A Quantum-Corrected Monte Carlo Study on Quasi-Ballistic Transport in Nanoscale MOSFETs

In this paper, the authors study a quasi-ballistic transport in nanoscale Si-MOSFETs based upon a quantum-corrected Monte Carlo device simulation to explore an ultimate device performance. It was found that, when a channel length becomes shorter than 30 nm, an average electron velocity at the source-end of the channel increases due to ballistic transport effects, and then, it approaches a ballistic limit in a sub-10-nm regime. Furthermore, the authors elucidated a physical mechanism creating an asymmetric momentum distribution function at the source-end of the channel and the influences of backscattering from the channel region. The authors also demonstrated that an electron injection velocity at a perfectly ballistic transport is independent of the channel length and corresponds well to a prediction from Natori's analytical model     

A flux-based study of carrier transport in thin-base diodes and transistors

Carrier transport in pn-junction is re-examined using McKelvey's flux method. A simple but physically based treatment of carrier transport leads to new expressions for the "law of the junction," quasi-Fermi level, I-V characteristics, base transit time, and probability of carrier backscattering from the space charge region, which are valid from the ballistic through the diffusive regimes. Comparison with Monte Carlo simulation shows that the deduced backscattering rate well describes the bias dependence. For silicon pn-junctions, the backscattering rate under reverse bias conditions is less than 5%, satisfying the Bethe condition of thermionic emission, while it rapidly increases with forward bias until drift-diffusion governs the transport. The effect of thin-base transport and backscattering on the current, carrier velocity, and distribution function is also investigated. It is found that for a base thickness less than 50 nm even silicon transistors enter the quasi-ballistic transport regime. These results should prove useful not only for fundamental understanding of the pn-junction transport, but also for careful design of advanced transistors.<>     

On the physical understanding of the kT-layer concept in quasi-ballistic regime of transport in nanoscale devices

The aim of this paper is to establish a well-defined theoretical background in which the kT-layer concept and the empirical expression of the backscattering coefficient introduced by Lundstrom and co-workers to describe the role of scattering in nanoscale devices under high-field conditions and quasi-ballistic transport regime are derived analytically. To this purpose, one-dimensional (1-D) Boltzmann transport equation is solved in the framework of a "relaxation length" approximation, leading to a set of drift-diffusion like transport equations. This set of equations is then solved in a template 1-D structure (with a linear potential energy profile), leading to an analytical expression for the backscattering coefficient, which is equal, in the limit of low- and high-electric fields, to the formulas proposed by Lundstrom This approach allows us to identify the exact assumptions and qualitative validity limits of these formulas.     

Scaling of strained-Si n-MOSFETs into the ballistic regime and associated anisotropic effects

The dependence of the strain-induced on-current improvement in n-MOSFETs on scaling and the crystallographic orientation of the channel is investigated by self-consistent full-band Monte Carlo simulation. For a channel orientation along the <110> direction, the enhancement decreases weakly from almost 40% to 30% as the effective gate length is reduced from 75 to 25 nm. For the <100> direction, the improvement is about 10% higher. The anisotropy of the drain current, which vanishes for small drain voltages, is attributed to the different band curvatures above 100 meV. This feature appears to be crucial for quasi-ballistic transport of the electrons in the high longitudinal field as they enter the source-side of the channel.     

Low-field mobility and carrier transport mechanism transition in nanoscale MOSFETs

This paper extends the flux scattering method to study the carrier transport property in nanoscale MOSFETs with special emphasis on the low-field mobility and the transport mechanism transition. A unified analytical expression for the low-field mobility is proposed, which covers the entire regime from drift-diffusion transport to quasi-ballistic transport in 1-D, 2-D and 3-D MOSFETs. Two key parameters, namely the long-channel low-field mobility (μ0) and the low-field mean free path (λ0), are obtained from the experimental data, and the transport mechanism transition in MOSFETs is further discussed both experimentally and theoretically. Our work shows that λ0 is available to characterize the inherent transition of the carrier transport mechanism rather than the low-field mobility. The mobility reduces in the MOSFET with the shrinking of the channel length; however, λ0 is nearly a constant, and λ0 can be used as the "entry criterion" to determine whether the device begins to operate under quasi-ballistic transport to some extent.     

Experimental Investigation on the Quasi-Ballistic Transport: Part II—Backscattering Coefficient Extraction and Link With the Mobility

Using a new extraction methodology taking into account multisubband population and carrier degeneracy, we have experimentally determined backscattering coefficients, ballistic ratios, and injection velocities of n- and p-FDSOI devices with gate lengths down to 30 nm in the saturated and, for the first time, in the linear regimes. The evolution of these quasi-ballistic parameters is examined as a function of the inversion charge in the channel and at temperatures ranging from 50 to 293 K, showing stronger ballistic ratios in the saturated regime than in the linear one. We particularly focus on the linear regime, and a model linking ballisticity ratios and effective mobility is proposed and validated experimentally for different gate lengths. According to the experimental evaluation of the device mean-free path and its evolution with both the inversion charge in the channel and the temperature, we investigate the mobility degradation with decreasing gate lengths, highlighting the importance of Coulomb scattering on this unexpected mobility behavior.      

Simulation of Silicon Nanowire Transistors Using Boltzmann Transport Equation Under Relaxation Time Approximation

An efficient approach for the simulation of electronic transport in nanoscale transistors is presented based on the multi-subband Boltzmann transport equation under the relaxation time approximation, which takes into account the effects of quantum confinement and quasi-ballistic transport. This approach is applied to the study of electronic transport in circular gate-all-around silicon nanowire transistors. Comparison with the nonequilibrium Green's function method shows that the new method gives reasonably accurate terminal characteristics. We study the influence of silicon body diameter and gate length on the terminal current and subthreshold slope (SS). We have found that the calculated ON current is inversely proportional to the gate length to the power 1/2, and that the silicon body diameter should be smaller than roughly 2/3 of the channel length in order to maintain the SS within 80 mV/dec.     

Influence of Elastic and Inelastic Phonon Scattering on the Drive Current of Quasi-Ballistic MOSFETs

In this paper, we study the influence of elastic and inelastic phonon scattering on the drive current of Si MOSFETs under quasi-ballistic transport. Inelastic phonon emission involving energy relaxation helps achieve ballistic current, even in the presence of scattering, if the channel length is scaled down to the 10-nm scale. This result agrees with Natori's previous predictions. However, for longer channel devices, inelastic phonon emission degrades the drain current due to space charge effects caused by charge accumulation. We also demonstrate that source-end potential engineering to electrically reduce the bottleneck barrier length can result in a ballistic current even in longer channel devices.      

Ballistic transport in high electron mobility transistors

A general ballistic FET model that was previously used for ballistic MOSFETs is applied to ballistic high electron mobility transistors (HEMTs), and the results are compared with experimental data for a sub-50 nm InAlAs-InGaAs HEMT. The results show that nanoscale HEMTs can be modeled as an intrinsic ballistic transistor with extrinsic source/drain series resistances. We also examine the "ballistic mobility" concept, a technique proposed for extending the drift-diffusion model to the quasi-ballistic regime. Comparison with a rigorous ballistic model shows that under low drain bias the ballistic mobility concept, although nonphysical, can be used to understand the experimental phenomena related to quasi-ballistic transport, such as the degradation of the apparent carrier mobility in short channel devices. We also point out that the ballistic mobility concept loses validity under high drain bias. The conclusions of this paper should be also applicable to other nanoscale transistors with high carrier mobility, such as carbon nanotube FETs and strained silicon MOSFETs.     

An explanation of the dependence of the effective saturation velocity on gate voltage in sub-0.1 μm metal-oxide-semiconductor transistors by quasi-ballistic transport theory

In 1992, Takagi and Toriumi reported that the electron saturation velocities decrease with the density of inversion charge in metal-oxide-semiconductor transistor test structures with effective channel length of 9.5-1.5 μm. Their results implied that the electron saturation velocity will decrease with gate voltage. We found that our sub-0.1 μm n-channel MOS transistors do not behave in this way. In this letter, we will explain our experimental results based on quasi-ballistic transport theory.     

The Monte Carlo approach to transport modeling in deca-nanometer MOSFETs

In this paper, we review recent developments of the Monte Carlo approach to the simulation of semi-classical carrier transport in nano-MOSFETs, with particular focus on the inclusion of quantum-mechanical effects in the simulation (using either the multi-subband approach or quantum corrections to the electrostatic potential) and on the numerical stability issues related to the coupling of the transport with the Poisson equation. Selected applications are presented, including the analysis of quasi-ballistic transport, the determination of the RF characteristics of deca-nanometric MOSFETs, and the study of non-conventional device structures and channel materials.     

自顶向下制备硅纳米线环栅MOSFET新工艺

准弹道输运特征的环栅纳米线MOSFET由于具备很强的栅控能力和抑制短沟道效应的能力,被认为是未来22nm技术节点以下半导体发展路线最有希望的候选者之一。采用传统CMOS工艺在SOI和体硅衬底上制备环栅纳米线MOSFET,解决了许多关键的技术难点,获得了许多突破性进展。文章综述了目前各种新颖的自顶向下制备方法和各种工艺的优缺点,以及优化的方向。     

Quasi-Ballistic Transport in Nanowire Field-Effect Transistors

In this paper, we investigate quasi-ballistic transport in nanowire field-effect transistors (NW-FETs). In order to do so, we address the 1-D Boltzmann transport equation (BTE) and find its exact analytical solution for any potential profile with the constraint of dominant elastic scattering. A simulation code implementing a self-consistent SchrÖdinger–Poisson solver in the transverse direction and the present BTE solution in the longitudinal direction is worked out, providing the $I$–$V$ characteristics of the NW-FET. Such characteristics are compared with those computed using a numerical BTE solver accounting for both inelastic and elastic collisions, and the two of them turn out to agree very nicely. From this comparison, it may be concluded that inelastic scattering plays a minor role for small-diameter FETs with device lengths in the decananometer range. Next, a methodology for the calculation of the transmission and backscattering coefficients is worked out for the first time starting from the scattering probabilities. The aforementioned coefficients turn out to be functions of the ratio between the carrier transit time and a suitably averaged momentum-relaxation time. Therefore, one of the main conclusions of this paper is that, so long as inelastic collisions are negligible, the so-called $kT$ layer plays no role in 1-D quasi-ballistic carrier transport.      

Quantum-aperture formation in a quasi-ballistic MESFET by neutron irradiation

The effect of neutron irradiation with a fluence reaching 5 × 10¹⁵ cm^?2 on a quasi-ballistic MESFET is studied theoretically and experimentally. A marked improvement is observed in the device performance; it is attributed to quantum effects. The experimental results are interpreted by the Monte Carlo simulation of defect formation and carrier transport in the channel of irradiated devices. It is shown that the positive effect of irradiation may be linked to the transformation of the channel into a set of quantum-size gaps (short quantum wires) between radiation-defect clusters. The higher degree of control over the drain current should result from the influence of the gate voltage on the gap diameter (and hence on the electron energy levels associated with the gaps).     

Critical current–gate voltage characteristics in short- and long-gated Josephson junctions

We studied the gate controllability of the critical current and the normal resistance in superconductor–semiconductor–superconductor junctions. The junctions used a two-dimensional electron gas (2DEG) in the InAs-inserted InAlAs/InGaAs heterostructure. It is shown that the interface barrier between the superconductor and the 2DEG affects the controllability in a short-gated junction. In a split-gated junction, the critical current–normal resistance product is almost constant against gate voltage. This is due to quantization of both the critical current and the conductance in a narrow and short semiconductor channel. The long-gated junction in the quasi-ballistic transport regime shows rapid suppression of the critical current by gate voltage.     

频域滤波抑制海水后向散射的带宽研究

在激光雷达探测海水水下目标的应用中,基于频域滤波的信号处理技术可以抑制海水后向散射噪声.其中接收系统中滤波器带宽的大小不仅对后向散射起到抑制作用,而且对目标信号产生抑制作用.为了确定带宽取值范围,分析了调制脉冲激光雷达探测信号水下传输模型,研究了海水后向散射信号以及目标反射信号的频域特性.通过对接收信号在频域上进行仿真,归纳出接收系统滤波器带宽的最优设计方法.该研究结果为系统达到最佳信噪比奠定理论基础.     

Analysis on Conductivity of nc—Si：H Films

A conduction channel model is propsed to explain the high conductivity property of nc-Si:H.Detailed energy band diagram is developed based on the analysis and calculation ,and the conductivity of the nc-Si:H was then analysed on the basis of energy band theory.It is assumed that the conductivity of the nc-Si:H stems from two parts:the conductance of the interface,where the transport mechanism is identified as a thermal -assisted tunneling process,and the conductance along the channel around the grain,which mainly determined the high conductivity of the nc-Si:H.The conductivity of nc-Si:H is calculated and compared with the experiment data .The theory is in agreement with the experiment.     

一种基于混合加密的移动代理安全传输模型

该文分析了目前移动代理系统存在的主要安全问题及现有的解决方案,随后提出了一种基于混合加密的移动代理安全传输模型(HESTM)。该模型主要分成两部分： (1)利用混合加密算法加密移动代理;(2)利用TLS加密通信信道。仿真与性能分析表明,HESTM模型的确能有效地保护移动代理的传输安全,从而有效地提高了整个系统的安全性和稳健性。该箅法已成功地应用在作者开发的原型系统-基于移动代理的入侵检测系统中。     

Two-dimensional numerical modeling of lightly doped nano-scale double-gate MOSFET

A two-dimensional numerical solution of electrostatic potential and electric field profiles are presented for lightly doped nano-scale Double-Gate Metal-Oxide-Semiconductor-Field-Effect-Transistor (DG-MOSFET). We have developed quasi-static (QS) model for evaluating bulk and inversion charges based on symmetric linearization model. We have also shown the non-quasi-static (NQS) effect on the charge due to a time varying gate voltage. It is seen that various symmetries of DG-MOSFET characteristics with respect to source/drain interchange is maintained in quasi-static as well as non-quasi-static version of the symmetrically linearized model. The variation of the threshold voltage with the varying width of the device is evaluated and presented. The results have been compared and contrasted with reported analytical model for QS condition for the purpose of verification of the model. The variation of threshold voltage along the width of the device is also predicted. This numerical model can be extended to analyze the transport phenomenon in sub 30 nm channel length DG-MOSFETs.