Charge transfer devices

A charge transfer device is a semiconductor structure in which discrete charge packets are removed. it finds wide applications in shift registers imaging systems dynamic memories and high speed filtering . there are two main methods of constructing CTD namely 1. Charge coupled Device CDD 2.Bucket Brigade Device.

charge coupled  device (ccd)

            a charge coupled device is a shift register formed by closely spaced Mos capacitors A CCD can store and transfer analog signals either electrons or holes which may be introduced electrically or optically . A cross sectional view of three phase charge coupled device is shown. the structure consists of a serial of metal gate electrodes separated from a p ( or an N ) type semiconducting silicon substrate  ( for an N channel ) by a thin silicon dioxide layer on the top of silicon dioxide is an array of metallised electrodes which are connected to signal voltages V1,V2  and V3.

            A three phase clocked voltage pulse system supplied to the gate ensures that the charge is transferred serially between gates and its direction is controlled as given below. the first phase connects a positive voltage V1 say + 10 V to G1 where V1 is greater than either V2 or V3 a depletion layer is formed in typically less than 1 microseconds. This produces the potential well into which information in the form of minority electrons is stored. During the second phase the adjacent gate G2 is biased to a greater voltage V2 say +15V to produce a deeper well under it is shown. The stored charge then transfers into the deeper potential  well by diffusion down the potential gradient which incidentally can be a relatively slow process.the charge is stored in the well under G2 the voltage on G1 is reduced say + 5 V and that on G2 to a sustaining level of +10 V

A third phase transfers a + 15 V pulse to the next gate G2.and the charge is transferred from G2 to the well under it as shown the voltages on G2 and G3 can be relaxed to complete one cycle of frequency. The charge transferred from under G1 to under G3 in one cycle of the clocked three phase pulse which causes a series of voltages in the sequence of  +15,+10,+5,+15 etc., to be applied to each. The charge in the substrate is transferred under one electrode to the next and so on. As charge is moved out of one set of three electrode G3 to G4 then the input gate is again put in a state to receive further bit of information .

            The CCD structure behaves as a dynamic shift register and charge as to be transferred to less than one ms.
Applications:-
CCDs can transferred up to 20 MHz.
Dynamic shift register in computers and solid state imaging video camera

BUCKET BRIGADE DEVICES (BBD) are formed by connecting a series of capacitors with switches FET. A single storage element consists of two capacitor switch units.

The BBD operates on the basis of charge transfer to the adjacent wells.the BBD structure cosists of a series of MOS transistors (MOSFET) as shown in fig. In which drain of the first transistor acts as the source of the adjacent transistor and so on. In a p channel BBD the source and drain regions are P+  of charge . the device is structured such that capacitance C associated with the left hand cell L is much larger due to the larger gate overlap than that of the right hand cell Cl >>Cr

Assume at time to when all channels are off a signal charge is stored in a diffused region L charging it to a potential Vo
Q = Cl Vo
If L has no charge to start with no charge transfer will take place.at t1 the phase 1 and phase 2 is low. The charge transfer takes place from all odd numbered diffused regions to even numbered diffused regions behave like L-type regions as shown

All odd numbered diffused regions behave like L type regions and transfer charge to the even numbered regions R. when phase 1 comes on cells L and R get connected. Since Cl >>Cr Ql =0 the charge  flow will continue till the left hand cell L contains practically no charge.

At t2 all channels are off and large potential barrier exists preventing any carrier flow. At t3 the phase phase 2 is high and phase 1 is low.qll even numbered regions behave like L type region and tranfer charge to theR type  regions.one pair of even and odd numbered regions constitute a bit or cell. The charge transfer takes place only from L to R not  from R to L . since there is a potential barrier between them. During one complete clock period T charge is transferred by one bit.

BBDs operate on lower maximum frequencies than CCDs because of transfer speed is limited by the charge flow through the channel. BBDs have large cell area .BBDs are used in audio delay lines to implement reverberation.BBDs can transfer at 1 MHz.