Charge-Coupled Devices

Summary

Originally conceptualized as a form of optical volatile memory for computers, charge-coupled devices were also seen to be useful for capturing light and images. The ability to capture images that can be read by a computer in digital form was profoundly useful, and charge-coupled devices quickly became known for their imaging abilities. Besides their utility in producing digital images that could be analyzed and manipulated by computers, charge-coupled devices also proved to be more sensitive to light than film. As technology advanced, charge-coupled devices became less expensive and more capable, and eventually, they replaced film as the primary means of taking photographs.

Definition and Basic Principles

A charge-coupled device (CCD) is an electronic array of devices fabricated at the surface of a semiconductor chip. The typical operation of a CCD is to collect electrical charge in specific locations laid out as an array on the surface of the chip. This electrical charge is normally produced by light shining onto the chip, producing an electrical charge. Each collecting area is called a pixel and consists of an electrical potential well that traps charge carriers. The charge in one area of the array is read, and then the charges on other parts of the chip are shifted from potential well to potential well until all have been read. The brighter the light shining on a pixel, the more charge it will have. Thus, an image can be constructed from the data collected by each pixel. When the CCD is placed at the focal plane of an optical system, such as a lens, then the image on the chip will be the same as the image seen by the lens; thus CCD chips can be used as the heart of a camera system. Because the data is collected electronically in digital form for each pixel, the image from a CCD is inherently a digital image and can be processed by a computer. For this reason, CCDs (and digital cameras) have become more popular than film as a way to take photographs.

Background and History

The charge-coupled device was invented based on an idea by Willard S. Boyle and George E. Smith of AT&T Bell Laboratories for volatile computer memory. They were seeking to develop an electrical analog to magnetic bubble memory. Boyle and Smith postulated that electric charge could be used to store data in a matrix on a silicon chip. The charge could be moved from location to location within that array, shifting from one holding cell to another by applying appropriate voltages. The name “charge-coupled device” stemmed from this shifting of electrical charge around on the device. Initial research aimed to perfect the CCD as computer memory, but the inventors soon saw that it might be even more useful in converting light values into electrical charges that could be read. This property led to the development of the CCD as an imaging array. Subsequent developments made pixel sizes smaller, the arrays larger, and the price lower. The CCD became the detector at the heart of digital cameras, and their ease of use quickly made them popular. By the early 2000s, digital cameras and their CCDs had become more popular than film cameras.

How It Works

At the heart of a charge-coupled device is a semiconductor chip with electrical potential wells created by doping select areas in an array on the surface of the device. Doping of a semiconductor involves fabricating it with a select impurity that would tend to have either an extra electron or one too few electrons for the normal lattice structure of the semiconductor. Fabricating the CCD with select areas so doped would tend to make any electrical charge created at the surface of the semiconductor chip want to stay in the region of the potential well. By applying the proper electrical voltage between adjacent wells, however, charge is permitted to flow from one well to another. In this way, charge anywhere in the array can be moved from well to well to a location where it can be read.

The Photoelectric Effect. The key to using a charge-coupled device is to put charge on it. This is done by shining light onto the surface of the semiconductor chip. When photons of sufficient energy shine on a material, the absorbed light can knock electrons from atoms. This is known as the photoelectric effect, a physical phenomenon first observed by German physicist Heinrich Hertz in 1887 and explained by another German physicist, Albert Einstein, in 1905. While the photoelectric effect works in many materials, in a semiconductor it produces a free electron and a missing electron, called a hole. The hole is free to move just like an electron. Normally, the electron and hole recombine (the electron going back to the atom it came from in the semiconductor lattice) after the electron-hole pair are created. The key to using a charge-coupled device is to put a charge on it by shining light onto the surface of the semiconductor chip. When photons of sufficient energy shine on a material, the absorbed light can knock electrons from atoms. This is known as the photoelectric effect—a physical phenomenon first observed by German physicist Heinrich Hertz in 1887 and explained by physicist Albert Einstein in 1905. While the photoelectric effect works in many materials, in a semiconductor, it produces a free electron and a missing electron called a hole. The hole is free to move, just like an electron. Normally, the electron and hole recombine (the electron returning to the atom it came from in the semiconductor lattice) after the electron-hole pair is created. However, if an electric field is present, the electron may go one way in the semiconductor and the hole another. Such an electric field is present in an operating CCD, so the electrons will congregate in regions of lower electrical potential (electrical potential wells), building up an electrical charge proportional to the intensity of the light shining on the chip. The potential wells are arranged in an array across the surface of the CCD. The individual wells serve as the individual picture elements in a digital image and are called pixels. A nearly identical technology to the CCD uses different materials and is called the complementary metal oxide semiconductor (CMOS).

Shifting Charges. Once the charge is produced and captured on the CCD chip, it must be measured for the device to be of any use. Typically, the charge is read-only from one location on the CCD. After that charge is read, all the charges in all the pixels on a row are shifted to the adjacent pixel to allow the following charge to be read from the readout position. This process repeats until all charges in the row have been read. All columns of pixels are then shifted down one row, shifting the charges in the second row into the row that was just read. Reading that row continues until all charges are read and all the columns shift down again, repopulating the empty row of charges with new charges. All the charges are shifted until they have been read. If a pixel has too much charge (being overexposed), then the charge can bleed between adjacent pixels. This creates a spurious surplus charge on charges throughout a column and appears as vertical lines running through the image. This image defect is known as blooming.

Color Images. Charge-coupled devices respond to light intensity, not color. Therefore, all images are inherently black and white. There are several techniques to get color images from CCDs. The simplest and most cost-effective method is to take pictures using different color filters and then to create a final image by adding those images in the colors of the filters. This is the technique usually used in astronomy. The disadvantage of this technique, though, is that it requires at least three images taken in succession. This does not permit “live” or action photography. A separate technique is to have filters constructed over each pixel on the CCD array. This permits one chip to take multiple images simultaneously. The disadvantage of this technique, besides the cost of constructing such an array of filters, is that each image uses only about one-third of the pixels on the chip, thus losing image quality and detail. Most color digital cameras use this technique. Though other strategies exist for making color images with CCDs, these two are the most common.

Applications and Products

CCDs and CMOSs are the detectors in virtually every digital imaging system. CCDs have also displaced the imaging tubes in television cameras.

Cameras. Because CCDs produce digital images, they can be directly viewed using computers, sent by email, coded into websites, and stored in electronic media. Digital images can also be viewed almost immediately after taking them, unlike film, which must first be developed. This made digital cameras very popular. Additionally, CCDs can be manufactured quite small. This allowed cameras to be integrated into cell phones, computers, tablets, and other devices. The pixels in CCDs were originally larger than the grains in film, so digital image quality suffered. However, technological advances permitted CCD pixels to be made much smaller, and by 2010, most commercial CCD camera systems compared very favorably with film systems in terms of image quality, with high-end CCD cameras often performing better than most film cameras.

Scientific Applications. Some of the first applications of CCDs were in the scientific community. CCDs permit very accurate images, but these images also contain precise data about light intensity. In chemistry, spectral lines can be studied using CCD detectors. CCDs are also important in astronomy, where digital images can be studied and measured using computers. Astronomical observatories were among the first to adopt CCD imaging systems, and they remain leaders in working with companies to develop new, more powerful, and larger CCD arrays.

Satellite Images. Early satellite cameras used film that had to be dropped from orbit to be collected below. Soon, television cameras displaced film, but as soon as the first reliable charge-coupled devices capable of imaging became available, that technology was far better than any other. Since 2011, nearly all satellite imaging systems, both civilian and military, have relied on a type of CCD technology.

Careers and Course Work

Charge-coupled devices (CCDs) have become the most common imaging system. Thus, any career that uses images will come into contact with CCDs. For most people using CCDs, no different training or coursework is needed than would be needed to use any camera.

However, manufacturing CCDs or developing new CCD technology requires specialized training. CCDs are semiconductor devices, so their manufacture requires a background in semiconductor manufacturing technology. Two- and four-year degrees in semiconductor manufacturing technology exist, and these degrees require courses in electronics, semiconductor physics, semiconductor manufacturing technology, mathematics, and related disciplines. Manufacturing CCDs is not much different from manufacturing any other semiconductor device.

Construction of equipment, such as cameras, to hold CCDs involves optics and electronics. Manufacture of such equipment requires little detailed knowledge of these areas other than what is required for any other camera or electronic device. Most of the components are manufactured and ready to be assembled. However, specialized electronics knowledge is needed to design the circuit boards to operate the CCDs.

Developing improved CCD technology or researching better CCDs or technologies to replace CCDs requires much more advanced training, generally a postgraduate degree in physics or electrical engineering. These degrees require extensive physics and mathematics courses, along with electrical engineering coursework and courses in chemistry, materials science, electronics, and semiconductor technology. Most jobs related to research and development are in university or corporate laboratory settings.

Social Context and Future Prospects

CCDs were once very esoteric devices. Early digital cameras had CCDs with large pixels, producing pictures of inferior quality to even modest film cameras. However, CCDs quickly became less expensive and of higher quality and soon became ubiquitous. Most cameras sold by 2005 were digital cameras. Most cell phones now come equipped with cameras with CCD technology. Since most people own a cell phone, this puts a camera in the hands of almost everyone all the time. The number of photographs and videos being made has skyrocketed far beyond what has ever existed since the invention of the camera. Since these images are digital, they can easily be shared on the Internet and through social media.

Though charge-coupled devices are superior to film, other competing technologies that can do the same thing are being developed. A very similar technology (so similar that it is often grouped with CCD technology) is the complementary metal oxide semiconductor (CMOS). CMOS technology works the same as CCD technology for the end user, but the chip operation and manufacture details are different. However, as CCD technology becomes commonplace, other imaging technological developments will likely follow.

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