Direct observation of the hysteretic Fermi level modulation in monolayer MoS2 field effect transistors

The hysteresis in the transfer curve of MoS₂ has significant impact on the device performance. However, the hysteresis mechanism is still not clear. Here, we investigate the hysteresis of the monolayer MoS₂ by probing the local Fermi level variations as a function of the back gate voltage in different atmosphere using the Kelvin probe microscopy. While the Fermi level of the MoS₂ in air is much lower than that in vacuum, both the MoS₂ devices in vacuum and air show large Fermi level hysteresis. The Fermi level hysteresis direction is clock-wise, identical to that observed in the transfer curves. Both the hysteresis in Fermi level and transfer curve can be explained consistently by taking into account the charge trapping. Our findings confirm that carrier density modulation in MoS₂ plays a vital role in the hysteresis, and provide insight into the hysteresis mechanism for the optimization of the device performance.     

An analytic model for gate-all-around silicon nanowire tunneling field effect transistors

An analytical model of gate-all-around(GAA) silicon nanowire tunneling field effect transistors(NW-TFETs) is developted based on the surface potential solutions in the channel direction and considering the band to band tunneling(BTBT) efficiency. The three-dimensional Poisson equation is solved to obtain the surface potential distributions in the partition regions along the channel direction for the NW-TFET, and a tunneling current model using Kane’s expression is developed. The validity of the developed model is shown by the good agreement between the model predictions and the TCAD simulation results.     

缺陷对单层MoS2电子结构的影响

为了研究缺陷对单层MoS2的电子结构, 本文基于密度泛函理论框架下的第一性原理, 采用数值基组的方法计算了MoS2的Mo位缺陷、S位缺陷的能带结构和态密度.结果发现：Mo位缺陷、S位缺陷的MoS2的能带结构中的价带顶与导带底都在Q点, 为直接带隙材料; 其中Mo位缺陷体的禁带区域都出现5条新能级, S位缺陷体的禁带区域出现了3条新能级; 缺陷体能带结构的能量下降与体系中未成键的电子有关.对于态密度而言, Mo位缺陷体的费米能级处出现了峰值, 表明Mo位缺陷会对其光电性质带来影响.同时分析电荷分布发现, Mo缺陷周围存在着负电荷聚集的现象, S缺陷周围存在正电荷聚集的现象.     

缺陷对单层MoS_2电子结构的影响

为了研究缺陷对单层MoS_2的电子结构,本文基于密度泛函理论框架下的第一性原理,采用数值基组的方法计算了MoS_2的Mo位缺陷、S位缺陷的能带结构和态密度.结果发现:Mo位缺陷、S位缺陷的MoS_2的能带结构中的价带顶与导带底都在Q点,为直接带隙材料;其中Mo位缺陷体的禁带区域都出现5条新能级,S位缺陷体的禁带区域出现了3条新能级;缺陷体能带结构的能量下降与体系中未成键的电子有关.对于态密度而言,Mo位缺陷体的费米能级处出现了峰值,表明Mo位缺陷会对其光电性质带来影响.同时分析电荷分布发现,Mo缺陷周围存在着负电荷聚集的现象,S缺陷周围存在正电荷聚集的现象.     

Petal-like MoS2 nanostructures with metallic 1 T phase for high performance supercapacitors

Herein, we report the metallic 1 T phase MoS₂ petal-like nanostructures (MP-LNs), synthesized by the solvothermal method, for applications in supercapacitor electrodes. X-ray photoelectron spectroscopy (XPS) verified the composition and distribution of Mo and S, illustrating that the 1 T metallic phase is predominant in the MP-LNs. Electrochemical analyses were performed to explore the supercapacitor applications of the MP-LN material, demonstrating a superior cyclic voltammetry (CV), high specific capacitance, good stability. MP-LN-based supercapacitors (MP-LNS) show high specific capacitances of 811 F/g and 400 F/g at current densities of 0.1 A/g and 10 A/g, respectively. The long-term cycling stability was also studied to investigate the reproducible nature of MP-LNS and was found to display excellent specific capacitance retention of 49.3% (at 0.1 A/g) and 82.7% (at 10 A/g) after 1000 charge-discharge cycles, which indicates good reversibility of the galvanostatic charge-discharge (GCD) of the electrode material. These findings highlight the potential use of MP-LNs in supercapacitors.     

Magnetotransport in a zigzag monolayer MoS2 nanoribbon with ferromagnetic electrodes

In this research, using the three-band tight-binding model combined with a non-equilibrium Green's function technique, spin-dependent electron transport was investigated in the quantum structure of zigzag monolayer molybdenum disulfide with ferromagnetic electrodes. It was shown that in parallel configurations, the conductance exhibited a quantized oscillating phenomenon, while in the antiparallel configurations with increasing magnetization, the conductance showed a zero platform in a large-energy region. We observed a giant magnetoresistance effect. Moreover, the length of the central part of the structure had a certain influence on the magnetoresistance ratio. It was found that as the length of the middle region increased, the magnetoresistance ratio decreased gradually. The results not only extended our understanding of novel electronic structures of monolayer MoS₂ but also provided the possibility for the technological applications of spintronics device.     

Co对单层MoS_2电子结构与磁性的影响

为了研究Co对单层MoS_2电子结构和磁性的影响,本文基于第一性原理,采用数值基组的方法计算了Co吸附式掺杂、Co替代式掺杂单层MoS_2的能带结构、态密度以及分析了其结构的稳定性.结果发现:Co替换式掺杂体系的形成能较低,实验上容易实现;Co在Mo位吸附的稳定性强于在S位吸附;Mo位吸附体系的总磁矩为0.999μB,其磁矩的主要来源于Co原子的吸附所贡献的0.984μB,Co原子的掺杂体系总磁矩为1.029μB,其磁矩的主要由Co原子替代掉一个Mo原子所贡献的磁矩为0.9444μB,相比于吸附体系,Co原子对磁矩的贡献率有所降低;无论是Co吸附在单层MoS_2表面还是Co直接替代掉Mo原子的掺杂体系,Co原子3d轨道的引入是引起单层MoS_2体系磁性的主要原因.     

Na、 Be、 Mg掺杂单层MoS_2的第一性原理研究

本文基于第一性原理研究了Na、 Be、 Mg掺杂单层MoS_2的稳定性、能带结构、态密度以及电荷分布.得到Be掺杂单层MoS_2体系在实验上较容易实现,在三者掺杂体系中稳定性最强.与此同时,掺杂体系的带隙值都降低,有利于电子的跃迁,增强了导电性能;掺杂原子打破了原体系的平衡关系,导致周边S原子p轨道上的多余的电子会与近邻Mo原子d轨道上的电子产生相互作用;平衡的打破,也导致了杂质原子周围存在着电荷聚集和损失的现象.     

Evaluation of thermal resistance constitution for packaged AlGaN/GaN high electron mobility transistors by structure function method

The evaluation of thermal resistance constitution for packaged AlGaN/GaN high electron mobility transistor (HEMT) by structure function method is proposed in this paper.The evaluation is based on the transient heating measurement of the AlGaN/GaN HEMT by pulsed electrical temperature sensitive parameter method.The extracted chip-level and package-level thermal resistances of the packaged multi-finger AlGaN/GaN HEMT with 400-μm SiC substrate are 22.5 K/W and 7.2 K/W respectively,which provides a non-invasive method to evaluate the chip-level thermal resistance of packaged AlGaN/GaN HEMTs.It is also experimentally proved that the extraction of the chiplevel thermal resistance by this proposed method is not influenced by package form of the tested device and temperature boundary condition of measurement stage.     

Optical modulation of the effective channel thickness in GaAs field effect transistors with a Schottky gate (MESFETs)

Using a model relating the change in drain current with modulation of the effective channel thickness upon IR illumination of GaAs field effect transistors with a Schottky gate (MESFETs), it is shown that it is possible to determine the change in channel thickness and the concentration profile for deep centers in an FET channel. Profiles are given for the distribution of deep centers in the channels of GaAs MESFETs with different noise temperatures. __________ Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 3, pp. 349–353, May–June, 2006.     

Model and analysis of drain induced barrier lowering effect for 4H--SiC metal semiconductor field effect transistor

Based on an analytical solution of the two-dimensional Poisson equation in the subthreshold region, this paper investigates the behavior of DIBL (drain induced barrier lowering) effect for short channel 4H--SiC metal semiconductor field effect transistors (MESFETs). An accurate analytical model of threshold voltage shift for the asymmetric short channel 4H--SiC MESFET is presented and thus verified. According to the presented model, it analyses the threshold voltage for short channel device on the L/a (channel length/channel depth) ratio, drain applied voltage V_DS and channel doping concentration N_D, thus providing a good basis for the design and modelling of short channel 4H--SiC MESFETs device.     

顶栅石墨烯离子敏场效应管的表征及其初步应用

传统的液栅型石墨烯场效应管虽然灵敏度高, 但是石墨烯沟道极易被污染, 致使器件的稳定性减小, 不能被重复利用. 为此, 我们设计制造了一种顶栅石墨烯离子敏场效应管, 以化学气相沉积生长的石墨烯为沟道, 通过原子层沉积在石墨烯表面沉积绝缘层HfO₂/Al₂O₃, 其中Al₂O₃作为敏感膜, HfO₂/Al₂O₃作为石墨烯及电极的保护膜. 经过一系列的电学表征和测试发现, 相较于液栅型石墨烯场效应管, 顶栅石墨烯场效应管具有更高的信噪比、更好的稳定性. 为了利用顶栅石墨烯进行生物分子的检测, 我们将单链DNA修饰在Al₂O₃表面, 成功检测到了修饰DNA前后的信号差异, 并结合荧光修饰的表征验证了顶栅石墨烯场效应管用于生物传感器的可行性.     

单层MoS_2表面吸附Ag_6团簇电子结构的第一性原理计算

本文运用第一性原理研究了单层MoS_2在S位吸附Ag_6团簇的稳定性、能带结构和态密度.结果表明,Ag_6团簇在S位单点位吸附的稳定性强于双点位吸附、三点位吸附;其中吸附体系禁带中产生了2条杂质能级,原因在于Ag原子与S形成共价键下的施主能级与受主能级;Ag_6团簇在单层MoS_2的吸附导致态密度峰值在费米能级处发生劈裂,说明Ag_6团簇的吸附会增强单层MoS_2的光电特性;单层MoS_2的能带结构可以通过表面吸附Ag_6团簇以及金属团簇进行调控;在实际的生产应用中依据不同的金属团簇吸附于单层MoS_2表面得到需要的的半导体器件.     

N2O处理对背沟刻蚀金属氧化物薄膜晶体管性能的影响

通过采用稀土元素镨掺杂铟锡锌氧化物半导体作为薄膜晶体管沟道层,成功实现了基于铝酸的湿法背沟道刻蚀薄膜晶体管的制备.研究了N₂O等离子体处理对薄膜晶体管背沟道界面的影响,对其处理功率和时间对器件性能的影响做了具体研究.结果表明,在一定的功率和时间处理下能获得良好的器件性能,所制备的器件具有良好的正向偏压热稳定性和光照条件下负向偏压热稳定性.高分辨透射电镜结果显示,该非晶结构的金属氧化物半导体材料可以有效抵抗铝酸的刻蚀,未发现明显的成分偏析现象.进一步的X射线光电能谱测试表明, N₂O等离子体处理能在界面处形成一个富氧、低载流子浓度的界面层.其一方面可以有效抵抗器件在沉积氧化硅钝化层时等离子体对背沟道的损伤;另一方面作为氢的钝化体,抑制了低能级施主态氢的产生,为低成本、高效的薄膜晶体管性能优化方式提供了重要参考.     

DC and analog/RF performance of C-shaped pocket TFET (CSP-TFET) with fully overlapping gate

A C-shaped pocket tunnel field effect transistor (CSP-TFET) has been designed and optimized based on the traditional double-gate TFETs by introducing a C-shaped pocket region between the source and channel to improve the device performance. A gate-to-pocket overlapping structure is also examined in the proposed CSP-TFET to enhance the gate controllability. The effects of the pocket length, pocket doping concentration and gate-to-pocket overlapping structure on the DC and analog/RF characteristics of the CSP-TFET are estimated after calibrating the tunneling model in double-gate TFETs. The DC and analog/RF performance such as on-state current (I_on), on/off current ratio (I_on/I_off), subthreshold swing (SS) transconductance (g_m), cut-off frequency (f_T) and gain-bandwidth product (GBP) are investigated. The optimized CSPTFET device exhibits excellent performance with high I_on (9.98×10 ^{- 4} A/μm), high I_on/I_off (～ 10¹¹), as well as low SS (～ 12 mV/dec). The results reveal that the CSP-TFET device could be a potential alternative for the next generation of semiconductor devices.     

Novel attributes of AlGaN/AlN/GaN/SiC HEMTs with the multiple indented channel

In this paper, a high performance AlGaN/AlN/GaN/SiC High Electron Mobility Transistor (HEMT) with the multiple indented channel (MIC-HEMT) is proposed. The main focus of the proposed structure is based on reduction of the space around the gate, stop of the spread of the depletion region around the source–drain, and decrement of the thickness of the channel between the gate and drain. Therefore, the breakdown voltage increases, meanwhile the elimination of the gate depletion layer extension to source/drain decreases the gate–source and gate–drain capacitances. The optimized results reveal that the breakdown voltage and the drain saturation current increase about 178% and 46% compared with a conventional HEMT (C-HEMT), respectively. Therefore, the maximum output power density is improved by factor 4.1 in comparison with conventional one. Also, the cut-off frequency of 25.2 GHz and the maximum oscillation frequency of 92.1 GHz for the MIC-HEMT are obtained compared to 13 GHz and 43 GHz for that of the C-HEMT and the minimum figure noise decreased consequently of reducing the gate–drain and gate–source capacitances by about 42% and 40%, respectively. The proposed MIC-HEMT shows a maximum stable gain (MSG) exceeding 24.1 dB at 3.1 GHz which the greatest gain is yet reported for HEMTs, showing the potential of this device for high power RF applications.     

Novel attributes in the performance and scaling effects of carbon nanotube field-effect transistors with halo doping

In this paper, we performed a comprehensive scaling study of a carbon nanotube field-effect transistor (CNTFET) with halo doping (HD) using self-consistent and atomistic scale simulations. Our simulation results demonstrate that drain induced barrier lowering (DIBL) diminishes in the HD-CNTFET due to a step in the potential of the CNT at the interface of p-doped and undoped regions in the channel. Also, the hot carrier effect minimizes with reduction of the peak of the electric field at the drain side of the HD-CNTFET. Moreover, the features of the HD-CNTFET can be controlled by the length and concentration engineering of the HD region. Leakage current, on–off current ratio and subthreshold swing improve with an increase of the length and concentration of the HD region, due to the increment of the threshold voltage and the barrier height of the p–n junction near the source. Therefore, this work can provide an incentive for further experimental exploration.     

双轴应变调控二维单层C2X2TM（X = O, H; TM=Fe, Cr） 的多铁性研究

基于密度泛函理论的第一性原理计算,系统地研究了过渡金属原子插层的单层氧化/氢化石墨烯的磁学性质和铁电性质.在考虑了电子在位库仑作用和自旋轨道耦合作用下,得到了过渡金属Fe、Cr插层形成的C₂X₂TM二维单层膜的稳定结构以及基态磁性结构,研究了不同应变作用下C₂X₂TM的磁性、能带、铁电极化以及电子结构的变化.结果发现,对于任何应变下的C₂X₂TM其基态磁性都为手性逆时针反铁磁结构.在无应变时体系存在一个较大的离子翻转势垒,通过外加双轴应变,可有效调控体系的势垒高度和能隙,发现25%应变下C₂O₂Cr和30%应变时C₂O₂Fe单层薄膜具有与GeS等二维铁电材料相近的铁电极化和翻转势垒,这些研究结果表明C₂O₂Fe(Cr)单层薄膜是一种新型二维多铁性材料.     

提高单层GeSe对H2O气体传感性能的第一性原理研究

采用第一性原理计算方法,通过在单层GeSe上施加双轴向应变、外加电场、掺Ag等途径来探索提高单层GeSe对H₂O分子传感性能的有效方法,并从微观角度阐明内在机理.计算结果表明,-1.0 V/?的外加电场能有效降低H₂O分子在单层GeSe的吸附能并使二者之间的电荷转移量增加11倍,显著提高了单层GeSe对H₂O分子的响应速度和敏感性.研究结果为进一步设计并制成二维GeSe基湿度传感器提供了理论依据.     

阶梯槽交错双栅慢波结构高频特性理论和模拟

本文基于一种阶梯槽交错双栅周期慢波结构模型,获得了该结构中的电磁场分布公式.通过场匹配方法,求出了该结构的高频色散方程和耦合阻抗表达式.以W波段行波管用的阶梯槽交错双栅为例,利用本文公式和CST-MWS电磁软件比较计算了色散和耦合阻抗特性,分析了阶梯尺寸参数对高频特性(基模色散、+1次空间谐波归一化相速和耦合阻抗)的影响.结果表明,理论和CST-MWS软件模拟有很好的一致性;相对矩形交错双栅,改善了色散特性,拓展了基模带宽,同时具有足够大的耦合阻抗和适合工程应用的机械强度,在一定程度上可以弥补矩形交错双栅周期慢波结构的不足.