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Volume 42, Issue 1, Jan 2021
Special Issue on Beyond Moore: Resistive Switching Devices for Emerging Memory and Neuromorphic Computing
RESEARCH HIGHLIGHTS
Ion migration in perovskite solar cells
Xiaoxue Ren, Lixiu Zhang, Yongbo Yuan, Liming Ding
J. Semicond.  2021, 42(1): 010201  doi: 10.1088/1674-4926/42/1/010201

COMMENTS AND OPINIONS
Embracing the era of neuromorphic computing
Yanghao Wang, Yuchao Yang, Yue Hao, Ru Huang
J. Semicond.  2021, 42(1): 010301  doi: 10.1088/1674-4926/42/1/010301

A pioneer in magnetic semiconductors — Professor Stephan von Molnár
Jianhua Zhao, Yongqing Li, Peng Xiong
J. Semicond.  2021, 42(1): 010302  doi: 10.1088/1674-4926/42/1/010302

SHORT COMMUNICATION
A chlorinated copolymer donor demonstrates a 18.13% power conversion efficiency
Jianqiang Qin, Lixiu Zhang, Chuantian Zuo, Zuo Xiao, Yongbo Yuan, Shangfeng Yang, Feng Hao, Ming Cheng, Kuan Sun, Qinye Bao, Zhengyang Bin, Zhiwen Jin, Liming Ding
J. Semicond.  2021, 42(1): 010501  doi: 10.1088/1674-4926/42/1/010501

D18, an eximious solar polymer!
Ke Jin, Zuo Xiao, Liming Ding
J. Semicond.  2021, 42(1): 010502  doi: 10.1088/1674-4926/42/1/010502

EDITORIAL
Preface to the Special Issue on Beyond Moore: Resistive Switching Devices for Emerging Memory and Neuromorphic Computing
Yue Hao, Huaqiang Wu, Yuchao Yang, Qi Liu, Xiao Gong, Genquan Han, Ming Li
J. Semicond.  2021, 42(1): 010101  doi: 10.1088/1674-4926/42/1/010101

REVIEWS
Towards engineering in memristors for emerging memory and neuromorphic computing: A review
Andrey S. Sokolov, Haider Abbas, Yawar Abbas, Changhwan Choi
J. Semicond.  2021, 42(1): 013101  doi: 10.1088/1674-4926/42/1/013101

Resistive random-access memory (RRAM), also known as memristors, having a very simple device structure with two terminals, fulfill almost all of the fundamental requirements of volatile memory, nonvolatile memory, and neuromorphic characteristics. Its memory and neuromorphic behaviors are currently being explored in relation to a range of materials, such as biological materials, perovskites, 2D materials, and transition metal oxides. In this review, we discuss the different electrical behaviors exhibited by RRAM devices based on these materials by briefly explaining their corresponding switching mechanisms. We then discuss emergent memory technologies using memristors, together with its potential neuromorphic applications, by elucidating the different material engineering techniques used during device fabrication to improve the memory and neuromorphic performance of devices, in areas such as ION/IOFF ratio, endurance, spike time-dependent plasticity (STDP), and paired-pulse facilitation (PPF), among others. The emulation of essential biological synaptic functions realized in various switching materials, including inorganic metal oxides and new organic materials, as well as diverse device structures such as single-layer and multilayer hetero-structured devices, and crossbar arrays, is analyzed in detail. Finally, we discuss current challenges and future prospects for the development of inorganic and new materials-based memristors.

Resistive random-access memory (RRAM), also known as memristors, having a very simple device structure with two terminals, fulfill almost all of the fundamental requirements of volatile memory, nonvolatile memory, and neuromorphic characteristics. Its memory and neuromorphic behaviors are currently being explored in relation to a range of materials, such as biological materials, perovskites, 2D materials, and transition metal oxides. In this review, we discuss the different electrical behaviors exhibited by RRAM devices based on these materials by briefly explaining their corresponding switching mechanisms. We then discuss emergent memory technologies using memristors, together with its potential neuromorphic applications, by elucidating the different material engineering techniques used during device fabrication to improve the memory and neuromorphic performance of devices, in areas such as ION/IOFF ratio, endurance, spike time-dependent plasticity (STDP), and paired-pulse facilitation (PPF), among others. The emulation of essential biological synaptic functions realized in various switching materials, including inorganic metal oxides and new organic materials, as well as diverse device structures such as single-layer and multilayer hetero-structured devices, and crossbar arrays, is analyzed in detail. Finally, we discuss current challenges and future prospects for the development of inorganic and new materials-based memristors.
A review of in situ transmission electron microscopy study on the switching mechanism and packaging reliability in non-volatile memory
Xin Yang, Chen Luo, Xiyue Tian, Fang Liang, Yin Xia, Xinqian Chen, Chaolun Wang, Steve Xin Liang, Xing Wu, Junhao Chu
J. Semicond.  2021, 42(1): 013102  doi: 10.1088/1674-4926/42/1/013102

Non-volatile memory (NVM) devices with non-volatility and low power consumption properties are important in the data storage field. The switching mechanism and packaging reliability issues in NVMs are of great research interest. The switching process in NVM devices accompanied by the evolution of microstructure and composition is fast and subtle. Transmission electron microscopy (TEM) with high spatial resolution and versatile external fields is widely used in analyzing the evolution of morphology, structures and chemical compositions at atomic scale. The various external stimuli, such as thermal, electrical, mechanical, optical and magnetic fields, provide a platform to probe and engineer NVM devices inside TEM in real-time. Such advanced technologies make it possible for an in situ and interactive manipulation of NVM devices without sacrificing the resolution. This technology facilitates the exploration of the intrinsic structure-switching mechanism of NVMs and the reliability issues in the memory package. In this review, the evolution of the functional layers in NVM devices characterized by the advanced in situ TEM technology is introduced, with intermetallic compounds forming and degradation process investigated. The principles and challenges of TEM technology on NVM device study are also discussed.

Non-volatile memory (NVM) devices with non-volatility and low power consumption properties are important in the data storage field. The switching mechanism and packaging reliability issues in NVMs are of great research interest. The switching process in NVM devices accompanied by the evolution of microstructure and composition is fast and subtle. Transmission electron microscopy (TEM) with high spatial resolution and versatile external fields is widely used in analyzing the evolution of morphology, structures and chemical compositions at atomic scale. The various external stimuli, such as thermal, electrical, mechanical, optical and magnetic fields, provide a platform to probe and engineer NVM devices inside TEM in real-time. Such advanced technologies make it possible for an in situ and interactive manipulation of NVM devices without sacrificing the resolution. This technology facilitates the exploration of the intrinsic structure-switching mechanism of NVMs and the reliability issues in the memory package. In this review, the evolution of the functional layers in NVM devices characterized by the advanced in situ TEM technology is introduced, with intermetallic compounds forming and degradation process investigated. The principles and challenges of TEM technology on NVM device study are also discussed.
Electrolyte-gated transistors for neuromorphic applications
Heyi Huang, Chen Ge, Zhuohui Liu, Hai Zhong, Erjia Guo, Meng He, Can Wang, Guozhen Yang, Kuijuan Jin
J. Semicond.  2021, 42(1): 013103  doi: 10.1088/1674-4926/42/1/013103

Von Neumann computers are currently failing to follow Moore’s law and are limited by the von Neumann bottleneck. To enhance computing performance, neuromorphic computing systems that can simulate the function of the human brain are being developed. Artificial synapses are essential electronic devices for neuromorphic architectures, which have the ability to perform signal processing and storage between neighboring artificial neurons. In recent years, electrolyte-gated transistors (EGTs) have been seen as promising devices in imitating synaptic dynamic plasticity and neuromorphic applications. Among the various electronic devices, EGT-based artificial synapses offer the benefits of good stability, ultra-high linearity and repeated cyclic symmetry, and can be constructed from a variety of materials. They also spatially separate “read” and “write” operations. In this article, we provide a review of the recent progress and major trends in the field of electrolyte-gated transistors for neuromorphic applications. We introduce the operation mechanisms of electric-double-layer and the structure of EGT-based artificial synapses. Then, we review different types of channels and electrolyte materials for EGT-based artificial synapses. Finally, we review the potential applications in biological functions.

Von Neumann computers are currently failing to follow Moore’s law and are limited by the von Neumann bottleneck. To enhance computing performance, neuromorphic computing systems that can simulate the function of the human brain are being developed. Artificial synapses are essential electronic devices for neuromorphic architectures, which have the ability to perform signal processing and storage between neighboring artificial neurons. In recent years, electrolyte-gated transistors (EGTs) have been seen as promising devices in imitating synaptic dynamic plasticity and neuromorphic applications. Among the various electronic devices, EGT-based artificial synapses offer the benefits of good stability, ultra-high linearity and repeated cyclic symmetry, and can be constructed from a variety of materials. They also spatially separate “read” and “write” operations. In this article, we provide a review of the recent progress and major trends in the field of electrolyte-gated transistors for neuromorphic applications. We introduce the operation mechanisms of electric-double-layer and the structure of EGT-based artificial synapses. Then, we review different types of channels and electrolyte materials for EGT-based artificial synapses. Finally, we review the potential applications in biological functions.
Multiply accumulate operations in memristor crossbar arrays for analog computing
Jia Chen, Jiancong Li, Yi Li, Xiangshui Miao
J. Semicond.  2021, 42(1): 013104  doi: 10.1088/1674-4926/42/1/013104

Memristors are now becoming a prominent candidate to serve as the building blocks of non-von Neumann in-memory computing architectures. By mapping analog numerical matrices into memristor crossbar arrays, efficient multiply accumulate operations can be performed in a massively parallel fashion using the physics mechanisms of Ohm’s law and Kirchhoff’s law. In this brief review, we present the recent progress in two niche applications: neural network accelerators and numerical computing units, mainly focusing on the advances in hardware demonstrations. The former one is regarded as soft computing since it can tolerant some degree of the device and array imperfections. The acceleration of multiple layer perceptrons, convolutional neural networks, generative adversarial networks, and long short-term memory neural networks are described. The latter one is hard computing because the solving of numerical problems requires high-precision devices. Several breakthroughs in memristive equation solvers with improved computation accuracies are highlighted. Besides, other nonvolatile devices with the capability of analog computing are also briefly introduced. Finally, we conclude the review with discussions on the challenges and opportunities for future research toward realizing memristive analog computing machines.

Memristors are now becoming a prominent candidate to serve as the building blocks of non-von Neumann in-memory computing architectures. By mapping analog numerical matrices into memristor crossbar arrays, efficient multiply accumulate operations can be performed in a massively parallel fashion using the physics mechanisms of Ohm’s law and Kirchhoff’s law. In this brief review, we present the recent progress in two niche applications: neural network accelerators and numerical computing units, mainly focusing on the advances in hardware demonstrations. The former one is regarded as soft computing since it can tolerant some degree of the device and array imperfections. The acceleration of multiple layer perceptrons, convolutional neural networks, generative adversarial networks, and long short-term memory neural networks are described. The latter one is hard computing because the solving of numerical problems requires high-precision devices. Several breakthroughs in memristive equation solvers with improved computation accuracies are highlighted. Besides, other nonvolatile devices with the capability of analog computing are also briefly introduced. Finally, we conclude the review with discussions on the challenges and opportunities for future research toward realizing memristive analog computing machines.
Neuromorphic vision sensors: Principle, progress and perspectives
Fuyou Liao, Feichi Zhou, Yang Chai
J. Semicond.  2021, 42(1): 013105  doi: 10.1088/1674-4926/42/1/013105

Conventional frame-based image sensors suffer greatly from high energy consumption and latency. Mimicking neurobiological structures and functionalities of the retina provides a promising way to build a neuromorphic vision sensor with highly efficient image processing. In this review article, we will start with a brief introduction to explain the working mechanism and the challenges of conventional frame-based image sensors, and introduce the structure and functions of biological retina. In the main section, we will overview recent developments in neuromorphic vision sensors, including the silicon retina based on conventional Si CMOS digital technologies, and the neuromorphic vision sensors with the implementation of emerging devices. Finally, we will provide a brief outline of the prospects and outlook for the development of this field.

Conventional frame-based image sensors suffer greatly from high energy consumption and latency. Mimicking neurobiological structures and functionalities of the retina provides a promising way to build a neuromorphic vision sensor with highly efficient image processing. In this review article, we will start with a brief introduction to explain the working mechanism and the challenges of conventional frame-based image sensors, and introduce the structure and functions of biological retina. In the main section, we will overview recent developments in neuromorphic vision sensors, including the silicon retina based on conventional Si CMOS digital technologies, and the neuromorphic vision sensors with the implementation of emerging devices. Finally, we will provide a brief outline of the prospects and outlook for the development of this field.
ARTICLES
Study of short-term synaptic plasticity in Ion-Gel gated graphene electric-double-layer synaptic transistors
Chenrong Gong, Lin Chen, Weihua Liu, Guohe Zhang
J. Semicond.  2021, 42(1): 014101  doi: 10.1088/1674-4926/42/1/014101

Multi-terminal electric-double-layer transistors have recently attracted extensive interest in terms of mimicking synaptic and neural functions. In this work, an Ion-Gel gated graphene synaptic transistor was proposed to mimic the essential synaptic behaviors by exploiting the bipolar property of graphene and the ionic conductivity of Ion-Gel. The Ion-Gel dielectrics were deposited onto the graphene film by the spin coating process. We consider the top gate and graphene channel as a presynaptic and postsynaptic terminal, respectively. Basic synaptic functions were successfully mimicked, including the excitatory postsynaptic current (EPSC), the effect of spike amplitude and duration on EPSC, and paired-pulse facilitation (PPF). This work may facilitate the application of graphene synaptic transistors in flexible electronics.

Multi-terminal electric-double-layer transistors have recently attracted extensive interest in terms of mimicking synaptic and neural functions. In this work, an Ion-Gel gated graphene synaptic transistor was proposed to mimic the essential synaptic behaviors by exploiting the bipolar property of graphene and the ionic conductivity of Ion-Gel. The Ion-Gel dielectrics were deposited onto the graphene film by the spin coating process. We consider the top gate and graphene channel as a presynaptic and postsynaptic terminal, respectively. Basic synaptic functions were successfully mimicked, including the excitatory postsynaptic current (EPSC), the effect of spike amplitude and duration on EPSC, and paired-pulse facilitation (PPF). This work may facilitate the application of graphene synaptic transistors in flexible electronics.
Voltage-dependent plasticity and image Boolean operations realized in a WOx-based memristive synapse
Jiajuan Shi, Ya Lin, Tao Zeng, Zhongqiang Wang, Xiaoning Zhao, Haiyang Xu, Yichun Liu
J. Semicond.  2021, 42(1): 014102  doi: 10.1088/1674-4926/42/1/014102

The development of electronic devices that possess the functionality of biological synapses is a crucial step towards neuromorphic computing. In this work, we present a WOx-based memristive device that can emulate voltage-dependent synaptic plasticity. By adjusting the amplitude of the applied voltage, we were able to reproduce short-term plasticity (STP) and the transition from STP to long-term potentiation. The stimulation with high intensity induced long-term enhancement of conductance without any decay process, thus representing a permanent memory behavior. Moreover, the image Boolean operations (including intersection, subtraction, and union) were also demonstrated in the memristive synapse array based on the above voltage-dependent plasticity. The experimental achievements of this study provide a new insight into the successful mimicry of essential characteristics of synaptic behaviors.

The development of electronic devices that possess the functionality of biological synapses is a crucial step towards neuromorphic computing. In this work, we present a WOx-based memristive device that can emulate voltage-dependent synaptic plasticity. By adjusting the amplitude of the applied voltage, we were able to reproduce short-term plasticity (STP) and the transition from STP to long-term potentiation. The stimulation with high intensity induced long-term enhancement of conductance without any decay process, thus representing a permanent memory behavior. Moreover, the image Boolean operations (including intersection, subtraction, and union) were also demonstrated in the memristive synapse array based on the above voltage-dependent plasticity. The experimental achievements of this study provide a new insight into the successful mimicry of essential characteristics of synaptic behaviors.

Neutron irradiation-induced effects on the reliability performance of electrochemical metallization memory devices
Ye Tao, Xuhong Li, Zhongqiang Wang, Gang Li, Haiyang Xu, Xiaoning Zhao, Ya Lin, Yichun Liu
J. Semicond.  2021, 42(1): 014103  doi: 10.1088/1674-4926/42/1/014103

In this work, electrochemical metallization memory (ECM) devices with an Ag/AgInSbTe (AIST)/amorphous carbon (a-C)/Pt structure were irradiated with 14 MeV neutrons. The switching reliability performance before and after neutron irradiation was compared and analyzed in detail. The results show that the irradiated memory cells functioned properly, and the initial resistance, the resistance at the low-resistance state (LRS), the RESET voltage and the data retention performance showed little degradation even when the total neutron fluence was as high as 2.5 × 1011 n/cm2. Other switching characteristics such as the forming voltage, the resistance at the high-resistance state (HRS), and the SET voltage were also studied, all of which merely showed a slight parameter drift. Irradiation-induced Ag ions doping of the a-C layer is proposed to explain the damaging effects of neutron irradiation. The excellent hard characteristics of these Ag/AIST/a-C/Pt-based ECM devices suggest potential beneficial applications in the aerospace and nuclear industries.

In this work, electrochemical metallization memory (ECM) devices with an Ag/AgInSbTe (AIST)/amorphous carbon (a-C)/Pt structure were irradiated with 14 MeV neutrons. The switching reliability performance before and after neutron irradiation was compared and analyzed in detail. The results show that the irradiated memory cells functioned properly, and the initial resistance, the resistance at the low-resistance state (LRS), the RESET voltage and the data retention performance showed little degradation even when the total neutron fluence was as high as 2.5 × 1011 n/cm2. Other switching characteristics such as the forming voltage, the resistance at the high-resistance state (HRS), and the SET voltage were also studied, all of which merely showed a slight parameter drift. Irradiation-induced Ag ions doping of the a-C layer is proposed to explain the damaging effects of neutron irradiation. The excellent hard characteristics of these Ag/AIST/a-C/Pt-based ECM devices suggest potential beneficial applications in the aerospace and nuclear industries.