CCD (charge coupled device)
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Charge Coupled Device.
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It is a charge transfer device.
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CCD is a shift register formed by a string of
closely placed MOS capacitors.
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It stores and transfers analog signals which
are generated electrically or optically.
Construction
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Series of metal gate electrodes separated from a p type substrate (if N
channel).
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On top it SiO2 an array of metalized electrodes
are placed which are connected to various voltages V1, V2,….
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| CCD Structure ( pic courtesy: Electron Deviecs By Salivahanan) |
Operation
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3 phase clock is applied to CCD
First Phase
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A positive voltage V1 (10 V) is applied to G1.
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V1>V2,V3 so a depletion region is formed in
less than 10microsecs.
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A potential well is formed where the minority
charge carrier is stored (electrons).
Second Phase
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The adjacent gate G2 is biased with higher
voltage (15V)
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A deeper well is created and the stored
electrons moves into it.
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This process is relatively slow.
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To ensure transfer of charge the potential
gradient must overlap. This can be done by keeping the distance between the
gates as small as possible.
Third phase
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The charge on g1 and G2 can be slightly relaxed
and G3 is given with 15V.
 |
| CCD Operation (pic Courtesy:Vik Dhillon) |
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Thus the charges has been transferred in one
cycle of 3 phase.
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So one cycle of transfer can store one bit
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As charge or cycle moves to next gate G3 to G4
it is ready to receive the next bit.
Characteristics
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CCD behaves like a dynamic shift register.
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Charge has to be transferred in less than 1ms.
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Inversion has to take place very soon else the
charge will be disappeared and information is lost.
Applications
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transfer speed 20 MHz
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Dynamic shift register
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Solid state imaging(video camera)
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Photosensors
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Consumes very little power and is capable of
operating at high frequencies.
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