The Desired Memristor for Circuit Designers

Memristors are two-terminal devices with varying resistance, where the behavior is dependent on the history of the device. In recent years, different physical phenomena of resistive switching have been linked with the theoretical concept of a memristor, and several emerging memory devices (e.g., Phase Change Memory, Resistive RAM, STT-MRAM) are now considered as memristors. Memristors hold promise for use in diverse applications such as memory, digital logic, analog circuits, and neuromorphic systems. Important characteristics of memristors include high speed, low power, good scalability, data retention, endurance, and compatibility with conventional CMOS in terms of manufacturing and operating voltages. One interesting property of some memristors is a nonlinear response to current or voltage. Nonlinear memristors exhibit a current or voltage threshold, such that the resistance is affected only by currents or voltages which exceed the threshold, while the resistance of a linear memristor changes with small perturbations in device current. Different applications exploit different characteristics of a memristor. In this article, the desired characteristics for different applications are presented from the viewpoint of an integrated circuit designer. Understanding the desired characteristics for different applications can assist device and material engineers in providing the appropriate behavior when developing memristive devices, thereby optimizing these devices for different applications.

I. Introduction

Memristors have many different facets. A memristor can be considered as the theoretical missing fundamental element originally proposed by Leon Chua in 1971 [1]. This theoretical device is a resistor with varying resistance, where the resistance changes according to the charge passed through the memristor over its entire history. Chua extended the theory of memristors to ‘memristive devices’ in 1976 with his student, Steve (Sung Mo) Kang [2]. A memristive device is basically any resistor with a resistance that only changes due to the voltage across the device or, alternatively, the current flowing through the device. Since the resistance does not change when there is no voltage applied across the device, memristive devices are nonvolatile. It is acceptable to use the term ‘memristor’ to describe a ‘memristive device.’