Tunable memory behavior in light stimulated artificial synapse based on ZnO thin film transistors

Abstract Optoelectronic synapses are inevitable for realizing neuromorphic vision systems, which require the integration of image recognition, memory and image processing into a single platform. In this work, we present a three terminal optoelectronic synapse created using zinc oxide (ZnO) thin film transistor. The persistent photoconductivity (PPC) of ZnO thin film is utilized to demonstrate the synaptic behavior. The change in conductance of the device under UV illumination has been interpreted as the weight change in the synapse. The basic synaptic functions such as sensory memory, short term memory, long term memory, duration-time-dependent plasticity and paired pulse facilitation (PPF) have been successfully demonstrated. The device shows a PPF index of 160%, comparable to other optoelectronic synapses reported in literature. Further, to corroborate the existing theory that PPC is caused by oxygen vacancies, additional characterizations are carried out and the presence of oxygen vacancies is detected in the fabricated ZnO device. Subsequently, pattern recognition of MNIST handwritten dataset has been performed using the conductance tuning curves of the proposed ZnO TFT based synapses in a neural network architecture, thereby demonstrating their feasibility to be used in neuromorphic applications.