Closely mimicking synapses: diffusive memristors

October 04, 2016 // By Julien Happich
Led by Professor Jianhua Yang from the Department of Electrical and Computer Engineering at the University of Massachusetts, Amherst, a team of international researchers has designed a novel type of CMOS-compatible memristors which they say more closely mimic the functional behaviour of biological synapses.

The seemingly simple two-terminal device presented in the Nature Materials journal in a paper titled "Memristors with diffusive dynamics as synaptic emulators for neuromorphic computing" is not only capable of emulating spike-timing-dependent plasticity (STDP) but also paired-pulse facilitation (PPF) followed by paired-pulse depression (PPD) and when combined with a non-volatile element (a drift-type memristor), spike-rate-dependent plasticity (SRDP) could be obtained. All these phenomenon are observed in biological synapses, initiating both short- and long-term plasticity of the synapses and forming the basis of memory and learning.

As well as enabling a substantial reduction in footprint, complexity and energy consumption compared to three-terminal CMOS synaptic circuits, the two-terminal device doesn't require complex circuitry to simulate synaptic behaviour.

The diffusive memristors described in the paper consist of two platinum or gold inert electrodes sandwiching a switching layer of a dielectric film with embedded silver nanoclusters (SiOxNy:Ag, HfOx:Ag or MgOx:Ag). Devices were first built with a footprint of 10x10µm, the researchers then demonstrated similar switching behaviours for nano-device only 100nm by 100nm.

Fig. 1: Pseudo-colour scanning electron micrograph of a crossbar device. Top electrodes are depicted by the red dashed line and bottom contacts by the blue dashed lines. Biasing is applied on the top electrode with the bottom electrode grounded. The inset shows an atomic force micrograph of the junction (source University of Massachusetts, Amherst).

The resistance ratio between the conducting and insulating states was five orders of magnitude in SiOxNy:Ag and over ten orders in HfOx:Ag devices, the highest ever reported for a threshold switching device, the researchers wrote. They also reported very sharp turn-on slopes, around 10mV per decade in MgOx:Ag and SiOxNy:Ag and around 1mV per decade in HfOx:Ag.