Issue 1, March 2001
How It Works: The Charged-Coupled Device, or CCD
Biology and Physics, Georgetown University
In this article,
written for a non-specialized audience, she explains what a CCD
is, how it works, and why it is so important.
allows us to see to the edges of the Universe, yet also lets us
explore and build objects smaller than a hundredth of the width
of a human hair? Here is another hint: it has revolutionized astronomy
in a little over two decades, and is utilized in many imaging devices.
Give up? The answer is the charge-coupled device, or the CCD.
This small, electrical device is familiar to astronomers, physicists
and engineers but now even some biologists and chemists are beginning
use CCDs in their research. You have likely encountered it before,
as CCDs are used in facsimile machines, photocopiers, bar-code readers,
closed-circuit television cameras, video cameras, regular photographic
cameras, or other sorts of sensitive light detectors. In fact, CCDs
have a wide range of applications - everything from reconnaissance
and aerial mapping to medicine, microtechnology and astronomy.6,7
What is a CCD?
of a CCD Chip
A CCD is an electrical device that is used to create images of objects,
store information (analogous to the way a computer stores information),
or transfer electrical charge (as part of larger device). It receives
as input light from an object or an electrical charge. The CCD takes
this optical or electronic input and converts it into an electronic
signal - the output. The electronic signal is then processed by some
other equipment and/or software to either produce an image or to give
the user valuable information.1,2
A CCD chip is a metal oxide semiconductor (MOS) device. This means
that its base, which is constructed of a material which is a good
conductor under certain conditions, is topped with a layer of a metal
oxide. In the case of the CCD, usually silicon is used as the base
material and silicon dioxide is used as the coating. The final, top
layer is also made of silicon - polysilicon.1,7
This silicon that forms the base and the top layer, however, is special
in nature. It is a silicon material that is doped with, or made to
contain, a small amount of some other material. Doping endows materials
with special properties that can be exploited through different electrical
To understand why doped silicon would have special properties and
how those properties can be exploited, consider how silicon normally
forms chemical bonds. Like carbon, a silicon atom can form up to four
bonds with adjacent atoms. This is because silicon has four valence
electrons that it can share t form bonds. In a crystal of pure silicon,
all atoms (not on the surface of the crystal) would be perfectly bonded
to four neighboring atoms; in this case, there are no extra electrons,
but also no places where electrons are missing. You can see this by
drawing the Lewis structures.
we introduce into the perfect crystal an element with only three electrons
available for bonding, this atom will form three normal bonds and
one bond with a "hole", meaning that it is missing an electron.
What is interesting here is that this "hole" actually can
move around the entire crystal. An electron nearby can move to fill
in the original hole, but in eliminating the original hole, it has
created a new hole. Effectively, this hole is able to move around
just a freely as a mobile electron. Such a material, one that contains
extra holes, is called a p-type material.
with extra electrons is called an n-type material. In a n-type material,
the "contaminating" element has five available electrons,
so it makes the four usual bonds, but then has an extra electron left
over. It is important to note that these materials are all neutral,
and that extra electrons or extra holes in this case do not make the
materials charged but merely come from what is left over or needed
for a neutral atom to form four bonds.
application of the right stimulus, the movement of the hole can be
directed. This is one of the fundamental keys to the operation of
the CCD. An electron is repelled by negative charge and is attracted
by positive charge. A hole, however, is repelled by the positive charge
and is attracted by negative charge. In this way, we can think of
a hole as sort of a "positive" electron, even though it
is not. As we can control the motion of electrons by applying different
electrical fields or charges in the vicinity, so can we control the
motion of holes.
If a p-type and an n-type material are brought into contact, a p-n
junction is formed and a very interesting result occurs. Extra electrons
from the n-type material will diffuse to the p-type material and fill
in some of the extra holes from the p-type material. The diffusion
and recombination of electron-hole pairs across the boundary directly
results in the n-type material becoming positively charged and the
p-type material becoming negatively charged. Recall that before the
two materials were brought into contact and before diffusion occurred,
they were both neutral. As diffusion occurs and the n-type and p-type
materials become increasingly charged, an electric field is generated
around the contact boundary. This electric field eventually slows
and stops the diffusion of charge across the boundary. When diffusion
stops, there are no more extra electrons or holes around the boundary;
they have all recombined. This region surrounding the boundary in
which electrons and holes have recombined is called the depletion
region. Outside of the depletion region, extra electrons still remain
in the n-type material and extra electrons remain in the p-type material.
The depletion region is the key area which can be used to create electrical
devices. By applying a voltage to the depletion region, we can either
increase or decrease the electric field across the depletion region.
If the electric field is increased by an applied voltage (reverse
bias), the depletion region is increased and less of any applied current
can flow through the two materials. If the electric field is decreased
by an applied voltage (forward bias), the depletion region is decreased
and more applied current is allowed to flow through the two materials.
The importance of
applying voltages to the depletion region (called biasing the p-n
junction) is that it precisely allows us to control applied current
through any p-n material. When the p-n junction is reversed-biased,
an only infinitesimal amount of applied current can flow, which for
all practical purposes is zero. This corresponds to the "off"
state. When the p-n junction is forward-biased, current easy flows
through the junction (because the smaller electric field does not
impede the flow of charges as much). In fact, by plotting a graph
of applied voltage versus current flow - an I-V Curve - we can see
that the dependence of current flow on applied voltage across the
junction is exponential. Forward-bias corresponds to the "on"
state. Thus, biasing of the junction through the application of voltages
can be used to precisely control the motion of electrical charge.
How does the CCD work?
In a CCD, the electrical
field at different parts of the surface is controlled by an array
or matrix of electrodes; these electrodes are called the gates.
(CCD arrays can be either one-dimensional or two-dimensional, but
here we will consider the one-dimensional array in detail, and then
apply that information to understand the two-dimensional array.)
This array of electrodes biases each small part of the surface differently,
which allows any flow of charge on the CCD to be controlled. 1,2
The surface of the CCD is further broken down into smaller regions
called pixels, or picture elements.8 This name is appropriate
because they represent a single "grain" of the imaged
object (just like you can see that your TV images appear to be made
up of tiny "grains"). The array of electrodes apply a
positive potential, (+Vg, a positive electric field)
to two-thirds of each pixel, thus forward-biasing that portion of
the pixel. Let's represent the first third of the pixel by (1, the
second third by (2, and the last third by (3. So, (1 and (3 are
at a positive potential of +Vg, and (2 is at a lower
When light or photons of high enough energy strike the surface,
electrons are usually liberated from the surface.2 (The
quantum efficiency or the ratio of electrons liberated per incident
photon is about 0.70-0.80.4) For every electron liberated,
a hole is created simply by the act of the electron leaving. Thus,
incident photons create electron-hole pairs.4,5 The hole,
being effectively positive, is repelled by the applied positive
potential (1 and (3, and eventually escapes into the base of the
chip.4 The electron, however, is captured in the nearest
potential well (2. The more light incident on a pixel, the more
electrons captured in the potential wells. Thus, differences in
the intensity of incoming light are "recorded" by the
number of electrons collected in each potential well.1,4,5
So now the challenge is to extract information from these "electron-collecting
bins" (which may also be thought of as tiny capacitors). To
do this, the charge packets (the collection of electrons in each
well) must be transferred to another device for data processing.
This is accomplished by sequentially changing the applied voltage
at the three parts of each pixel. First, the level of the potential
barrier (V3) closest to the data processing device is
lowered to the same potential as (2. This causes the electrons to
divide between the two wells. The primary mechanism for this electron
diffusion is induced self-drift from Coulomb repulsions, which acts
to separate the charge. Then, the potential of (2 is raised over
a finite time interval (corresponding to the diffusion rate of the
electrons from (2 to (3) so that (2 now becomes a potential barrier.
The remaining charge is transferred from (2 to (3 by these changes
in potential. (1 is maintained at constant potential during this
entire process to keep the charge packets separate from one another.
Now, each charge packet in the row has moved over one-third of a
pixel closer to the data processing device. This cycle is repeated
over and over in fractions of a second to transfer all the charge
off the chip to a detector which usually uses a load resistance
to measure the amount of charge collected in each "bin".
This is how the three-phase CCD works. 1,3,4,7 The term
charge-coupling in charged-coupled device comes from the coupling
of electrical potentials.4
A two-dimensional CCD is composed of channels, or rows along which
charge is transferred. Each channel is essentially a one-dimensional
CCD array. Charge is prevented from moving sideways by channel stops,
which are the narrow barriers between the closely spaced channels
of the CCD.4
is the CCD chip so great?
The answer to the
above question lies in its resolution.8 CCDs provide
extraordinary detail for objects either very far away or very small
in size - resolution which was hitherto impossible to attain.4
This resolution is a result of the large number of pixels in the
CCD array - the more pixels, the finer the detail that can be achieved.2,5
Typically, modern CCDs comprise anywhere from about one thousand
to about half a million pixels.8 As each pixel is a few
microns square, the active area of a two-dimensional CCD array is
usually a few millimeters by a few millimeters.1,3,4,6,8
How do CCDs record color or distinguish among photons of
In imagers or CCD
cameras, CCDs are only part of the whole device. A lens is required
to properly focus the incident radiation from the object onto the
array. For astronomical applications, the lens is an optical telescopic
lens. In addition, since the pixels themselves are monochrome, there
must be a way to select for the wavelengths of light incident on
the array. Colored filters are thus used to record colors in the
case of visible light. In most digital cameras, a color filter array
registers the intensity of a single color at each pixel. By interpolation,
algorithms use the color intensities at nearby pixels to estimate
the intensity of a color at each pixel. This is how a full-color
image is generated.5 A single picture made by a CCD imager
that is only 500 pixels by 500 pixels holds the same amount of raw
information as a 100,000 word book!4 In addition, the
number of electrons collected is proportional to the energy of the
incident photons. So mathematically, the energies of the photons
liberated can be calculated.
What if an electron is "randomly" liberated? (
i.e. Aren't There Sources of Noise?)
itself during two main processes: the collection of electrons and
the transfer of charge packets. During the collection of electrons,
noise stems from thermal processes, light pollution, and the generation
of electron-hole pairs in the depletion regions.1,4,7,8
During charge-transfer, noise stems from transfer inefficiency.7
During the collection of electrons, thermal noise or dark current
is one of the biggest sources of error. Over time, thermal processes
fill the depletion region with electrons, which masks stored information.1,7
To minimize the effects of thermal noise, CCDs are cooled to very
low temperatures, which are typically around 150 K.4
Other sources of error present during input include random noise
from electron-hole pair generation in the depletion region and light
from sources other than the desired one (light pollution).7,8
During charge transfer, efficiency is a major concern. Whenever
charge is transferred, a small amount is left behind.7
This "residue" blurs the image.5 Charge transfer
efficiency values greater than 99.9% are common, which translates
into only 10% of the original charge being lost after 100 transfers.7
Electron traps - which are most common at the surface - collect
charge when exposed to a large charge packet and also pose a problem
for efficient charge transfer. Over time, they slowly release the
charge that they have trapped during later cycles of smaller charge
packets. This smears the image. This problem, however, can be minimized
by "fat zero". In fat zero, anywhere between 10-25% of
the total capacity of the well is filled with electrons before data
is even collected. This reduces trapping to tolerable levels. Alternatively,
the buried channel CCD structure - a different construction from
the surface channel CCD (the one so far described) - can reduces
the occurrence of electron traps, since in this design, charge is
not exposed to surface-state traps.7
What are some applications of the CCD?
CCDs are used in
a variety of different imaging devices. Linear imaging devices (LIDs)
employing CCDs are typically used in facsimile machines, photocopiers,
mail sorters, and bar code readers, in addition to being used for
aerial mapping and reconnaissance. Area imaging devices (AIDs) employing
CCDs are used for closed-circuit television cameras, video cameras,
and vision systems for robotics and as film for astronomical observations.7
CCDs are also used in drug discovery in combination with DNA arrays
in a process called Multi-Array Plate Screening (MAPS).6
CCD Image of M31
In astronomy, CCD cameras have revolutionized the way that astronomers
take and record images. They have extended the range of faint objects
astronomers can study, and thus how far across the Universe astronomers
are able to "see". CCD cameras have supplanted photographic
plates, which were previously used to record astronomical images.
One of the other huge advantages to using CCD cameras in astronomy
is the ability to convert the gathered analogue information into digital
information, which can be analyzed using computer software.4
Detection of very faint objects is achieved by minimizing noise from
thermal photons by cooling the CCD to temperatures around 160 K. When
the CCD operates at such low temperatures, it is said to be functioning
in "slow scan" mode, which permits long exposures of up
to several hours.7,8 For galaxies and other astronomical
objects as faint as 25 (visual magnitude), this is the only way to
detect the light emitted by these objects.4 For comparison,
the human eye can only see objects brighter than about 5 or 6 (visual
Applications: Multi-Array Plate Screening (MAPS)
In MAPS, potential
drugs are applied to their targets in a microtiter plates. Their
degree of binding to the target - and thus their potential ability
as effective drugs - is assessed by the amount of light emitted
from the well through either laser-induced fluorescence, radioactive
scintillation, or chemiluminescence. As for astronomical applications,
the CCD must be cooled to cryogenic temperatures of about -100°C
to minimize noise from thermal photons. CCD cameras are able to
detect both RNA and DNA in amounts as low as 30,000 molecules, which
is about 5.0 x 10-20 moles. The benefit of using CCD cameras in
MAPS is that no amplification of the nucleotides is required, which
reduces cost and error and saves time.6
What Are the Non-Imaging Applications of the CCD?
of CCDs include signal processing, memory applications, video integration,
transversal filtering, and MTI filtering for radar. Again, non-imaging
applications fall under the categories of either signal processing
or delay line applications.1
What Does the Future Hold for CCDs?
The future of imaging
devices, however, is not likely to be the CCD. The CMOS or Complementary
Metal Oxide Semiconductor image sensor appears to be the future
of imaging, because it is fabricated using essentially the same
CMOS process as the large majority of modern integrated circuits,
which include microprocessors and dynamic random-access memories
(RAMs). CCDs, on the other hand, are fabricated using a variant
of practically obsolete N-MOS fabrication technology, which is basically
only used now in the fabrication of CCDs. What has kept CMOS image
devices from replacing CCDs is the trade-off in image quality; there
is more noise in CMOS devices, and unwanted signals from various
sources degrade the input signal. For many applications requiring
that every photon counts, the CMOS is simply not sensitive enough.
For the meantime, CCDs will be used in imaging devices where resolution
is very important, and CMOS imaging devices will be used when their
cheaper cost outweighs the benefit of higher resolution.5
1 Burt, D.J. "Basic Operation of the Charge Coupled Device".
International Conference on Technology and Applications of Charge
Coupled Devices. September 1974. Edinburgh: University of Edinburgh,
Centre for Industrial Consultancy and Liaison, 1974.
2 Fink, John. "Television". The New Book of Popular Science:
Deluxe Library Edition. Vol. 6. 1996 ed. Grolier Inc.: Chicago, 1996.
3 Hobson, G.S. Charge-Transfer Devices. John Wiley & Sons, Inc.: New
4 Kristian, Jerome and Morley Blouke. "Charged-Coupled Devices
in Astronomy". Scientific American. October 1982 ed. 247(4):
5 McCreary, Michael D. "Digital Cameras". Scientific American.
June 1998 ed.
6 Slaughter, Charles. "Cooled Cameras Help Discover Drugs".
Laser Focus World. November 2000. pp. 103-107.
7 Weisner, David E. "Charged-Coupled Devices". McGraw-Hill
Encyclopedia of Science and Technology. 7th ed. McGraw Hill Inc.:
New York, 1992.
8 World Book Encyclopedia of Science: Astronomy. 1997 ed. Scott Fetzer
Company: Chicago, 1997.
9 Zorpette, Glenn. "Seeing the Light: CMOS Image Sensors Are
Poised to Take on CCDs". Scientific American. May 1998 ed.
of Young Investigators. 2001. Volume Three.
Copyright © 2001 by Courtney Peterson and JYI. All rights reserved.