like modern camcorders or digital still cameras), the film is replaced by the CCD. The CCD is essentially an array of optical detectors (or a type of microchip) that forms a photographic image using digital processing. It consists of a silicon wafer (the major portion of which is composed of the N -TYPE silicon mounted on the P-TYPE silicon) divided into an array of small regions called picture elements, more commonly known as pixels. Each pixel consists of three small electrodes separated from the N-TYPE semiconductor by a very thin layer of Silicon Dioxide insulator. Initially the potential of the center electrode of each pixel is maintained at +10V and the other two are maintained at +2V. Incident photons of light from the object being photographed create electron hole pairs in the N-TYPE semiconductor by photoelectric phenomenon. So the image focused on the chip becomes an identical pattern of electrons in the semiconductor. These electrons are attracted by the +10V electrode and are held against the layer of silicone dioxide under the electrode.

Each electrode is connected to one of the three voltage terminals which provide repeated +10V pulses in three phases. The effect is to shift the charge gathered under one electrode to the next electrode and then to the next one until they arrive at the output electrode. Thus the output electrode produces a stream of pulses.

A Detailed Structure of a CCD

The main advantage of the CCDs is that, CCDs are very much efficient. Photographic film uses less than 4% of the photons reaching it. CCDs can make use of about 70% of the incident photons due to greater sensitivity of pixels than photographic chemicals/grains. (Photoelectric phenomenon is instantaneous whereas chemical reaction is relatively time consuming). This explains the extensive use of CCDs in astronomical telescopes where sensitivity is a prime issue.

An Array of CCDs In a Digital Sky Survey Telescope

Being more sensitive means that CCDs require much less exposure time than photographic films. But photographic films do have the advantage that light sensitive grains are smaller than CCD pixels and so they can produce images with better resolution.

The CCD is a more or less new invention with developments on it still at their infancy. The potentials of the device are far reaching and I believe we are yet to unleash the real power the CCD has to offer. Ongoing research on the CCD technology is likely to make it more useful, efficient and widely available in the near future.
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By Mahmud Hasan
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The Aftermath Publications, Issue 1
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