The maximum storage capacity of open-loop written RRAM is around 4 bits

TL;DR

This paper uses information theory to analyze the theoretical maximum storage capacity of open-loop written RRAM devices, concluding it is around 4 bits due to inherent variations.

Contribution

It provides a comprehensive theoretical model for RRAM storage capacity considering device variations, clarifying the realistic limits of multi-bit storage.

Findings

Maximum capacity of RRAM is around 4 bits.

Conductance variation significantly limits storage capacity.

Number of levels and conductance range influence capacity.

Abstract

There have been a plethora of research on multi-level memory devices, where the resistive random-access memory (RRAM) is a prominent example. Although it is easy to write an RRAM device into multiple (even quasi-continuous) states, it suffers from the inherent variations that should limit the storage capacity, especially in the open-loop writing scenario. There have been many experimental results in this regard, however, it lacks a comprehensive analysis of the valid multi-bit storage capability, especially in theoretical terms. The absence of such an insight usually results in misleading conclusions that either exaggerate or underestimate the storage capacity of RRAM devices. Here, by the concept of information theory, we present a model for evaluating the storage capacity of open-loop written RRAM. Based on the experimental results in the literature and the test results of our own devices, we have carefully examined the effects of number of pre-defined levels, conductance variation, and conductance range, on the storage capacity. The analysis leads to a conclusion that the maximum capacity of RRAM devices is around 4 bits.