Charge Transport and Storage in Metal-Nitride-Oxide-Silicon (MNOS) Structures

Abstract

A simple physical model that predicts charge accumulation at the dielectric interface of metal-nitride-oxide-silicon (MNOS) structures is proposed and verified experimentally. The model is based on the presence of steady-state current flow in the dielectric structure. Interface-charge accumulation is shown to be determined by the requirement for continuity of current through the structure under steady-state conditions. Continuity of current is established by accumulation of either positive or negative charge for a given polarity of charging voltage, depending on the relative current-field characteristics of the silicon nitride and silicon dioxide layers. Due to the exponential nature of the current-field characteristics, the time required to reach steady state is a strong function of the applied charging voltage. This leads to the observed charge storage property of MNOS devices. The hysteresis characteristic observed in MNOS structures is shown to be time-dependent with a tendency to merge into a single-valued dependence of accumulated charge on charging voltage as the steady-state condition is approached. The validity of the theoretical model for both steady-state and transient behavior is confirmed by current-voltage, capacitance-voltage, and turn-on measurements of MNOS capacitors and transistors for different dielectric thickness ratios and over a wide temperature range. The underlying concept that charge accumulation establishes current continuity in a two-layer dielectric structure should be valid, in general, for any two-dielectric structure.