Charge Transfer in Charge-Coupled Devices

Abstract

The transfer characteristics of charge-coupled devices have been investigated theoretically and simulated experimentally. The transport of minority carriers along the Si–SiO2 interface can be described by a diffusion equation where the effective diffusion coefficient is composed of two terms representing the drift and the diffusion currents. There are two time constants associated with the two current components. For typical operating conditions, the drift time constant is about 100 times smaller than the diffusion time constant. The charge transfer can theoretically be separated into a charging and a discharging process, and the final stages are described by the drift and the diffusion time constants, respectively. Transfer efficiencies for charge-coupled devices have been calculated from the discharging phenomena. The drift current increases the transfer efficiency significantly. For high transfer efficiencies (>99%), however, the increase of the transfer efficiency with transfer time is limited by the diffusion time constant. A simple scheme which overcomes this limitation is described. Experimentally, the charging and discharging conditions have been simulated by using gated diodes. Good agreement between theory and experiment has been found by a reasonable adjustment of the surface mobility alone.