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.

翻译：关于多级存储器件的研究已大量涌现，其中电阻式随机存取存储器（RRAM）是一个典型代表。尽管将RRAM器件写入多个（甚至准连续）状态较为容易，但其固有的变异特性会限制存储容量，尤其在开环写入场景下。这方面已有许多实验结果，但缺乏对有效多比特存储能力的综合分析，特别是在理论层面。这种认知的缺失常导致误导性结论，要么高估要么低估RRAM器件的存储容量。本文基于信息论概念，提出了评估开环写入RRAM存储容量的模型。结合文献中的实验结果及自主器件的测试数据，我们系统研究了预定义电平数量、电导变异和电导范围对存储容量的影响。分析表明，RRAM器件的最大存储容量约为4比特。