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I have developed "tennis elbow" from lugging this book around the past four weeks, but it is worth the pain, the effort, and the aspirin. It is also worth the (relatively speaking) bargain price. Including appendixes, this book contains 894 pages of text. The entire panorama of the neural sciences is surveyed and examined, and it is comprehensive in its scope, from genomes to social behaviors. The editors explicitly state that the book is designed as "an introductory text for students of biology, behavior, and medicine," but it is hard to imagine any audience, interested in any fragment of neuroscience at any level of sophistication, that would not enjoy this book. The editors have done a masterful job of weaving together the biologic, the behavioral, and the clinical sciences into a single tapestry in which everyone from the molecular biologist to the practicing psychiatrist can find and appreciate his or

Principles of Neural Science

Memristive devices are electrical resistance switches that can retain a state of internal resistance based on the history of applied voltage and current. These devices can store and process information, and offer several key performance characteristics that exceed conventional integrated circuit technology. An important class of memristive devices are two-terminal resistance switches based on ionic motion, which are built from a simple conductor/insulator/conductor thin-film stack. These devices were originally conceived in the late 1960s and recent progress has led to fast, low-energy, high-endurance devices that can be scaled down to less than 10 nm and stacked in three dimensions. However, the underlying device mechanisms remain unclear, which is a significant barrier to their widespread application. Here, we review recent progress in the development and understanding of memristive devices. We also examine the performance requirements for computing with memristive devices and detail how the outstanding challenges could be met.

Memristive devices for computing

In this paper, recent progress of binary metal-oxide resistive switching random access memory (RRAM) is reviewed. The physical mechanism, material properties, and electrical characteristics of a variety of binary metal-oxide RRAM are discussed, with a focus on the use of RRAM for nonvolatile memory application. A review of recent development of large-scale RRAM arrays is given. Issues such as uniformity, endurance, retention, multibit operation, and scaling trends are discussed.

Metal–Oxide RRAM

Despite much progress in semiconductor integrated circuit technology, the extreme complexity of the human cerebral cortex, with its approximately 10(14) synapses, makes the hardware implementation of neuromorphic networks with a comparable number of devices exceptionally challenging. To provide comparable complexity while operating much faster and with manageable power dissipation, networks based on circuits combining complementary metal-oxide-semiconductors (CMOSs) and adjustable two-terminal resistive devices (memristors) have been developed. In such circuits, the usual CMOS stack is augmented with one or several crossbar layers, with memristors at each crosspoint. There have recently been notable improvements in the fabrication of such memristive crossbars and their integration with CMOS circuits, including first demonstrations of their vertical integration. Separately, discrete memristors have been used as artificial synapses in neuromorphic networks. Very recently, such experiments have been extended to crossbar arrays of phase-change memristive devices. The adjustment of such devices, however, requires an additional transistor at each crosspoint, and hence these devices are much harder to scale than metal-oxide memristors, whose nonlinear current-voltage curves enable transistor-free operation. Here we report the experimental implementation of transistor-free metal-oxide memristor crossbars, with device variability sufficiently low to allow operation of integrated neural networks, in a simple network: a single-layer perceptron (an algorithm for linear classification). The network can be taught in situ using a coarse-grain variety of the delta rule algorithm to perform the perfect classification of 3 × 3-pixel black/white images into three classes (representing letters). This demonstration is an important step towards much larger and more complex memristive neuromorphic networks.

Training and operation of an integrated neuromorphic network based on metal-oxide memristors

The multilevel capability of metal oxide resistive switching memory was explored for the potential use as a single-element electronic synapse device. TiN/HfOx/AlOx/ Pt resistive switching cells were fabricated. Multilevel resistance states were obtained by varying the programming voltage amplitudes during the pulse cycling. The cell conductance could be continuously increased or decreased from cycle to cycle, and about 105 endurance cycles were obtained. Nominal energy consumption per operation is in the subpicojoule range with a maximum measured value of 6 pJ. This low energy consumption is attractive for the large-scale hardware implementation of neuromorphic computing and brain simulation. The property of gradual resistance change by pulse amplitudes was exploited to demonstrate the spike-timing-dependent plasticity learning rule, suggesting that metal oxide memory can potentially be used as an electronic synapse device for the emerging neuromorphic computation system.

An Electronic Synapse Device Based on Metal Oxide Resistive Switching Memory for Neuromorphic Computation

HfOx/AlOx bilayer resistive switching devices were fabricated for the study of the switching dynamics of the metal oxide memory An exponential voltage-time relationship was experimentally observed as follows: the programming pulse widths need for switching exponentially decreased with the increase in the programming pulse amplitudes Two following programming schemes were proposed to modulate the high resistance state values: (1) exponentially increase the programming pulse width; (2) linearly increase the programming pulse amplitude Although both of these schemes were effective to achieve the target resistance, the transient current response measurements suggest the second scheme consumes considerably less energy in the programming A field-driven oxygen ions migration model was utilized to elucidate the above experimentally observed phenomenon

Investigating the switching dynamics and multilevel capability of bipolar metal oxide resistive switching memory

A dynamic Verilog-A resistive random access memory (RRAM) compact model, including cycle-to-cycle variation, is developed for circuit/system explorations. The model not only captures dc and ac behavior, but also includes intrinsic random fluctuations and variations. A methodology to systematically calibrate the model parameters with experiments is presented and illustrated with a broad set of experimental data, including multilayer RRAM. The physical meanings of the various model parameters are discussed. An example of applying the RRAM cell model to a ternary content-addressable-memory (TCAM) macro is provided. Tradeoffs on the design of RRAM devices for the TCAM macro are discussed in the context of the energy consumption and worst case latency of the memory array.

A Compact Model for Metal–Oxide Resistive Random Access Memory With Experiment Verification

Al2O3-based RRAM devices were fabricated using atomic layer deposition under 100°C and 300°C deposition temperatures, respectively, and their resistance-switching behaviors were investigated. Both devices show unipolar switching if the top electrode (TE) is made of Ti/Al, whereas the bipolar phenomenon is observed when TE is pure aluminum. Devices fabricated at higher temperature give better uniformity and higher resistance ratio. Ultralow RESET current (~1 μA) was obtained, together with adequate voltage margin.

Yi Wu

Papers

Metal–Oxide RRAM

An Electronic Synapse Device Based on Metal Oxide Resistive Switching Memory for Neuromorphic Computation

Investigating the switching dynamics and multilevel capability of bipolar metal oxide resistive switching memory

A Compact Model for Metal–Oxide Resistive Random Access Memory With Experiment Verification

$\hbox{Al}_{2}\hbox{O}_{3}$ -Based RRAM Using Atomic Layer Deposition (ALD) With 1- $\mu\hbox{A}$ RESET Current