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Author

Toshitsugu Sakamoto

Other affiliations: Korea University
Bio: Toshitsugu Sakamoto is an academic researcher from NEC. The author has contributed to research in topics: Crossbar switch & Electrode. The author has an hindex of 29, co-authored 221 publications receiving 3435 citations. Previous affiliations of Toshitsugu Sakamoto include Korea University.


Papers
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Journal ArticleDOI
TL;DR: In this article, the authors describe a nanometer-scale switch that uses a copper sulfide film and demonstrate its performance, which is a chalcogenide semiconductor sandwiched between copper and metal electrodes.
Abstract: We describe a nanometer-scale switch that uses a copper sulfide film and demonstrate its performance. The switch consists of a copper sulfide film, which is a chalcogenide semiconductor, sandwiched between copper and metal electrodes. Applying a positive or negative voltage to the metal electrode can repeatedly switch its conductance in under 100 μs. Each state can persist without a power supply for months, demonstrating the feasibility of nonvolatile memory with its nanometer scale. While biasing voltages, copper ions can migrate in copper sulfide film and can play an important role in switching.

513 citations

Journal ArticleDOI
13 Sep 2004
TL;DR: In this paper, a reconfigurable LSI employing a nonvolatile nanometer-scale switch, called NanoBridge, is proposed, and its basic operations are demonstrated, and operational tests with them have confirmed the switch's potential for use in programmable logic arrays.
Abstract: A reconfigurable LSI employing a nonvolatile nanometer-scale switch, NanoBridge, is proposed, and its basic operations are demonstrated. The switch, composed of solid electrolyte copper sulfide, has a <30-nm contact diameter and <100-/spl Omega/ on-resistance. Because of its small size, it can be used to create extremely dense field-programmable logic arrays. A 4 /spl times/ 4 crossbar switch and a 2-input look-up-table circuit are fabricated with 0.18-/spl mu/m CMOS technology, and operational tests with them have confirmed the switch's potential for use in programmable logic arrays. A 1-kb nonvolatile memory is also presented, and its potential for use as a low-voltage memory device is demonstrated.

279 citations

Journal ArticleDOI
TL;DR: In this article, the authors examined the electronic transport of a solid electrolyte resistive switch and deduced that the conductive path is composed of Cu metal precipitated in the liquid by an electrochemical reaction, and observed Coulomb blockade phenomena at 4K when the switch was in the off state.
Abstract: The authors examined the electronic transport of a solid electrolyte resistive switch. Using element analysis and the temperature dependence of its electronic transport, they deduced that the conductive path is composed of Cu metal precipitated in the solid electrolyte film by an electrochemical reaction. Furthermore, they observed Coulomb blockade phenomena at 4K when the switch was in the off state. Their observations and experimental results suggest that the metallic conductive path consists of metallic islands separated by tunneling barriers and that switching between the on and off states originates from modulation in the tunneling barriers.

221 citations

Journal ArticleDOI
TL;DR: In this paper, a novel solid-electrolyte nonvolatile switch that was developed for programmable large-scale-integration circuits turns on or off when a conducting Cu bridge is formed or dissolved in the solid electrolyte.
Abstract: A novel solid-electrolyte nonvolatile switch that we previously developed for programmable large-scale-integration circuits turns on or off when a conducting Cu bridge is formed or dissolved in the solid electrolyte. Cu+ ion migration and an electrochemical reaction are involved in the switching process. For logic applications, we need to adjust its turn-on voltage (V ON), which was too small to maintain the conductance state during logic operations. In this paper, we clarified that V ON is mainly affected by the rate of Cu+ ion migration in the solid electrolyte. Considering the relationship between the migration rate and V ON, we replaced the former electrolyte, Cu2-alphaS, with Ta2O5, which enabled us to appropriately adjust V ON with a smaller Cu+ ion diffusion coefficient.

118 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated quantum effects in electrically variable shallow junction metal-oxide-semiconductor field effect transistors with an 8 nm long gate and showed that the direct tunneling current will exceed the thermal current and will become detrimental to lowvoltage operation of MOSLSIs in the 5 nm gate generation.
Abstract: We investigated quantum mechanical effects in electrically variable shallow junction metal–oxide–semiconductor field-effect transistors with an 8 nm long gate. We clearly observed the direct tunneling current from the source to the drain below 77 K, in good agreement with the calculation. We also showed that the direct tunneling current will exceed the thermal current and will become detrimental to low-voltage operation of MOSLSIs in the 5 nm gate generation.

90 citations


Cited by
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Journal ArticleDOI
TL;DR: A coarse-grained classification into primarily thermal, electrical or ion-migration-induced switching mechanisms into metal-insulator-metal systems, and a brief look into molecular switching systems is taken.
Abstract: Many metal–insulator–metal systems show electrically induced resistive switching effects and have therefore been proposed as the basis for future non-volatile memories. They combine the advantages of Flash and DRAM (dynamic random access memories) while avoiding their drawbacks, and they might be highly scalable. Here we propose a coarse-grained classification into primarily thermal, electrical or ion-migration-induced switching mechanisms. The ion-migration effects are coupled to redox processes which cause the change in resistance. They are subdivided into cation-migration cells, based on the electrochemical growth and dissolution of metallic filaments, and anion-migration cells, typically realized with transition metal oxides as the insulator, in which electronically conducting paths of sub-oxides are formed and removed by local redox processes. From this insight, we take a brief look into molecular switching systems. Finally, we discuss chip architecture and scaling issues.

4,547 citations

Proceedings Article
14 Jul 1996
TL;DR: The striking signature of Bose condensation was the sudden appearance of a bimodal velocity distribution below the critical temperature of ~2µK.
Abstract: Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

3,530 citations

Journal ArticleDOI
TL;DR: The performance requirements for computing with memristive devices are examined and how the outstanding challenges could be met are examined.
Abstract: 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.

3,037 citations

Journal ArticleDOI
07 May 2015-Nature
TL;DR: 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).
Abstract: 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.

2,222 citations