The properties of surface-channel charge-coupled shift registers are analysed taking into account all four mechanisms involved in the transfer of charge, namely thermal diffusion, charge gradient-induced drift, fringing-field drift, and interface state trapping. A nonlinear equation for the simulation of the signal transfer and degradation in charge-coupled shift registers in the presence of interface states is derived. It is shown that under some realistic assumptions, the nonlinear properties of both 2-phase and 3-phase devices are governed by the same equation. The theoretical results are used to investigate, by computer simulation, the input/output properties of digital registers. Among the discussed results, special emphasis is placed on worst case output signals. Small-signal and large-signal degradation parameters are also discussed and numerical results are presented.