Understanding synaptic characteristics of nonvolatile analog redox transistor based on mobile ion-modulated-electrolyte thickness model for neuromorphic applications

Linear and symmetric updates of the channel current of the redox transistor are involved in bulk mobile ion motion. In this study, we introduce the concept of a variable effective electrolyte thickness (teff) precisely tuned by gate voltage-driven ions into the drain current equation of a conventional transistor. In order to understand the switching characteristics of a nonvolatile analog redox transistor that serves as an artificial synapse for neuromorphic systems, we developed a physics-based model in MATLAB. The simulated synaptic update curves obtained using identical gate pulses were in good agreement with the fabricated Cu-ion-actuated CuOx/HfOx/WOx redox transistor. We then analyzed the impact of geometrical and material-related parameters on the synaptic behavior, taking into account the ion speed and the degree of allowable electric field through the electrolyte. In addition, we performed Monte Carlo simulation to create a non-uniformly changed teff circumstance. With this, we reproduced the fluctuated update of the channel current every gate pulse, which is occasionally observed experimentally when mobile ions are easily moved randomly. Our simulation results revealed that the redox transistor immune to the unevenly changed teff can be achieved by lowering the ion velocity.