A Nonlinear Diffusion Analysis of Charge-Coupled-Device Transfer

The charge-coupled device, as conceived by Boyle and Smith 1 and realized by Amelio, Tompsett, and Smith,2-3 consists of a series of 1721 1722 T H E BELL SYSTEM TECHNICAL J O U R N A L , J U L Y - A U G U S T 1971 metal oxide semiconductor (MOS) capacitors that are driven by clock pulses into deep depletion. In this condition, the MOS structures are capable of holding minority carriers in the potential wells beneath the plates (pads). In order to establish a transfer of charge from one plate to another, it is necessary to make the potential well beneath the second plate deeper than that beneath the first. This is illustrated in Fig. 1. In the absence of charge, the potential configuration beneath any one plate would be essentially flat, and finite electric fields would exist only in that span between two CCD electrodes. When charge is present, its transfer is driven predominately by the electrostatic forces associated with the presence of the charge in the CCD and by the thermal forces responsible for diffusion. It is the purpose of this paper to describe the transport of charge under the influence of these forces and, using this description, to make predictions concerning the operation of CCD's. In order to do this, it will be necessary first to derive the equation governing the transport of charge and solve it. Then the solutions will be applied to some particular CCD situations. I I . DERIVATION OF TRANSPORT EQUATION