Charge coupled image sensor CCIS
- an overview of the charge coupled image sensor or CCIS that is
based on the charge coupled device CCD and is used for many image sensing
applications, especially within cameras
The charge coupled image sensor ( CCIS ) is a specialised
form of its parent device, the charge coupled device ( CCD ). So popular is the
CCIS now that it is often referred to as a CCD.
The concept of the CCIS has been known for many years. In
fact the first paper on its parent device the charge coupled device or CCD in
1970 mention the idea of using the CCD as an image sensor. With the widespread
applications of the CCD, a large amount of research was soon started with one of
the main aims being to develop a charge coupled image sensor.
Structure
The CCIS is effectively a modified form of CCD. The main difference is that the
gates are made to be semi-transparent so that light can reach the active region
of the charge coupled image sensor. Often the gate material used is polysilicon.
It is also possible to illuminate the active area of the CCIS
from the opposite direction to avoid the light absorption in the gate region.
When this approach is adopted it is necessary to thin the semiconductor of the
image sensor to ensure that the level of light absorbed within the body of the
device is as small as possible. In this way the maximum level of light will
reach the depletion region which is at the top of the CCIS, close to the gate
region. Thinning the semiconductor also ensures that resolution is not lost.
This is a particularly important consideration because each pixel within the
CCIS may be less than ten microns across.
Silicon is the most commonly used material for charge coupled
image sensor ( CCIS ) devices. When using silicon, the most widely used
insulator is thermally grown oxide.
Operation
There are two basic mechanisms used within the charge coupled image sensor (
CCIS ). The first is the shift register function that it inherits from its CCD
parenthood, and the second is that of light detection.
The charge generation as a result of the light impact is of
particular importance for the charge coupled image sensor. As photons of light
impact on the light sensitive area, it is found that the photons of light that
hit the structure, electrons are released, thereby generating a charge. This
charge is stored in the cell which acts as a metal-insulator-silicon capacitor.
The actual exposure time can vary considerably and may be anywhere between 100
microseconds and 100 milliseconds.
The charge in each cell is stored until the shift register
action of the device moves the stored charge along the cells to enable the
information to be accessed externally.
Applications
Charge coupled image sensors are used in a wide variety of applications. They
provide a very method of capturing images where there is a large array of
information. The fact that the information is accessed in a serial fashion means
that interfacing to them is considerably simplified and as a result is
comparatively cheap. As a result they are widely used for still cameras,
camcorders, broadcast cameras, image analysis systems, robotic systems where
video information is required, and a variety of other imaging, guidance and
control systems. In view of this, charge coupled image sensors are widely used.
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