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Stochastic Spiking Neural Networks Enabled by Magnetic Tunnel Junctions: From Nontelegraphic to Telegraphic Switching Regimes

Chamika M. Liyanagedera, Abhronil Sengupta, Akhilesh Jaiswal, and Kaushik Roy
Phys. Rev. Applied 8, 064017 – Published 15 December 2017
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Abstract

Stochastic spiking neural networks based on nanoelectronic spin devices can be a possible pathway to achieving “brainlike” compact and energy-efficient cognitive intelligence. The computational model attempt to exploit the intrinsic device stochasticity of nanoelectronic synaptic or neural components to perform learning or inference. However, there has been limited analysis on the scaling effect of stochastic spin devices and its impact on the operation of such stochastic networks at the system level. This work attempts to explore the design space and analyze the performance of nanomagnet-based stochastic neuromorphic computing architectures for magnets with different barrier heights. We illustrate how the underlying network architecture must be modified to account for the random telegraphic switching behavior displayed by magnets with low barrier heights as they are scaled into the superparamagnetic regime. We perform a device-to-system-level analysis on a deep neural-network architecture for a digit-recognition problem on the MNIST data set.

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  • Received 22 June 2017

DOI:https://doi.org/10.1103/PhysRevApplied.8.064017

© 2017 American Physical Society

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Authors & Affiliations

Chamika M. Liyanagedera*, Abhronil Sengupta, Akhilesh Jaiswal, and Kaushik Roy

  • Purdue University, West Lafayette, Indiana 47906, USA

  • *cliyanag@purdue.edu

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Vol. 8, Iss. 6 — December 2017

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Images

  • Figure 1
    Figure 1

    (a) High resistive antiparallel state of a MTJ. (b) Low-resistive parallel state of a MTJ. (c) A SHE-MTJ device structure where the MTJ is switched by passing charge current through the underlying heavy metal. The charge current flowing through the heavy metal leads to spin splitting, thereby creating a perpendicular spin current and switching the magnetization direction of the free layer.

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  • Figure 2
    Figure 2

    Decoupled read- and write-current paths of the MTJ with a HM. The output of the inverter is high when the MTJ is in the P state, low when the MTJ is in the AP state.

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  • Figure 3
    Figure 3

    The two operating states of a MTJ. The two states are thermally stable if the barrier height of the magnet, EB, is large enough.

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  • Figure 4
    Figure 4

    (a) Switching characteristics of a MTJ with varying EB values at T=300K for a write-cycle duration of 0.5 ns. (b) MTJ switching-probability characteristics as a function of IIbias, normalized by a factor Io. The data closely resemble the sigmoid function. (c) Variation of the bias current, Ibias, and the normalizing factor, Io, with varying EB values. Both Ibias and Io decrease with a decreasing EB value. (d) Failure probability during a read cycle of 1 ns (in logarithm scale) with varying EB values.

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  • Figure 5
    Figure 5

    Switching characteristics of a MTJ with a 1kBT barrier height. (a) When the current flowing through the HM is zero, the MTJ is equally likely to be in a parallel or antiparallel state. (b) When 1.5μA is flowing through the HM layer, the MTJ is more likely to be in the antiparallel state. (c) When 1.5μA is flowing through the HM layer, the MTJ is more likely to be in the parallel state.

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  • Figure 6
    Figure 6

    (a) Average inverter output over a duration of 2μs with and without the impact of the read current. (b) Variations of the inverter average output over durations of 2μs with magnitudes of the write current for different EB values. (c) Inverter average output over a duration of 2μs for a nominal corner and for the worst-case conditions of ±1σ and ±2σ variations in the threshold voltages of the transistors. (d) A typical plot of the output voltage of the inverter stage of the read circuit as a function of time under zero external input current.

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  • Figure 7
    Figure 7

    Crossbar architecture connecting the inputs of one layer to the neurons of the corresponding layer. Horizontal bars provide the input voltage for the synapses. The summation of weighted synaptic currents along the columns of the crossbar array are then provided as inputs to the MTJ neurons.

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  • Figure 8
    Figure 8

    Variation of classification accuracy of the proposed network with time for (a) synchronous and (b) asynchronous implementations.

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  • Figure 9
    Figure 9

    (a) Energy consumption of the MTJ neuron. (b) Energy consumption of the read circuit. (c) Energy consumption of the synapses. (d) Total energy consumption per image classification (for an accuracy of 96%) for the asynchronous (1 and 2 kBT) and synchronous (10 and 20 kBT) networks.

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  • Figure 10
    Figure 10

    Average classification accuracy (measured over 50 independent Monte Carlo simulations) with variations in the resistive synapses (variation percentage in σ) for the (a) synchronous and (b) asynchronous designs.

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  • Figure 11
    Figure 11

    Average classification accuracy (measured over 50 independent Monte Carlo simulations) with variations in the supply voltage (up to 25-mV variations) for the (a) synchronous and (b) asynchronous designs.

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  • Figure 12
    Figure 12

    Average classification accuracy for the worst-case corner, with variations in the CMOS read circuit (up to a ±2σ variation) for the asynchronous design.

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  • Figure 13
    Figure 13

    Classification accuracy with varying operating temperature for the (a) synchronous and (b) asynchronous designs.

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  • Figure 14
    Figure 14

    Average inverter output under different temperatures for (a) 1- and (b) 2kBT magnets.

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