A Rational Design of All-Optically Controlled Optoelectronic Synapses with Bidirectional STP-to-LTP Learning Capabilities

An all-optically controlled optoelectronic synapse can regulate the synaptic plasticity purely by light, leveraging the advantages of light signals in speed, bandwidth, and parallelism. Currently, optoelectronic synapses can only emulate limited functionalities of biological synapses, such as generating positive or negative photocurrents to mimic the excitatory or inhibitory postsynaptic potential, while the emulation of the more complex learning plasticity is often missing. In contrast, the biological synapse in the hippocampus can generate long-term plasticity (LTP) after frequent stimuli, which is closely related to the crucial learning behaviors and memory capabilities of human beings. In this work, an optoelectronic synapse with bidirectional learning plasticity is rationally designed by TCAD, demonstrating bidirectional short-term plasticity (STP) to LTP transition after high-frequency light stimulation. This optoelectronic synapse adopts a distinct phototransistor architecture featuring two semiconductor floating gates. Both short-term and long-term photocurrents controlled by stimulation frequency can be induced by exploiting the carrier generation-recombination dynamics and the mutual capacitive coupling between the adjacent floating gates. An all-optically controlled optoelectronic synapse with bidirectional STP-to-LTP learning plasticity is enabled by designing a double-sided dual-floating-gate phototransistor, providing an efficient all-optically driven hardware platform for potential neuromorphic computing applications.