Sensor Sense: Charge-coupled devices

Nov. 8, 2007
In simplest terms, the image sensor or imager in vision systems is an XY grid of photosites that convert light into electricity.

Edited by Robert Repas

Most often the imager is a charge-coupled device or CCD, though CMOS imagers are rapidly gaining ground.

Charge-coupled devices gained fame as the primary imager in electronic cameras. Invented in the 1960s, CCDs were initially developed as a new type of computer memory circuit. While its memory use soon faltered, it became apparent that there were many other potential areas for it in signal processing and imaging. Of special interest to researchers was the sensitivity the device displayed to light.

In its simplest form, a CCD uses a substrate layer of p-type silicon. Embedded within the substrate are channels of n-type silicon over which is laid a silicon-oxide insulator. Placed atop the insulating layer at right angles to the n-type channels are strips of transparent electrode material. The point at which the electrode material crosses the n-channel region forms an area known as a gate. Each gate forms a photosite that corresponds to a picture element or pixel.

During an image acquisition or exposure, an electrical charge is applied to the electrodes forming potential barriers and wells in the n-channel material. Photons striking the n-channel material release energy creating photoelectrons. The photoelectrons remain trapped in the potential well and continue to accumulate as long as light keeps hitting the sensor. The number of photoelectrons held in the potential well corresponds to the number of photons striking the sensor at that point.

By changing the voltage applied to the gates, the electrons stored in each well shift from one well to the next in a bucket-brigade fashion. Electrons at the end of the channel feed a charge converter that changes electron quantity to a voltage output. Other circuits in the camera convert that voltage into the output signal from the camera.

Sensor voltage corresponds only to the intensity of the light that strikes each potential well. Thus, the output of the sensor forms only a black-and-white image. For color images, a color filter is placed over the sensor. The Bayer filter is the most common. It consists of four color quadrants: one red, one blue, and two green. Each color covers one photosite, so the camera must combine colors between each photosite to create the overall image.

 

Control voltages applied to the gates of a CCD create potential barriers and wells in an n-type silicon channel embedded in the substrate. When a photon of light strikes the channel during an exposure, it gives up energy to create a photoelectron that remains trapped in the well. By shifting gate polarities, the well transfers the photoelectrons along the channel until a charge converter at the end converts them to an output voltage.

About the Author

Robert Repas

Robert serves as Associate Editor - 6 years of service. B.S. Electrical Engineering, Cleveland State University.

Work experience: 18 years teaching electronics, industrial controls, and instrumentation systems at the Nord Advanced Technologies Center, Lorain County Community College. 5 years designing control systems for industrial and agricultural equipment. Primary editor for electrical and motion control.

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