Versatile Functionality of Four-Terminal TiO_2–x Memristive Devices as Artificial Synapses for Neuromorphic Computing

Brain-inspired computing systems, which emulate the activity of biological synapses and neurons, are becoming more and more essential owing to their potential ability to solve the von Neumann bottleneck. Various types of memristive devices have been proposed to achieve information processing through synaptic functions. However, biological synapses have many complicated functions, such as heterosynaptic plasticity and the related neuromodulation, that are difficult to implement in conventional two-terminal memristors because of the requirement for multiple inputs. In this study, simple four-terminal memristive devices consisting of epitaxial thin films of TiO_2–x are fabricated and their capacity to implement synaptic functions are explored. These devices utilize the two-dimensional variation of the oxygen vacancy distribution in the TiO_2–x film to realize advanced heterosynaptic functionality, including tunable spike-timing-dependent plasticity, heterosynaptic plasticity mimicking habituation and sensitization, and Pavlovian conditioning, through multiterminal voltage inputs. The present results demonstrate the striking versatility of our four-terminal device for implementing the diverse and complex functions of artificial synapses on a single memristive passive element.