Computer modeling of charge-coupled device characteristics

Properties of various charge-coupled device (CCD) configurations are investigated by means of a computer model. The model is based on a numerical solution of the Poisson equation for a unit cell of the CCD structure. The surface potential and the tangential surface electric field are obtained to an estimated accuracy of one percent and used to calculate transfer characteristics. On this basis various devices are compared as a function of both geometrical and electrical parameters. The geometrical parameters include oxide thickness, gap length, and electrode length. The electrical parameters include such things as doping density, fixed interface charge, and applied voltage. The influence of surface states is omitted from the treatment. The principal results indicate that (i) for dimensions of practical interest electrode lengths of the same order as the interelectrode spacings are desirable, (ii) moderately thick oxides enhance the tangential surface electric fields and increase the effectiveness of the channel-stop diffusion, (iii) lightly doped p-substrates are more resistant to the formation of electrostatic barriers in the gaps and yield faster devices because p-type conductivity silicon has a higher minority carrier mobility, and (iv) fixed charge at the Si-SiO
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interface can have a significant influence on the device characteristics. It is concluded that proper choice of both geometrical and electrical parameters is essential in obtaining optimum CCD performance; however, for such an optimized design, the transfer efficiency is for all practical purposes not limited by electrostatic considerations and is probably limited only by surface states. Theoretical limits of transfer efficiency based on these calculations are reported.