Charge-transfer devices

Abstract

Shift registers of simple form have been known since the days of vacuum tube circuits. Bipolar and MOS transistor versions of static and dynamic registers using discrete components followed in due course. The concept of a shift register involves the passage of charge along a line of capacitors by the sequential switching of transistors in response to clock pulses. This class of circuit has acquired the name bucket brigade since the action is reminiscent of the transfer of water buckets down a line of people. In 1970 integrated versions of these circuits were shown to be practical for delay and other applications. A MOS FET version of an integrated bucket-brigade circuit is shown in Fig. 10.1(a). The storage regions are p islands in the n substrate and the metal gates are offset. When the gates are negatively pulsed, conducting channels are created that link adjacent p-storage regions. The storage condition (V 1=V 2) is shown in Fig. 10.1(b) and the transfer condition (V 2 more negative than V 1) during which time positive charge flows from well 1 to well 2 is shown in Fig. 10.1(c). The equivalent circuit, considered as discrete components, is given in Fig. 10.1(d) and consists of a line of p-channel MOS FETs with the storage capacitors between the gates and drains.