Frequency modulation of conductance level in PCM device for neuromorphic applications

In this study we report for the first time the control of conductance level in PCM cells by means of a frequency modulation of progressive SET pulses. We show that by applying a train of progressive SET pulses, the conductance increases gradually and eventually saturates to a value Gsat. The latter can be tailored by changing the duty cycle of the pulse train. We propose a simple physics-based model to explain this effect. First, we simulated the thermal condition in the active region and showed that the increase of conductance was due to nucleation of a spherical hollow region around a central core, where re-amorphization takes place during programming. Based on this we illustrate that the frequency modulation can be analytically described by an equilibrium equation where the increase of conductance is balanced by the intrinsic chalcogenide resistance drift. The model is in very good agreement with data and shows that a fine tuning of the PCM device can be achieved. A frequency blind modulation of the programming pulse is believed to be much easier to be implemented in neuromorphic circuits as synaptic device [1] [2].