A Model for Multilevel Phase-Change Memories Incorporating Resistance Drift Effects

Phase change memories are emerging as a most promising technology for future nonvolatile, solid-state, electrical storage. However, to compete effectively in mainstream storage applications, a multilevel cell capability is most desirable. Unfortunately, phase-change memories exhibit a temporal drift in programmed resistance (and in threshold switching voltage) which appears to be a fundamental and universal property of the amorphous or partially amorphous phase. Phase-change device models should therefore include these drift effects in a realistic way so that circuit and systems designers can assess the likely performance of multilevel phase-change memories in a variety of potential applications. In this paper, therefore, we present a comprehensive SPICE-based model for phase-change devices that includes the capability for programming into multiple resistance levels, the prediction of the drift of cell resistance (and threshold voltage) with time, and the capability for modeling the randomness inherent to the resistance drift phenomenon. Simulations of multilevel programming and drift phenomena using the model are presented and compared to experimental results, with which there is very good agreement.