Study On Two Kinds Of Synaptic Transistors And Their Plasticity

With the development of computer technology,Von Neumann bottleneck will limit the computer for higher computing speed,and cause higher power consumption.In order to solve this problem,a neural network architecture that can emulate the human brain functions has been proposed.The basic unit of the new architecture is synaptic device which can process neuromorphic computing.Synaptic devices could also be used to simulate the organs of human body,like e-skin and retina,which may have many applications in robots and medical treatment in the future.Many synaptic devices have been studied by different mechanisms.One of most promising architectures in synaptic devices is synaptic transistor which has advantages of easy integration and relatively independent relationship between the operating end and the reading end.In the commonly used signal sources,the electricity is the most widely used input signal in the synaptic devices,while the light is the latter.The synaptic devices stimulated by optical signal can imitate the retinal neurons in human eyes and improve the bandwidth,which the synaptic devices stimulated by electricity only is inaccessible.The functions of memory and learning of synaptic devices are based on the synaptic plasticity.Having a wide adjustable range of synaptic plasticity is the key to achieving high-capacity and high-efficiency operation in neural network architecture.But currently reported lightstimulated synaptic devices have a smaller adjustable range of synaptic plasticity than the synaptic devices stimulated by electricity only.Therefore,it is so vital for lightstimulated synaptic devices to widen the adjustable range of synaptic plasticity that they can be used in various conditions.In this paper,pentacene is chosen as the research material to realize the light-stimulated synaptic transistor,and the synaptic plasticity is studied.The main research contents and results are as follows:1.A composite dielectric layer of 5 nm Ta2O5 and 3 nm Al2O3 had been grown onto the p++Si with 90 nm SiO2 by atomic layer deposition technology.And the surface was modified by spin-coating a polystyrene layer.Then 40 nm pentacene and 80 nm Cu electrodes were deposited onto the oxide layers by thermal evaporation,respectively.The growth rate of pentacene is 0.2 A/s,and 0.5 A/s for Cu.The electrical properties of pentacene transistor are as follows:When working in the linear region,the on/off ratio reaches?3×106,the subthreshold swing is about 314 mV·dec-1,the mobility is 0.36 cm2V-1s-1.When working in the saturation region,the on/off ratio reaches?107,the subthreshold swing is about 365mV·dec-1,the mobility is 0.31cm2V-1s-1.This is better than most of the reported pentacene transistors,whose on/off ratio is less than 105,and mobility is less than 0.25 cm2V-1s-1.2.Numerically simulating the transfer curve of p-type field-effect transistor changed by changing the threshold voltage.The result shows,when the read gate voltage is set as 0 V,the current between the drain electrode and the source electrode can change from a very low value to a very high value by shifting the threshold voltage from a negative value to a positive value.Then a large adjustable range of synaptic plasticity can be obtained by this way.3.Using the characteristic of photogating effect of pentacene transistor,which can shift the threshold voltage by light-stimulating,to realize the light-stimulated synaptic transistor.The long-term potentiation(depression)is achieved.And the adjustable range of synaptic weight is as high as 108%,exceeding at least three orders of magnitude of the light-stimulated synaptic transistors currently available and about two orders of magnitude for the electrical synaptic transistors.The device is very robust,the change of synaptic weight can remain above 108%in 1000 s,and reach more than 108%for repeatedly testing 20 times with an average value of?6×108%.4.Single-layer and double-layers molybdenum sulfide with large area were successfully prepared by gold-mediated exfoliating method.The area of the biggest single-layer molybdenum sulfide is about?150×70?m2.A single layer MoS2 transistor was fabricated by maskless lithography,the width of the channel was 5?m and the length was 2?m,and the on/off ratio reached about 104.The preparation of single layer MoS2 transistor lays the foundation of further studying the light-stimulated synaptic devices based on MoS2 transistors in the later work.