Abstract
1. Introduction
With the advent of the big data era, computers store and calculate information with explosive growth, which will inevitably lead to the need for computers to increase their scales and invest more hardware devices at the current level of computing. Most computers today are still designed according to the architecture of "programmed storage, sharing data, sequential execution" proposed by Von Neumann[
In recent years, scholars have modified existing physical devices based on their understanding of the structure and the function of nerve synapses, and derived a series of neurosynaptic devices with different structures and mechanisms. With the application of massive optoelectronic materials, ionic liquids, ferroelectric materials, flexible materials, etc. to neurosynaptic devices, the development of this field gradually becomes multifunctional, diversified, intelligent, and biomimetic. Therefore, scholars have put forward higher requirements for stability, biocompatibility, flexibility, low power consumption and other performance characteristics. Nowadays, neurosynaptic devices have not been limited to imitate basic synaptic functions, but expanded to the aspects of mechanisms, materials, device structures, integration and applications. Although numerous literature has made comprehensive, timely and constructive summaries on the development of neurosynaptic devices, few scholars have noticed the important role played by neurosynaptic devices in simulating the human perception systems and the significant achievements that have been made. Hence, this article will focus on the introduction and summary of this field, hoping to arouse scholars' attention and interest. In order to describe neurosynaptic devices clearly, the basic principles and device types of neurosynaptic devices are systematically introduced before discussing the imitation of the human perception systems. Finally, a summary and a critical evaluation are also given based on our comprehension of this field.
2. Neurosynaptic devices
Before using physical devices to simulate synapses, the devices first need to achieve synaptic plasticity functionally. Synaptic plasticity describes the phenomenon that the effectiveness of synaptic information transmission among neurons increases or decreases with changes in their neural activities. The “synaptic weight” is used to measure the strength of signal transmission between synapses. Synaptic plasticity includes short-term plasticity (STP), long-term plasticity (LTP), spiking-timing-dependent plasticity (STDP), and spiking-rate-dependent plasticity (SRDP) etc.[
2.1. Two-terminal memristors
Due to the simple structure, memristors are easy to integrate on a large scale with a low cost. The memristor is generally composed of a top electrode, a resistive change layer, and a bottom electrode stacked on top of each other. The top electrode can be regarded as the presynaptic membrane, while the bottom electrode serves as the postsynaptic membrane. Synaptic response is achieved by adjusting the conductance between the two electrodes. The working mechanisms of memristors are usually divided into four types as follows: electrochemical metallization mechanism (ECM), valence change mechanism (VCM), and phase change mechanism (PCM) and ferroelectric mechanism (FeM), as illustrated in Fig. 1.
Figure 1.(Color online) The working mechanisms of two-terminal memristors. (a) Electrochemical metallization mechanism. (b) Valence change mechanism. (c) Phase change mechanism. (d) Ferroelectric mechanism.
2.2. Three-terminal synaptic transistors
A three-terminal synaptic transistor is usually composed of a semiconductor layer, a gate dielectric layer and a source/drain/gate electrode. The gate electrode and the conductive channel are regarded as the presynaptic terminal and the postsynaptic membrane, respectively. The synaptic signals of three-terminal synaptic devices can modulate the synaptic weight through the independent gate electrode. Since the transistor has one more electrode than the memristor, the transistor can more effectively realize the simulation of biological synapses and provide more synaptic functions and applications. At present, three-terminal synaptic transistors can be roughly divided into floating-gate transistors (FGTs), electrolyte-gate transistors (EGTs), ferroelectric field-effect transistors (FeFETs) and optoelectronic synaptic transistors (OSTs) according to their working mechanisms[
Figure 2.(Color online) The types of three-terminal synaptic transistors. (a) Floating-gate transistors. (b) Electrolyte-gate transistors. (c) Ferroelectric field-effect transistors. (d) Optoelectronic synaptic transistors.
A floating gate transistor is composed of a gate electrode, a blocking oxide layer, a floating gate layer, a tunneling oxide layer, a semiconductor layer, a source electrode and a drain electrode. When the gate voltage is applied, charges are easily injected into the floating gate through thermal excitation or quantum tunneling[
The electrolyte gate transistor is composed of a source electrode, a drain electrode, an electrolyte dielectric layer, a semiconductor layer and a gate electrode. Electrolyte gate transistors use the ions in the electrolyte dielectric layer to adjust the channel conductance of the device and achieve synaptic weight modulation as well. The working principle of electrolyte gate transistors can be simply divided into two types: electrostatic modulation and electrochemical doping[
The ferroelectric field-effect transistor is composed of a source electrode, a drain electrode, a semiconductor layer, a ferroelectric dielectric layer, and a gate electrode. The ferroelectric dielectric layer is the key to realize the synaptic function of the transistor. The ferroelectric materials have a spontaneous polarization state, and the carrier concentration of the ferroelectric field-effect transistors is precisely modulated by gate voltage to change the polarization state of the ferroelectric materials. The ferroelectric insulators switch between the two remnant polarization states, corresponding to the two digital states of the memory: “0” and “1”. The ferroelectric field-effect transistors have the advantages of multi-level conductance, large on-off ratio, high stability, lossless readout, low power consumption, high operating speed, small device size and simple manufacturing process[
The difference between the optoelectronic synaptic transistor and the synaptic devices described above is that it introduces light as another effective way to modulate the channel conductance besides the gate. Because light can provide synaptic devices with large bandwidth, low interconnection energy loss and ultra-fast signal transmission, optoelectronic synaptic transistors have become one of the most popular research directions for synaptic devices[
After nearly a decade of development, neurosynaptic devices are no longer simply satisfied with simulating synaptic plasticity. The advent of artificial intelligence, the internet of things, big data, and the era of bionic robots has put forward higher requirements for neurosynaptic devices. Massive functional materials and the innovation of device structures provide preconditions for the systematization, functionalization and intelligence of neurosynaptic devices. Moreover, neurosynaptic devices at present can achieve rapid response to various external stimuli such as light signals and electrical signals, which greatly expands the range of applications of neurosynaptic devices. Because the process of neurosynaptic devices responding to external stimuli is very similar to that of humans feeling the external world, neurosynaptic devices can effectively shape and imitate the human perception systems. Therefore, this review will summarize the achievements of the imitation of human perception systems from five parts: the visual system, the somatosensory system, the auditory system, the gustatory system and the olfactory system.
3. The imitation of human perception systems
3.1. Visual system
Among the information received by humans, more than 80% comes from the visual sense organs[
Optoelectronic synaptic devices have the advantages of both optoelectronic sensors and synaptic devices. Synaptic devices process, amplify and store the information of light intensity, color and frequency obtained by the optoelectronic sensor in real time[
Sun et al.[
Figure 3.(Color online) (a) Schematic diagram of phototransistor structure. (b) Schematic diagram of the heterojunction band before and after light. (c) Sensor array chip, wires bonding on printed circuit board (scale bar: 5mm). (d) Optical micrograph of 32 × 32 sensor array (scale bar: 500
Chen et al.[
Figure 4.(Color online) (a) Schematic diagram of bioinspired visual memory unit integrated by image sensor and storage device. (b) Characteristic
3.2. Somatosensory system
The somatosensory system is the largest and most widely distributed system of the human body. There are countless tactile receivers on every inch of skin, which receive real-time information such as external temperature, humidity, pressure, and stimulation, then transmit complex information to the brainstem and cerebral cortex for analysis[
Bao et al.[
Figure 5.(Color online) Comparison of the artificial afferent nervous system and biological afferent nervous system. (a) Biological afferent nerve stimulated by pressure. (b) An artificial afferent nerve made of pressure sensor, organic ring oscillator and synaptic transistor. (c) A photograph of an artificial afferent nerve system. Reproduced with permission[
Lee et al.[
Figure 6.(Color online) (a) Schematic diagram of flexible ferroelectric organic field-effect transistor structure. (b) Schematic diagram of 2 × 2 sensor array of artificial tactile nerve. (c) Infer the order of touch in the 2 × 2 sensor array based on synaptic weight. Reproduced with permission. Copyright 2020[
3.3. Auditory system
The human auditory system consists of the ears (outer ear, middle ear, inner ear), related nerve pathways (such as the auditory nerve), and the auditory cortex located in the brain. People can hear and recognize sounds in a frequency range from about 20 to 20 000 Hz frequency band[
Guo et al.[
Artificial auditory synapses composed of triboelectric sensors and ion gel-gated organic synaptic transistors (IGOST) was reported by Seo et al.[
Figure 7.(Color online) (a) Schematic illustration of the human auditory pathway. (b) Basic structure scheme of the TENG acoustic receptor. (c) A schematic configuration of the acoustic synaptic transistor and the acoustic processing with neuromorphic function. Reproduced with permission[
3.4. Gustatory system
Human taste perception allows humans to identify various flavors. There are about 2000–10 000 taste buds on the tongue, receiving various stimuli. Then the sensory nerve endings transmit the stimulus signals to the central nervous system to recognize and distinguish various tastes. In fact, the taste that humans feel is not only from the stimulation of the chemical signals received by the taste receptors, but the smell provided by the olfactory system[
Xu et al.[
Figure 8.(Color online) (a) Schematic diagram of biological synapse and p–i–n JST. (b) Postsynaptic current of p–i–n JST under negative and positive pulses mimics the different functions of dopamine and acetylcholine in the nervous system: excitement response and memory formation. (c) Schematic diagram of high salt aversion and low salt attraction caused by the synergy of different gustatory receptor neurons. Reproduced with permission[
3.5. Olfactory system
The human olfactory system includes 10 million specialized neurons and more than 400 olfactory receptor genes. The complex nervous system allows humans to distinguish nearly 10 000 odors[
Guo et al.[
Figure 9.(Color online) (a) Schematic diagram of an artificial olfactory inference system based on an RC system and a classifier for gas classification. (b) Temporal responses of the memristive devices to the spike trains of the response speeds. (c) Temporal responses of the memristive devices to the spike trains of the sensing responses. The complete output is segmented at 0.15 s interval, as presented in the inset of (b). (d) Classification accuracy for testing samples with two types of artificial synapses, that is, ideal and WO3-based ones. (e–g) Classification accuracy for testing samples with reduced (e) spatial, (f) temporal, and (g) spatial and temporal dimensions. Reproduced with permission[
Wang et al.[
Human perception systems involve a variety of physical and chemical signals. Although various sensors can effectively and conveniently convert external signals into electrical signals, the difficulty and key point of human perception systems is to detect, filter and simplify input signals efficiently, sensitively and in real time, consequently forming short-term and long-term memories. Neurosynaptic devices have natural advantages, but for complex systems, it is necessary to solve crucial problems such as integration, arraying, power consumption, flexibility, sensitivity, and life span. Nevertheless, multifunctional neurosynaptic devices have gained fruitful and excellent achievements in imitating human perception systems, which were summarized in Table 1 for reference.
4. Summary and prospects
This review introduces the development of neurosynaptic devices and summarizes the working mechanisms, advantages and disadvantages of neurosynaptic devices from two-terminal memristors to three-terminal transistors. It has been more than ten years since the birth of the first neurosynaptic device. During this period, neurosynaptic devices have been diversified and functionalized to mimic synaptic plasticity and various functions. With the development of big data, Internet of Things and artificial intelligence, people continue to expect that neurosynaptic devices can move towards more complex systems, using their excellent memory and calculation properties to achieve multi-functionality, intelligence and using physical devices to simulate human perception systems. At present, there are many articles showing the exploration in the simulation of human perception systems, but few of them summarize their works. Therefore, this review classifies the current achievements from five aspects of vision, touch, hearing, taste and smell, and selects representative works among them to summarize, aimed to give readers an overall understanding about the development of neurosynaptic devices and enlighten their thoughts.
Since increasing achievements of neurosynaptic devices have been made year by year, it is delighting that optoelectronic synaptic devices and piezoelectric synaptic devices have been developed on the basis of the earliest electrical synaptic devices. The functions have become more abundant and the power consumption has been reduced to femtojoule level. In general, the basic framework of neurosynaptic devices has been built in the past five years and have been summarized by several reviews from different perspectives, such as small molecule materials[
We believe that there are still many unsolved problems and tough challenges for neurosynaptic devices on the way to shape the human perception systems. First, the development of neurosynaptic devices is obviously dependent on the intrinsic properties of materials, including stability, conductivity, photoelectric properties, mechanical properties, etc. However, few materials can perfectly meet the requirements of the applications. Improving the performance of materials and optimizing the device structures are prerequisites for realizing more complex applications. Second, the functions of synaptic devices to simulate the human perception systems are still simple. For example, photo-synaptic arrays can only recognize and memorize simple numbers and images, and cannot process large amounts of dynamic information in real time. Achieving more intelligent simulation requires more complex device structures and integration, which is a big challenge for neurosynaptic devices. Third, as people's expectation for human–computer interaction and the internet of things becomes stronger, people hope that neurosynaptic devices can be attached to the human body and even replace the human sense organs in the future. Therefore, biocompatible, flexible and stretchable synaptic devices are required for future developments. However, most materials with excellent performance are inorganic materials, but their applications are limited by toxicity and rigidity. Although organic materials have many advantages such as tunable energy levels, diverse structures, and flexibility, there are many problems that need to be resolved in terms of stability and electrical properties. Fourth, one of the ultimate goals of neurosynaptic devices is to achieve the integration of sensing, storage and computing, which means the process of collection, conversion, storage, calculation and feedback of multiple signals. Therefore, the most convenient and feasible method is to construct sensors, actuators and memristors separately and connect them with a network, so that they can make full use of their superiorities respectively. Nevertheless, the selection of materials, the construction of devices, and the effective connection between various components still requires further exploration.
The artificial perception systems are currently in the initial stage of imitating the peripheral nervous system. In the future, we will engage in the development of multi-functional and complex artificial perception systems. Through the combination with the fields of chemistry, materials engineering, computer science, and medicine, the synaptic electronics will be widely used and contribute to the development of interdisciplinary fields.
Acknowledgements
The authors acknowledge the financial support from the National Key R&D Program of China (Grant No. 2018YFA0703200), the National Natural Science Foundation of China (Grant Nos. 91833304, 91833306, 21922511, 61890940, 21633012, and 51873216), the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB30000000), the CAS Key Research Program of Frontier Sciences (Grant No. QYZDYSSW-SLH029) and the CAS-Croucher Funding Scheme for Joint Laboratories.
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