Synaptic behaviors of electromigrated Au nanogaps

Keita Sakai,Jun-Ichi Shirakashi,Mamiko Yagi,Soki Tani,Mitsuki Ito,Tomomi Sato

doi:10.1063/1.5096817

Abstract

Artificial electronic synapses or synaptic devices, which are capable of mimicking the functions of biological synapses in the human brain, are considered the basic building blocks for brain-inspired computing. Therefore, we investigated the emulation of synaptic functions in a simple Au nanogap. The synaptic functionality of neuromorphic hardware originates from a gradually modulated resistance. Previously, we investigated simple electromigration-based methods for controlling the tunnel resistance of nanogaps, called activation. In this study, a new type of artificial synaptic device based on planar Au nanogaps is demonstrated using a newly investigated activation procedure with voltage pulses. In the activation method with specific voltage pulses, the change in tunnel resistance of the Au nanogaps can be gradually controlled depending on the interval and amplitude of input voltage pulses. Moreover, Au inorganic synapses can emulate the synaptic functions of both short-term plasticity (STP) and long-term plasticity (LTP) characteristics. After the applied pulse is removed, the current decays rapidly at the beginning, followed by a gradual fade to a stable level. In addition, with repeated stimulations, the forgetting rate becomes decreases and the memory retention increases. Therefore, we observe an effect analogous to a memory transition from STP to LTP in biological systems. Our results may contribute to the development of highly functional artificial synapses and the further construction of neuromorphic computing architecture.

Highlights

Similar to a biological synapse, the conductance of synaptic devices can be gradually modulated by precisely regulated voltage pulses and by controlling the current through the devices.[7,8,9,10,11] In the last couple of years, demonstrations of synaptic functions have been realized in various memristive devices,[12] which can be implemented with magnetic tunnel junctions,[13,14] phase-change memories,[15,16] carbon nanotubes,[17,18] metal oxides,[19,20] and redox-based resistive switching.[3,21]
We show that a phenomenon similar to the short-term plasticity (STP)-to-long-term plasticity (LTP) transition can be achieved in Au nanogaps upon repeated stimulations, analogous to the effects observed in biological systems
In order to observe the transition from STP to LTP, we applied consecutive stimulation voltage pulses to the Au nanogaps with amplitudes V and intervals T of 20 V and 3 s, respectively

Summary

Introduction

Similar to a biological synapse, the conductance of synaptic devices can be gradually modulated by precisely regulated voltage pulses and by controlling the current through the devices.[7,8,9,10,11] In the last couple of years, demonstrations of synaptic functions have been realized in various memristive devices,[12] which can be implemented with magnetic tunnel junctions,[13,14] phase-change memories,[15,16] carbon nanotubes,[17,18] metal oxides,[19,20] and redox-based resistive switching.[3,21]. By adjusting the magnitude of the field scitation.org/journal/adv emission current during the activation procedure, the tunnel resistance of the nanogaps can be decreased gradually.[22–25] it is expected that the nanogaps controlled by activation have adaptive resistance, similar to the change in biological synaptic weight.

Results

Conclusion