Ionic-electronic dynamics in an electrochemical gate stack toward high-speed artificial synapses

Despite their great synaptic potential, the trade-off between programming speed and energy consumption of electrochemical random-access memory (ECRAM) devices are major hindrance to their incorporation into practical applications. In this work, we experimentally study the main limiting factor for high-speed programming of ECRAMs, the ionic current in the gate stack. We use two-terminal structures composed of LiCoO₂/Li₃PO₄/amorphous-Si to represent the ECRAM gate stack (reservoir/electrolyte/channel). We perform electrical characterization including impedance spectroscopy (small-signal) and large-signal transient measurements across nine orders of magnitude in the time domain. We find that at the sub-microseconds range, the current is governed by the energy barrier for Li⁺ ions at the electrolyte interfaces. After a period of ∼1 μs, ionic migration through the ∼80 nm electrolyte layer dictates the current. At ∼50 μs, the ionic double layer at the interface is fully charged and the gate current drops by several orders of magnitude, indicating that the Li₃PO₄/Si interface is saturated, and the measured current is dominated by the electronic leakage component. Furthermore, we evaluate ECRAM performance under various pulse parameters.