Persistent photoconductivity and Emulating Ebbinghaus forgetting curve via characterization of excitatory synaptic transmission in a ZnO-based optoelectronic synapse with ultra-low power (∼ fJ) consumption

Optical synapses provide high bandwidth operation with low power consumption for neuromorphic computing. ZnO is established as a potential semiconductor for optoelectronic synapses with its high photosensitivity in the ultraviolet (UV) region. In this work, we focus on emulating Ebbinghaus forgetting curves via conductance decay and memory retention in a lateral synaptic device based on ZnO nanoparticles. We use optical stimulation using UV light of wavelength 375 nm. Retention of memory can be seen up to 2500 sec showing long-term potentiation which is useful for memory and learning abilities. Long-duration memory retention using optical spiking is achieved and the decay characteristics of the memory are studied. A transition from short-term plasticity (STP) to long-term plasticity (LTP) can be induced by tuning the optical pulse width. Energy consumption is found to increase exponentially with the applied optical pulse width. The conductance decay follows the Ebbinghaus forgetting curve. We have demonstrated the effect of various measurement parameters such as pulse width, pulse frequency, and pulse amplitude on memory decay. The synaptic device can be operated with energy consumption as low as 1.2 fJ which paves the way for very low energy-consuming synaptic devices for neuromorphic applications.