Research On Synapse-Like Behavior Of Polymer Ferroelectric Thin Film

Synapse is the basic unit of information processing and transmission in the brain.Therefore,it’s very crucial to simulate synaptic behavior using artificial devices for constructing neuro computing system.Currently,researchers have fabricated different types of artificial synaptic devices based on different mechanisms and device structures.However,the study of artificial synaptic devices is still in the initial stage and there are some deficiencies in the relevant research.Thus,more attempts of novel device structural design and mechanism exploration are needed for enriching the research on synapse emulation.There are mainly two working modes in present artificial synaptic devices:current output and voltage output.With the help of multiple remnant polarization states,we fabricated two kinds of synaptic devices with current output and voltage output working modes respectively through the structural design of asymmetric top electrodes and floating bottom gate electrode.And we used the two kinds of synaptic devices to simulate synaptic behaviors like excitatory or inhibitory postsynaptic current and potential successfully.The specific contents of this paper mainly include the following three parts:(1)Research on synaptic device with current output working mode.In the past several years,researchers have made some artificial synaptic devices based on ferroelectric field-effect transistors.When these devices are in operation,the input of stimulus signal is usually applied on the gate electrode and the channel current between the drain and source electrodes is measured as the output signal.We fabricated "pseudo-two-terminals" synaptic devices with current output working mode by introducing a structural design of asymmetric top electrodes and floating bottom gate electrode in the ferroelectric field-effect transistor.And we used the device to simulate the synaptic potentiation and depression as well as paired-pulse facilitation which represents a typical synaptic short-term plasticity.Compared with the conventional artificial synaptic devices based on ferroelectric field-field transistors,the structural design of "pseudo-two-terminals" of our device is more similar to the synapse in the nervous system and makes the device integration more easily.(2)Research on synaptic device with voltage output working mode.Until now,the working mode of most artificial synaptic devices is current output and there are few reports on the artificial synaptic devices with voltage output working mode.In comparison with artificial synaptic devices with current output working mode,the behavior of artificial synaptic devices with voltage output working mode is more similar to the transmission and change of action potential in biological synapse.Thus,research on artificial synaptic devices with voltage output working mode is necessary.Meanwhile,the reported artificial synaptic devices with voltage output working mode have their own limitations such as complex device structure and needing assisted programming.So more efforts are needed to improve the performance of artificial synaptic devices with voltage output working mode.Based on the structural design of asymmetric top electrodes and floating bottom gate electrode of aforementioned synaptic device with current output working mode,we removed the semiconductor layer and used the floating gate voltage as output signal to fabricate the synaptic device with voltage output working mode.And we also used the device to simulate the synaptic potentiation and depression as well as paired-pulse facilitation.In addition,the simulation of handwritten digit recognition was realized by using the reversible weights of synaptic potentiation and depression.The device has a simple structure,multilevel synaptic weights and low power consumption.Furthermore,both the input and output signal are voltage,which makes the large-scale cascading of the device possible.Therefore,our synaptic device with voltage output working mode is a promising candidate for constructing artificial neural network in the future.(3)Research on microscopic characterization of the polarization switching in polymer ferroelectric thin film.For artificial synaptic devices,the gradual and reversible change of the output signal is used to simulate synaptic plasticity.In our synaptic devices with current output or voltage output working mode,the output signals are determined by the degree of polarization switching in polymer ferroelectric thin film which can be characterized by piezoresponse force microscopy.The gradual change of polarization switching in polymer ferroelectric thin film can be found when changing the amplitude or time of the voltage bias applied on the surface of polymer ferroelectric thin film.The lager the bias amplitude or the longer the bias is applied,the more complete the polarization switching in polymer ferroelectric thin film will be.This provides experimental basis of microscopic characterization for the gradual and reversible change of the output signals in synaptic devices with current output or voltage output working mode.In summary,two kinds of synaptic devices with current output and voltage output working modes,respectively,based on the multiple polarization states of polymer ferroelectric thin film were extensively studied.Based on the synaptic devices,the basic synaptic behaviors were successfully simulated.The results may provide a novel paradigm for the construction of artificial neural networks.