CCD (charge coupled device)

• Charge Coupled Device.

• It is a charge transfer device.

• CCD is a shift register formed by a string of closely placed MOS capacitors.

• It stores and transfers analog signals which are generated electrically or optically.

Construction

• Series of metal gate electrodes separated from a p type substrate (if N channel).

• On top it SiO2 an array of metalized electrodes are placed which are connected to various voltages V1, V2,….

CCD Structure ( pic courtesy: Electron Deviecs By Salivahanan)

Operation

• 3 phase clock is applied to CCD

First Phase

• A positive voltage V1 (10 V) is applied to G1.

• V1>V2,V3 so a depletion region is formed in less than 10microsecs.

• A potential well is formed where the minority charge carrier is stored (electrons).

Second Phase

• The adjacent gate G2 is biased with higher voltage (15V)

• A deeper well is created and the stored electrons moves into it.

• This process is relatively slow.

• 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

• The charge on g1 and G2 can be slightly relaxed and G3 is given with 15V.

CCD Operation (pic Courtesy:Vik Dhillon)

• Electrons are further transferred inside.

• Thus the charges has been transferred in one cycle of 3 phase.

• So one cycle of transfer can store one bit

• As charge or cycle moves to next gate G3 to G4 it is ready to receive the next bit.

Characteristics

• CCD behaves like a dynamic shift register.

• Charge has to be transferred in less than 1ms.

• Inversion has to take place very soon else the charge will be disappeared and information is lost.

Applications

• transfer speed 20 MHz

• Dynamic shift register

• Solid state imaging(video camera)

• Photosensors

• Consumes very little power and is capable of operating at high frequencies.

CCD (charge coupled device)