A New Transient Model for Recovery and Relaxation Oscillations in Phase-Change Memories

Phase-change memory (PCM) relies not only on phase transitions between the two structures of the chalcogenide materials but also on electronic switching in the amorphous material between OFF and ON conductive states and vice versa. Transient effects associated to both transitions, like the delay time for threshold switching, the switching time, and the recovery time for the ON to OFF transition, play a fundamental role in the operation of the PCM device, namely program, erase, and readout processes. This paper focuses on the transient modeling and simulation of the recovery effect in PCM cells. After providing physical insight into the recovery process, we present a numerical model able to account for the transient recovery of threshold voltage V(T) and of resistance R after the ON-OFF transition. The impact of recovery effect on program-verify loops used for accurate positioning of R is discussed. The combination of switching and recovery effects is shown to give rise to relaxation oscillations under suitable bias conditions, and the transient model for switching and recovery is applied to describe and predict oscillation behavior of PCM.