What is a CCD?
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
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
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 onedimensional
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 potential, Vs.1,2,3
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..
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 inite 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
الخميس مايو 13, 2010 3:57 pm
