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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.

By Ian Poole

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