Modern computers are based on the von Neumann architecture in which computation and storage are physically separated: data are fetched from the memory unit, shuttled to the processing unit (where computation takes place) and then shuttled back to the memory unit to be stored. The rate at which data can be transferred between the processing unit and the memory unit represents a fundamental limitation of modern computers, known as the memory wall. In-memory computing is an approach that attempts to address this issue by designing systems that compute within the memory, thus eliminating the energy-intensive and timeconsuming data movement that plagues current designs. Here we review the development of inmemory computing using resistive switching devices, where the two-terminal structure of the devices and the direct data processing in the memory can enable area-and energy-efficient computation. We examine the different digital, analogue, and stochastic computing schemes that have been proposed, and explore the microscopic physical mechanisms involved. Finally, we discuss the challenges in-memory computing faces, including the required scaling characteristics, in delivering next-generation computing.

International audienc

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Chalcogenide materials are receiving increasing interest for their many applications as active materials in emerging memories, such as phase-change memories, programmable metallization cells, and cross-point devices. The great advantage of these materials is the capability to appear in two different phases, the amorphous and the crystalline phases, with rather different electrical properties. The aim of this work is to provide a physically based model for conduction in the amorphous chalcogenide material, able to predict the current-voltage (I−V) characteristics as a function of phase state, temperature, and cell geometry. First, the trap-limited transport at relatively low currents (subthreshold regime) is studied, leading to a comprehensive model for subthreshold conduction accounting for (a) the shape of the I−V characteristics, (b) the measured temperature dependence, (c) the dependence of subthreshold slope on the thickness of the amorphous phase, and (d) the voltage dependence of the activation energy. The threshold switching mechanism is then explained by the nonequilibrium population in high-mobility shallow traps at high electric field and by the nonuniform field distribution along the amorphous layer thickness. A single analytical model is then shown which is able to account for subthreshold conduction, threshold switching, negative differential resistance region, and ON regime. The model can be applied for fast yet physically based computation of the current in chalcogenide-based devices (e.g., phase change memory cells and arrays) as a function of applied voltage, temperature, and programmed state.

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Silver/copper-filament-based resistive switching memory relies on the formation and disruption of a metallic conductive filament (CF) with relatively large surface-to-volume ratio. The nanoscale CF can spontaneously break after formation, with a lifetime ranging from few microseconds to several months, or even years. Controlling and predicting the CF lifetime enables device engineering for a wide range of applications, such as non-volatile memory for data storage, tunable short/long term memory for synaptic neuromorphic computing, and fast selection devices for crosspoint arrays. However, conflictive explanations for the CF retention process are being proposed. Here we show that the CF lifetime can be described by a universal surface-limited self-diffusion mechanism of disruption of the metallic CF. The surface diffusion process provides a new perspective of ion transport mechanism at the nanoscale, explaining the broad range of reported lifetimes, and paving the way for material engineering of resistive switching device for memory and computing applications.

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Chalcogenide glasses are widely used in phase-change nonvolatile memories and in optical recording media\ud for their ability to rapidly change their structure to crystalline, thus obtaining different electrical resistance and\ud optical reflectivity. Chalcogenide glasses universally display threshold switching, that is a sudden, reversible\ud transition from a high-resistivity state to a low-resistivity state observed in the current-voltage I-V characteristic.\ud Since threshold switching controls the operating voltage and speed of phase-change memories, the\ud predictability of the switching voltage, current, and speed is of critical importance for selecting the proper\ud chalcogenide material for memory applications. Although threshold switching has long been recognized to be\ud an electronic process with an intimate relation to localized states, its detailed physical mechanism is still not\ud clear. In this work, threshold switching is explained by the field-induced energy increase in electrons in their\ud hopping transport, moderated by the energy relaxation due to phonon-electron interaction. The energy increase\ud leads to an enhancement of conductivity and a collapse of the electric field within the amorphous chalcogenide\ud layer, accounting for the observed negative differential resistance at switching. Threshold switching is found to\ud obey to a constant electrical-power condition. The proposed model generally applies to low-mobility semiconductors\ud featuring a deep Fermi level and hopping-type conduction, and can predict the thickness, temperature,\ud and material dependence of threshold voltage and current