Review Articles
Complementary memtransistors for neuromorphic computing: How, what and why
Qi Chen, Yue Zhou, Weiwei Xiong, Zirui Chen, Yasai Wang, Xiangshui Miao, Yuhui He
J. Semicond.  2024, 45(6): 061701  doi: 10.1088/1674-4926/23120051

Memtransistors in which the source−drain channel conductance can be nonvolatilely manipulated through the gate signals have emerged as promising components for implementing neuromorphic computing. On the other side, it is known that the complementary metal-oxide-semiconductor (CMOS) field effect transistors have played the fundamental role in the modern integrated circuit technology. Therefore, will complementary memtransistors (CMT) also play such a role in the future neuromorphic circuits and chips? In this review, various types of materials and physical mechanisms for constructing CMT (how) are inspected with their merits and need-to-address challenges discussed. Then the unique properties (what) and potential applications of CMT in different learning algorithms/scenarios of spiking neural networks (why) are reviewed, including supervised rule, reinforcement one, dynamic vision with in-sensor computing, etc. Through exploiting the complementary structure-related novel functions, significant reduction of hardware consuming, enhancement of energy/efficiency ratio and other advantages have been gained, illustrating the alluring prospect of design technology co-optimization (DTCO) of CMT towards neuromorphic computing.

Memtransistors in which the source−drain channel conductance can be nonvolatilely manipulated through the gate signals have emerged as promising components for implementing neuromorphic computing. On the other side, it is known that the complementary metal-oxide-semiconductor (CMOS) field effect transistors have played the fundamental role in the modern integrated circuit technology. Therefore, will complementary memtransistors (CMT) also play such a role in the future neuromorphic circuits and chips? In this review, various types of materials and physical mechanisms for constructing CMT (how) are inspected with their merits and need-to-address challenges discussed. Then the unique properties (what) and potential applications of CMT in different learning algorithms/scenarios of spiking neural networks (why) are reviewed, including supervised rule, reinforcement one, dynamic vision with in-sensor computing, etc. Through exploiting the complementary structure-related novel functions, significant reduction of hardware consuming, enhancement of energy/efficiency ratio and other advantages have been gained, illustrating the alluring prospect of design technology co-optimization (DTCO) of CMT towards neuromorphic computing.
Flexible perovskite light-emitting diodes for display applications and beyond
Yongqi Zhang, Shahbaz Ahmed Khan, Dongxiang Luo, Guijun Li
J. Semicond.  2024, 45(5): 051601  doi: 10.1088/1674-4926/45/5/051601

The flexible perovskite light-emitting diodes (FPeLEDs), which can be expediently integrated to portable and wearable devices, have shown great potential in various applications. The FPeLEDs inherit the unique optical properties of metal halide perovskites, such as tunable bandgap, narrow emission linewidth, high photoluminescence quantum yield, and particularly, the soft nature of lattice. At present, substantial efforts have been made for FPeLEDs with encouraging external quantum efficiency (EQE) of 24.5%. Herein, we summarize the recent progress in FPeLEDs, focusing on the strategy developed for perovskite emission layers and flexible electrodes to facilitate the optoelectrical and mechanical performance. In addition, we present relevant applications of FPeLEDs in displays and beyond. Finally, perspective toward the future development and applications of flexible PeLEDs are also discussed.

The flexible perovskite light-emitting diodes (FPeLEDs), which can be expediently integrated to portable and wearable devices, have shown great potential in various applications. The FPeLEDs inherit the unique optical properties of metal halide perovskites, such as tunable bandgap, narrow emission linewidth, high photoluminescence quantum yield, and particularly, the soft nature of lattice. At present, substantial efforts have been made for FPeLEDs with encouraging external quantum efficiency (EQE) of 24.5%. Herein, we summarize the recent progress in FPeLEDs, focusing on the strategy developed for perovskite emission layers and flexible electrodes to facilitate the optoelectrical and mechanical performance. In addition, we present relevant applications of FPeLEDs in displays and beyond. Finally, perspective toward the future development and applications of flexible PeLEDs are also discussed.
Recent advances in two-dimensional photovoltaic devices
Haoyun Wang, Xingyu Song, Zexin Li, Dongyan Li, Xiang Xu, Yunxin Chen, Pengbin Liu, Xing Zhou, Tianyou Zhai
J. Semicond.  2024, 45(5): 051701  doi: 10.1088/1674-4926/45/5/051701

Two-dimensional (2D) materials have attracted tremendous interest in view of the outstanding optoelectronic properties, showing new possibilities for future photovoltaic devices toward high performance, high specific power and flexibility. In recent years, substantial works have focused on 2D photovoltaic devices, and great progress has been achieved. Here, we present the review of recent advances in 2D photovoltaic devices, focusing on 2D-material-based Schottky junctions, homojunctions, 2D−2D heterojunctions, 2D−3D heterojunctions, and bulk photovoltaic effect devices. Furthermore, advanced strategies for improving the photovoltaic performances are demonstrated in detail. Finally, conclusions and outlooks are delivered, providing a guideline for the further development of 2D photovoltaic devices.

Two-dimensional (2D) materials have attracted tremendous interest in view of the outstanding optoelectronic properties, showing new possibilities for future photovoltaic devices toward high performance, high specific power and flexibility. In recent years, substantial works have focused on 2D photovoltaic devices, and great progress has been achieved. Here, we present the review of recent advances in 2D photovoltaic devices, focusing on 2D-material-based Schottky junctions, homojunctions, 2D−2D heterojunctions, 2D−3D heterojunctions, and bulk photovoltaic effect devices. Furthermore, advanced strategies for improving the photovoltaic performances are demonstrated in detail. Finally, conclusions and outlooks are delivered, providing a guideline for the further development of 2D photovoltaic devices.
Light-emitting devices based on atomically thin MoSe2
Xinyu Zhang, Xuewen Zhang, Hanwei Hu, Vanessa Li Zhang, Weidong Xiao, Guangchao Shi, Jingyuan Qiao, Nan Huang, Ting Yu, Jingzhi Shang
J. Semicond.  2024, 45(4): 041701  doi: 10.1088/1674-4926/45/4/041701

Atomically thin MoSe2 layers, as a core member of the transition metal dichalcogenides (TMDs) family, benefit from their appealing properties, including tunable band gaps, high exciton binding energies, and giant oscillator strengths, thus providing an intriguing platform for optoelectronic applications of light-emitting diodes (LEDs), field-effect transistors (FETs), single-photon emitters (SPEs), and coherent light sources (CLSs). Moreover, these MoSe2 layers can realize strong excitonic emission in the near-infrared wavelengths, which can be combined with the silicon-based integration technologies and further encourage the development of the new generation technologies of on-chip optical interconnection, quantum computing, and quantum information processing. Herein, we overview the state-of-the-art applications of light-emitting devices based on two-dimensional MoSe2 layers. Firstly, we introduce recent developments in excitonic emission features from atomically thin MoSe2 and their dependences on typical physical fields. Next, we focus on the exciton-polaritons and plasmon-exciton polaritons in MoSe2 coupled to the diverse forms of optical microcavities. Then, we highlight the promising applications of LEDs, SPEs, and CLSs based on MoSe2 and their heterostructures. Finally, we summarize the challenges and opportunities for high-quality emission of MoSe2 and high-performance light-emitting devices.

Atomically thin MoSe2 layers, as a core member of the transition metal dichalcogenides (TMDs) family, benefit from their appealing properties, including tunable band gaps, high exciton binding energies, and giant oscillator strengths, thus providing an intriguing platform for optoelectronic applications of light-emitting diodes (LEDs), field-effect transistors (FETs), single-photon emitters (SPEs), and coherent light sources (CLSs). Moreover, these MoSe2 layers can realize strong excitonic emission in the near-infrared wavelengths, which can be combined with the silicon-based integration technologies and further encourage the development of the new generation technologies of on-chip optical interconnection, quantum computing, and quantum information processing. Herein, we overview the state-of-the-art applications of light-emitting devices based on two-dimensional MoSe2 layers. Firstly, we introduce recent developments in excitonic emission features from atomically thin MoSe2 and their dependences on typical physical fields. Next, we focus on the exciton-polaritons and plasmon-exciton polaritons in MoSe2 coupled to the diverse forms of optical microcavities. Then, we highlight the promising applications of LEDs, SPEs, and CLSs based on MoSe2 and their heterostructures. Finally, we summarize the challenges and opportunities for high-quality emission of MoSe2 and high-performance light-emitting devices.
Development of in situ characterization techniques in molecular beam epitaxy
Chao Shen, Wenkang Zhan, Manyang Li, Zhenyu Sun, Jian Tang, Zhaofeng Wu, Chi Xu, Bo Xu, Chao Zhao, Zhanguo Wang
J. Semicond.  2024, 45(3): 031301  doi: 10.1088/1674-4926/45/3/031301

Ex situ characterization techniques in molecular beam epitaxy (MBE) have inherent limitations, such as being prone to sample contamination and unstable surfaces during sample transfer from the MBE chamber. In recent years, the need for improved accuracy and reliability in measurement has driven the increasing adoption of in situ characterization techniques. These techniques, such as reflection high-energy electron diffraction, scanning tunneling microscopy, and X-ray photoelectron spectroscopy, allow direct observation of film growth processes in real time without exposing the sample to air, hence offering insights into the growth mechanisms of epitaxial films with controlled properties. By combining multiple in situ characterization techniques with MBE, researchers can better understand film growth processes, realizing novel materials with customized properties and extensive applications. This review aims to overview the benefits and achievements of in situ characterization techniques in MBE and their applications for material science research. In addition, through further analysis of these techniques regarding their challenges and potential solutions, particularly highlighting the assistance of machine learning to correlate in situ characterization with other material information, we hope to provide a guideline for future efforts in the development of novel monitoring and control schemes for MBE growth processes with improved material properties.

Ex situ characterization techniques in molecular beam epitaxy (MBE) have inherent limitations, such as being prone to sample contamination and unstable surfaces during sample transfer from the MBE chamber. In recent years, the need for improved accuracy and reliability in measurement has driven the increasing adoption of in situ characterization techniques. These techniques, such as reflection high-energy electron diffraction, scanning tunneling microscopy, and X-ray photoelectron spectroscopy, allow direct observation of film growth processes in real time without exposing the sample to air, hence offering insights into the growth mechanisms of epitaxial films with controlled properties. By combining multiple in situ characterization techniques with MBE, researchers can better understand film growth processes, realizing novel materials with customized properties and extensive applications. This review aims to overview the benefits and achievements of in situ characterization techniques in MBE and their applications for material science research. In addition, through further analysis of these techniques regarding their challenges and potential solutions, particularly highlighting the assistance of machine learning to correlate in situ characterization with other material information, we hope to provide a guideline for future efforts in the development of novel monitoring and control schemes for MBE growth processes with improved material properties.
Progress in efficient doping of Al-rich AlGaN
Jiaming Wang, Fujun Xu, Lisheng Zhang, Jing Lang, Xuzhou Fang, Ziyao Zhang, Xueqi Guo, Chen Ji, Chengzhi Ji, Fuyun Tan, Xuelin Yang, Xiangning Kang, Zhixin Qin, Ning Tang, Xinqiang Wang, Weikun Ge, Bo Shen
J. Semicond.  2024, 45(2): 021501  doi: 10.1088/1674-4926/45/2/021501

The development of semiconductors is always accompanied by the progress in controllable doping techniques. Taking AlGaN-based ultraviolet (UV) emitters as an example, despite a peak wall-plug efficiency of 15.3% at the wavelength of 275 nm, there is still a huge gap in comparison with GaN-based visible light-emitting diodes (LEDs), mainly attributed to the inefficient doping of AlGaN with increase of the Al composition. First, p-doping of Al-rich AlGaN is a long-standing challenge and the low hole concentration seriously restricts the carrier injection efficiency. Although p-GaN cladding layers are widely adopted as a compromise, the high injection barrier of holes as well as the inevitable loss of light extraction cannot be neglected. While in terms of n-doping the main issue is the degradation of the electrical property when the Al composition exceeds 80%, resulting in a low electrical efficiency in sub-250 nm UV-LEDs. This review summarizes the recent advances and outlines the major challenges in the efficient doping of Al-rich AlGaN, meanwhile the corresponding approaches pursued to overcome the doping issues are discussed in detail.

The development of semiconductors is always accompanied by the progress in controllable doping techniques. Taking AlGaN-based ultraviolet (UV) emitters as an example, despite a peak wall-plug efficiency of 15.3% at the wavelength of 275 nm, there is still a huge gap in comparison with GaN-based visible light-emitting diodes (LEDs), mainly attributed to the inefficient doping of AlGaN with increase of the Al composition. First, p-doping of Al-rich AlGaN is a long-standing challenge and the low hole concentration seriously restricts the carrier injection efficiency. Although p-GaN cladding layers are widely adopted as a compromise, the high injection barrier of holes as well as the inevitable loss of light extraction cannot be neglected. While in terms of n-doping the main issue is the degradation of the electrical property when the Al composition exceeds 80%, resulting in a low electrical efficiency in sub-250 nm UV-LEDs. This review summarizes the recent advances and outlines the major challenges in the efficient doping of Al-rich AlGaN, meanwhile the corresponding approaches pursued to overcome the doping issues are discussed in detail.
Research progress of alkaline earth metal iron-based oxides as anodes for lithium-ion batteries
Mingyuan Ye, Xiaorui Hao, Jinfeng Zeng, Lin Li, Pengfei Wang, Chenglin Zhang, Li Liu, Fanian Shi, Yuhan Wu
J. Semicond.  2024, 45(2): 021801  doi: 10.1088/1674-4926/45/2/021801

Anode materials are an essential part of lithium-ion batteries (LIBs), which determine the performance and safety of LIBs. Currently, graphite, as the anode material of commercial LIBs, is limited by its low theoretical capacity of 372 mA·h·g−1, thus hindering further development toward high-capacity and large-scale applications. Alkaline earth metal iron-based oxides are considered a promising candidate to replace graphite because of their low preparation cost, good thermal stability, superior stability, and high electrochemical performance. Nonetheless, many issues and challenges remain to be addressed. Herein, we systematically summarize the research progress of alkaline earth metal iron-based oxides as LIB anodes. Meanwhile, the material and structural properties, synthesis methods, electrochemical reaction mechanisms, and improvement strategies are introduced. Finally, existing challenges and future research directions are discussed to accelerate their practical application in commercial LIBs.

Anode materials are an essential part of lithium-ion batteries (LIBs), which determine the performance and safety of LIBs. Currently, graphite, as the anode material of commercial LIBs, is limited by its low theoretical capacity of 372 mA·h·g−1, thus hindering further development toward high-capacity and large-scale applications. Alkaline earth metal iron-based oxides are considered a promising candidate to replace graphite because of their low preparation cost, good thermal stability, superior stability, and high electrochemical performance. Nonetheless, many issues and challenges remain to be addressed. Herein, we systematically summarize the research progress of alkaline earth metal iron-based oxides as LIB anodes. Meanwhile, the material and structural properties, synthesis methods, electrochemical reaction mechanisms, and improvement strategies are introduced. Finally, existing challenges and future research directions are discussed to accelerate their practical application in commercial LIBs.
GaN based ultraviolet laser diodes
Jing Yang, Degang Zhao, Zongshun Liu, Yujie Huang, Baibin Wang, Xiaowei Wang, Yuheng Zhang, Zhenzhuo Zhang, Feng Liang, Lihong Duan, Hai Wang, Yongsheng Shi
J. Semicond.  2024, 45(1): 011501  doi: 10.1088/1674-4926/45/1/011501

In the past few years, many groups have focused on the research and development of GaN-based ultraviolet laser diodes (UV LDs). Great progresses have been achieved even though many challenges exist. In this article, we analyze the challenges of developing GaN-based ultraviolet laser diodes, and the approaches to improve the performance of ultraviolet laser diode are reviewed. With these techniques, room temperature (RT) pulsed oscillation of AlGaN UVA (ultraviolet A) LD has been realized, with a lasing wavelength of 357.9 nm. Combining with the suppression of thermal effect, the high output power of 3.8 W UV LD with a lasing wavelength of 386.5 nm was also fabricated.

In the past few years, many groups have focused on the research and development of GaN-based ultraviolet laser diodes (UV LDs). Great progresses have been achieved even though many challenges exist. In this article, we analyze the challenges of developing GaN-based ultraviolet laser diodes, and the approaches to improve the performance of ultraviolet laser diode are reviewed. With these techniques, room temperature (RT) pulsed oscillation of AlGaN UVA (ultraviolet A) LD has been realized, with a lasing wavelength of 357.9 nm. Combining with the suppression of thermal effect, the high output power of 3.8 W UV LD with a lasing wavelength of 386.5 nm was also fabricated.
A review of automatic detection of epilepsy based on EEG signals
Qirui Ren, Xiaofan Sun, Xiangqu Fu, Shuaidi Zhang, Yiyang Yuan, Hao Wu, Xiaoran Li, Xinghua Wang, Feng Zhang
J. Semicond.  2023, 44(12): 121401  doi: 10.1088/1674-4926/44/12/121401

Epilepsy is a common neurological disorder that occurs at all ages. Epilepsy not only brings physical pain to patients, but also brings a huge burden to the lives of patients and their families. At present, epilepsy detection is still achieved through the observation of electroencephalography (EEG) by medical staff. However, this process takes a long time and consumes energy, which will create a huge workload to medical staff. Therefore, it is particularly important to realize the automatic detection of epilepsy. This paper introduces, in detail, the overall framework of EEG-based automatic epilepsy identification and the typical methods involved in each step. Aiming at the core modules, that is, signal acquisition analog front end (AFE), feature extraction and classifier selection, method summary and theoretical explanation are carried out. Finally, the future research directions in the field of automatic detection of epilepsy are prospected.

Epilepsy is a common neurological disorder that occurs at all ages. Epilepsy not only brings physical pain to patients, but also brings a huge burden to the lives of patients and their families. At present, epilepsy detection is still achieved through the observation of electroencephalography (EEG) by medical staff. However, this process takes a long time and consumes energy, which will create a huge workload to medical staff. Therefore, it is particularly important to realize the automatic detection of epilepsy. This paper introduces, in detail, the overall framework of EEG-based automatic epilepsy identification and the typical methods involved in each step. Aiming at the core modules, that is, signal acquisition analog front end (AFE), feature extraction and classifier selection, method summary and theoretical explanation are carried out. Finally, the future research directions in the field of automatic detection of epilepsy are prospected.
A review on GaN HEMTs: nonlinear mechanisms and improvement methods
Chenglin Du, Ran Ye, Xiaolong Cai, Xiangyang Duan, Haijun Liu, Yu Zhang, Gang Qiu, Minhan Mi
J. Semicond.  2023, 44(12): 121801  doi: 10.1088/1674-4926/44/12/121801

The GaN HEMT is a potential candidate for RF applications due to the high frequency and large power handling capability. To ensure the quality of the communication signal, linearity is a key parameter during the system design. However, the GaN HEMT usually suffers from the nonlinearity problems induced by the nonlinear parasitic capacitance, transconductance, channel transconductance etc. Among them, the transconductance reduction is the main contributor for the nonlinearity and is mostly attributed to the scattering effect, the increasing resistance of access region, the self-heating effect and the trapping effects. Based on the mechanisms, device-level improvement methods of transconductance including the trapping suppression, the nanowire channel, the graded channel, the double channel, the transconductance compensation and the new material structures have been proposed recently. The features of each method are reviewed and compared to provide an overview perspective on the linearity of the GaN HEMT at the device level.

The GaN HEMT is a potential candidate for RF applications due to the high frequency and large power handling capability. To ensure the quality of the communication signal, linearity is a key parameter during the system design. However, the GaN HEMT usually suffers from the nonlinearity problems induced by the nonlinear parasitic capacitance, transconductance, channel transconductance etc. Among them, the transconductance reduction is the main contributor for the nonlinearity and is mostly attributed to the scattering effect, the increasing resistance of access region, the self-heating effect and the trapping effects. Based on the mechanisms, device-level improvement methods of transconductance including the trapping suppression, the nanowire channel, the graded channel, the double channel, the transconductance compensation and the new material structures have been proposed recently. The features of each method are reviewed and compared to provide an overview perspective on the linearity of the GaN HEMT at the device level.
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