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.
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.