Free charge propagation in a resistive-gate MOS transmission line

A detailed computer analysis of the charge-transport process in an MOS transmission line (MOSTL)operating in deep depletion is presented. The mathematical model is developed taking into account time-dependent gate voltages, field-dependent surface mobility, and voltage-dependent depletion capacitance. A distributed transmission line circuit model similar to that obtained in a previous work for charge-transfer devices is derived. The charge-propagation characteristics are discussed and interpreted. The contributions of the physical transport mechanisms are investigated from the standpoint of assessing the influence of the design parameters and operating conditions on the performances of the structure. Several driving techniques which use time-varying gate voltages are proposed and simulated. Special emphasis is directed toward the possible improvements in the performances of an MOSTL used as a bit line in a continuously charge-coupled RAM or as a variable delay line for digital signals. Under some simplifying assumptions, an approximate model which yields closed-form solutions for the operational parameters of interest that compare well with the numerical calculations is developed. To a certain extent, the theory derived in this paper is also valid for charge-transfer devices, in particular for the charge-transfer process in bucket-brigade devices and under the transfer gates of two-phase CCD's.