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CCD : Charge Coupled Devices

Table of Contents:

Written By: 

Anshul Thakur

 

October 6, 2009, Stockholm, It was a historic moment for digital cameras when their shutters opened for the fraction of a second to capture the committee, award the coveted Nobel Prize to the ones who invented the electronic eyes. Had it not been the duo Willard Boyle and George Smith, those cameras might still have been shuttering light on a photosensitive film and then drying them on rails in the dark room. By inventing the Charge Coupled Device and predicting its applications way back in 1969, Boyle and Smith opened up the domain of solid state devices for imaging and memory applications.
 
What is CCD - Charge Coupled Device
Though Boyle and Smith invented the method to convert light into an electrical signal, the entire research would not have been possible had it not been Albert Einstein’s groundbreaking explanation of the Photoelectric effect. Einstein explained how materials could absorb incident radiation and the eventual knocking out of the electrons from the surface. Boyle and Smith studied how this incidence of light caused disturbances in electrons and how it could be utilized. They succeeded in doing this by grouping a number of capacitors into an array, something which makes up the pixels of a digital image.
 
Initially named as Charge Bubble Devices, the device’s operation as a shift register and a linear imaging device was recorded. It was based on a principle similar to Bucket-Brigade Devices, where charge is transferred from one capacitor to another along the semiconductor surface. Bell labs succeeded in building the first solid state video camera in 1970. By 1971, Michael F. Tompsett and other researchers at Bell Labs succeeded in capturing images with linear CCD arrays. After that, many semiconductor majors like Texas Instruments, Fairchild Semiconductors, Sony etc. started investing in CCD technologies. One of the first commercial CCD devices was built by Fairchild in 1974 which was a 100x 100 pixel device having about 500 CCDs array. The first CCD based reconnaissance satellite KH-11 KENNAN was launched in 1976. By 1983, CCDs had started to replace photographic plates in astronomical telescopes too. Companies like Kodak had been manufacturing CCD based professional cameras since 1985, but by 1995, cost effective high resolution CCD cameras started flooding the markets.
A CCD can be thought of as a subset of Charge Transfer Devices. These are based upon Metal Oxide Semiconductor (MOS) capacitors. Two types, viz surface channel and buried channel MOS capacitors have been used in CCD, but primarily buried channel capacitors have been used for manufacturing since these do not have problems caused by surface irregularities at oxide-semiconductor interface. A thin n-type buried channel is formed on a p-type substrate through ion implantation. The Silicon Dioxide insulator layer is grown on top of the n-region, and to complete the capacitor, gates of metal or heavily doped polycrystalline silicon are placed on top of the insulating SiO2 using CVD process. To isolate the charge packets of one column from other, thermally grown oxide ‘channel stops’ are placed parallel to the channels.
 Construction and Elements of CCD - Charge Coupled Device
Buried Channel MOS
But if a CCD cell were to pass along charges, it would just be any other MOS capacitor. The additional property that it is sensitive to light makes a CCD unique. The response to photons is through the epitaxial layers of doped silicon grown on the substrate. When photons are incident on the semiconductor surface, they dislodge electrons which create charge that is proportional to the light falling on the surface. A single CCD cell performs four functions:
1.      Receive charge from the cell above it in the array.
2.      Hold that charge for sometime without much loss.
3.      Pass that charge to the cell below it in array.
4.      Respond to outside stimulus like light and generate its own charge.
 

Comments

good one.

great information

 

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