A generalized analytical model based on multistage scattering phenomena for estimating the impact ionization rate of charge carriers in semiconductors

A generalized analytical model based on multistage scattering phenomena has been developed in this paper for estimating the impact ionization rate of charge carriers in semiconductors. The probabilities of impact ionization initiated by electrons and holes have been calculated separately by taking into account all possible combinations of optical phonon scattering and carrier-carrier collisions prior to the impact ionization. Finally the analytical expressions of impact ionization rate of electrons and holes have been developed by using the aforementioned impact ionization probabilities. The impact ionization rates of electrons and holes in 4H-SiC have been calculated within the field range of \(2.5\times 10^{8}\) – \(6.5\times 10^{8}\hbox { V m}^{-1}\) by using the analytical expressions of those developed in the present paper. Those are also calculated by using the analytical expressions developed by some other researchers earlier without considering the multistage scattering phenomena. Finally the theoretical results obtained from the analytical model proposed in this paper and the analytical model developed by earlier researchers within the field range under consideration have been compared with the ionization rate values calculated by using the empirical relations fitted from the experimentally measured data. Closer agreement with the experimental data has been achieved when the impact ionization rate of charge carriers in 4H-SiC are calculated from the proposed model as compared to the earlier one.     

First Principles Calculations of Auger Recombination and Impact Ionization Rates in Semiconductors

A first-principles approach within the screened-exchange approximation to the density functional theory is proposed to simulate the impact ionization process in semiconductors. Results for InP are reported and discussed in comparison with those obtained for the most studied GaAs. The formalism is extended to the calculation of Auger recombination rates: the results for GaAs are found to be in good agreement with previous experimental and theoretical works. Moreover, Auger lifetimes calculated directly via Fermi's Golden Rule are shown to be in excellent agreement with lifetime values determined via detailed balance principles proceeding from calculated impact ionization rates.     

The long-wave instability of the front of planar waves of impact ionization in semiconductors

Theoretical analysis of the transversal stability of a planar front of a impact ionization wave in semiconductors having a finite thickness and doping concentrations, has been carried out. The formulas were developed for an increment s of an increase in long-wave perturbation in both the presence and the absence of background charge carriers ahead of the front. These formulas are applicable at arbitrary relationship between the wave vector of perturbation k and a distance L between the front and the electrode towards which the impact ionization wave propagates, if the thickness of a region in which the volume charge of electrons and holes is large, is far less than min(1/k, L). In the limiting cases, when kL ≪ 1 and kL ≫ 1, the theory leads to the results obtained previously. The practical result of the formulas developed is that at all actual values L perturbation with a wave vector k < 1/L decrease or increase much slower than short-wavelength perturbations do. Therefore it is just evolution of short-wave perturbations that should result in current pinching and limit the reliability of avalanche voltage sharpeners.     

Electrical characterization of polymer light-emitting diodes

This paper presents a device model for the current and light generation of polymer light-emitting diodes (PLEDs). The model is based on experiments carried out on poly(dialkoxy-p-phenylene vinylene) (PPV) devices. It is demonstrated that PLED's are fundamentally different as compared to conventional inorganic LEDs. The hole conduction in PPV is space-charge limited with a low-field mobility of only 5×10^-11 m²/Vs, which originates from the localized nature of the charge carriers. Furthermore, the hole mobility is highly dependent on the electric field and the temperature. The electron conduction in PPV is strongly reduced by the presence of traps. Combining the results of the electron- and hole transport a device model for PLEDs is proposed in which the light generation is due to bimolecular recombination between the injected electrons and holes. It is calculated that the unbalanced electron and hole transport gives rise to a bias dependent efficiency. By comparison with experiment it is found that the bimolecular recombination process is of the Langevin-type, in which the rate-limiting step is the diffusion of electrons and holes toward each other. This is in contrast to conventional semiconductors, in which the bimolecular recombination is governed by the joint density-of-states of electrons and holes and is not limited by charge transport. The occurrence of Langevin recombination explains why the conversion efficiency of current into light of a PLED is temperature independent. The understanding of the device operation of PLED's indicates directions for further improvement of the performance     

Scattering of electrons by ionized impurities in semiconductors: quantum-mechanical approach to third body exclusion

As applied to the numerical simulation of electron transport and scattering processes in semiconductors an efficient model describing the scattering of electrons by the ionized impurities is proposed. On the example of GaAs at 77 and 300 K and Si at 300 K the dependencies of low-field electron mobility on the donor impurity concentration in the semiconductors are calculated in the framework of proposed model as well as in the framework of such most frequently used applied models as the Conwel-Weisskopf model and the Brooks-Herring one. After comparing the calculation results with the well-known experimental data it has been ascertained that the best agreement between the theory and experiment is achieved with application of the proposed model.     

An optical field-induced ionization X-ray laser using a preformedplasma scheme

Soft X-ray amplification by optical field-induced ionization (OFI) of a preformed plasma is investigated. A small-signal gain of 20 cm^-1 and a gain-length product of 4 at 13.5 nm on the Lyman-α transition in hydrogen-like lithium have been obtained using the modified OFI scheme. Experimental and numerical results ensure that we have produced a plasma with a significantly lower electron temperature than what is expected by an above-threshold ionization model. To explain the results, a two component plasma model in which relatively high temperature electrons are produced by OFI in a cold electron bath of the preformed plasma is presented. The model indicates that the average electron temperature of the OFI plasma rapidly decreases since a high-temperature part of the electrons escapes from the focal volume without interaction. The initial electrons produced prior to the field ionization, which survive after the OEI, also significantly contribute to the rapid three-body recombination. Based on the ionization-induced refractive index change, the pulse propagation of a high-intensity pump laser during the OFI is also discussed     

Mathematical?Physical Model of Corona from Surges on High-Voltage Lines

A mathematical-physical model of corona current pulses from voltage surges on HV lines is presented. Distributed representation of corona along infinitely long transmission lines is suggested. The surge corona current pulse flows through the transmission line as a series of successive generations of electron avalanches, which develop in the ionization zone around the HV conductor. The initiatory electrons for triggering the current pulse under a positive applied surge are those available in the atmosphere surrounding the HV conductor. The two corona current components, the image component due to the presence of charge carriers (electrons and ions), and the convection component due to the motion of charge carriers in the interelectrode spacing were calculated. The calculated corona current pulse and the charge flow voltage curve agreed reasonably with those measured experimentally for a coaxial transmission line.     

Prebreakdown Process in Freon-Nitrogen Mixtures at Low Gas Pressures

Steady-state prebreakdown currents were measured under a uniform field in freon-nitrogen mixtures over the range 110? E/p?240 (E = electric field in V.cm -1torr -1, and p = gas pressure in torr) and at pressures ranging from 1-20 torr for different concentrations of freon. From the measured currents, the values of ionization and attachment coefficients and the ratio of diffusion coefficients to mobility for electrons (mean energy), were determined. The addition of a small concentration of freon (?10 percent) to nitrogen at high E/p values is found to increase the ionization rate considerably with a corresponding reduction in sparking potentials. A peak in the value of mean energy for electrons was also observed under this condition. This is attributed to a significant difference in the ionization potentials of freon and nitrogen molecules, which is capable of ionizing molecules.     

Percolation current in organic semiconductors

The charge transport in organic semiconductors has been investigated theoritically. A n analytical model describing the hopping current is developed. This model is based on VRH (variable range hopping) theory and percolation theory. The results of calculation are compared to the prediction of Mott’s formalism, which has been widely used to describe hopping transport in amorphous semiconductors. The model is also found to be in good agreement with recent experimental data.     

计及网络攻击影响的安全稳定控制系统风险评估方法

安全稳定控制系统(稳控系统)是保证电网可靠运行的重要防线,针对稳控系统的网络攻击会造成严重的物理后果。为了量化评估稳控系统遭受网络攻击的影响,解决现有风险评估方法未充分考虑网络攻击易发性的问题,文中提出一种计及网络攻击影响的稳控系统风险评估方法。文中首先分析了稳控系统的层次结构;然后,从攻击对象、攻击方式、攻击后果3个角度分析了稳控装置本体与稳控装置站间通信的网络攻击风险点;其次,基于模糊层次分析法对网络攻击易发性进行量化,并结合由Petri网建立的网络攻击防护单元模型建立针对稳控系统的网络攻击成功概率模型;最后,结合物理后果和攻击成功概率,对标准系统和实际系统进行风险评估,计算了正常运行和网络攻击2种情况下的风险值,验证了所提模型的有效性。     

Numerical simulation of time resolved charge transport in semiconductor structures for electronic devices

Excess charge carrier transport and relaxation in semiconductor layered structures have been numerically simulated by a finite-difference method. The standard mathematical model of charge transport in semiconductors has been used, consisting of two continuity, two diffusion and two drift equations and a Poisson equation involving electron, hole concentrations and potential. The relaxation process is described by means of Shockley-Read-Hall recombination statistics. The equations of the model are solved in a one-dimensional space domain and the time domain. The simulation program can be used to deduce semiconductor parameters like bulk lifetime, surface recombination velocity, diffusion coefficients and mobilities of electrons and holes from measurements of photoconductance decay. A technique for a direct measurement of the recombination velocity at a wafer’s surface or interface is presented. It is based on the measurement of the initial photoconductance decay after laser pulse excitation of excess charge carriers in a very shallow layer at the surface or interface. Under conditions to be fulfilled for a correct measurement, the initial decay process is dominated by recombination at the surface and the decay of the measured curve depends almost exclusively on the surface recombination velocity. In this case the simulation program is necessary to interpret the measurement data.     

Computer simulation on formation of space charge packets in XLPEfilms

The packet-like space-charge behaviour in an oxidized crosslinked polyethylene (XLPE) film doped with antioxidant is discussed through computer simulation. The model is based on the following assumptions, (1) both electrons and holes are injected from the electrodes and they migrate to the opposite electrodes by hopping transport, (2) holes are also supplied from acceptor sites depending on the field, (3) the holes are excited to the hopping level only via an intermediate level. In this model, a space-charge packet is not caused by the excess of mobile electrons but by the deficiency in mobile holes. The ionized acceptors left behind by hole migration form an apparent negative space charge packet and it propagates towards the anode as the high field region moves     

基于EMTP的水平接地体冲击时-频特性分析

提出了一种基于EMTP的接地体冲击电流作用下时-频特性分析方法,建立了考虑土壤火花放电的水平接地体时域传输线模型,利用该模型对水平接地体冲击特性进行了时域分析。通过对接地体注入电流、电压波形进行傅里叶变换,于频域内计算接地体不同频率响应下频域阻抗,详细分析了接地体冲击电流作用下的频域特性,并以此计算了接地体等效长度,分析了雷电流不同波头时间,幅值对接地体阻抗的影响。     

混合型模块化多电平换流器解析建模与功率运行区间分析

混合型模块化多电平换流器(MMC)在远距离大容量架空线输电领域具有十分广阔的应用前景。为定量研究混合型换流器的运行特性,文中提出了混合型MMC动态解析模型和稳态解析模型的建模方法。通过稳态解析模型求解与换流器内部电气量和控制量有关的非线性方程组,实现了在任意直流电压和功率运行点下换流器运行特性的完全解析求解。对比了不同直流电压水平下,电磁暂态模型仿真结果和稳态解析模型的计算结果,验证了稳态解析模型的精确性。研究了考虑多种运行约束条件时混合型MMC的功率运行区间计算方法,尤其考虑了半桥子模块的均压约束。计算了不同直流电压水平下的功率运行区间,分析了各约束条件以及子模块电容、桥臂电抗器、桥臂子模块比例等参数对功率运行区间的影响。     

Impact Ionization in GaAs within A Screened Exchange Density Functional Formalism

We present the results of a fully first-principles calculation of impact ionization rates in GaAs within the density functional theory formalism, using a screened-exchange approach and the highly accurate all-electron full-potential linearized augmented plane wave (FLAPW) method. The calculated impact ionization rates show a marked orientation dependence in k space, indicating the strong restrictions imposed by the conservation of energy and momentum. This anisotropy diminishes as the impacting electron energy increases. A Keldysh type fit performed on the energy-dependent rate shows a rather soft edge and a threshold energy greater than the direct band gap. The consistency with available Monte Carlo and empirical pseudopotential calculations shows the reliability of our approach and paves the way to ab-initio calculations of pair production rates in new and more complex materials.     

Al掺杂对Fe_2Si电子结构和磁学特性的影响

采用第一性原理的贋势平面波方法,计算了无掺杂和Al掺杂Fe_2Si体系的电子结构和磁学特性,并分析了Al掺杂对Fe_2Si体系电磁特性的影响。计算结果表明,未掺杂和Al掺杂Fe_2Si体系为半金属铁磁体,自旋向上的能带结构穿过费米面表现为金属特性,未掺杂Fe_2Si体系自旋向下的能带表现为间接带隙半导体特性,带隙值为0.464 eV;Al掺杂Fe_2Si体系自旋向下的能带表现为Z间的直接带隙半导体特性,带隙值为0.541 eV。Al掺杂使各原子磁矩和Fe_2Si体系的总磁矩均减小,体系的带隙值增加,相应的半金属隙也增加,并且使得体系自旋向下部分由间接带隙变为直接带隙半导体。Fe_2Si体系的半金属性和磁性主要来源于Fe-3d电子之间的d-d交换,Si-3p电子与Fe-3d电子之间的p-d杂化。综上所述,掺杂是调控半金属铁磁体Fe_2Si电磁特性的有效手段。     

A General Monte Carlo Model Including the Effect of the Acoustic Deformation Potential on the Transport Properties

In this paper we describe a model for a fully numerical Monte Carlo simulator and the required inputs, which include the calculation of numerical phonon scattering rates and the electronic bandstructure. The model is material-independent, and it therefore provides great flexibility in the ability to characterize many semiconductor materials, including the III–V and III-Nitride materials. All of the principal ingredients of the Monte Carlo model are determined numerically from first principles with one exception, that of the acoustic deformation potential. The determination of the acoustic deformation potential, which specifies the relative strength of the acoustic phonon scattering rate, affects the calculated results. We introduce a methodical approach for its selection and examine the sensitivity of the primary calculated transport quantities on its magnitude. Specifically, we examine how the choice of the acoustic deformation potential, in a completely numerical Monte Carlo model, affects the calculated velocity field curve, average energy, and impact ionization coefficients. Calculations are made for both bulk GaAs and 3C-SiC and for a representative GaAs MESFET structure. It is found that the acoustic deformation potential needs to be energy dependent and that it requires at least two values, one for intra-band transitions and one for inter-band transitions. It is further found that through the use of the fully numerical model introduced here, reliable transport-related results may be obtained for bulk material as well as representative devices using only the acoustic deformation potential as the single adjustable parameter.     

湖南省地方国有企业科技进步的Fuzzy积分评判模型

应用ｆｕｚｙ测度下的ｆｕｚｙ积分理论，结合湖南省的具体情况，建立了评判地方国有企业科技进步的ｆｕｚｚｙ积分模型．本模型和方法一改以往将各项指标值简单平均的方法，而是既考虑企业各评价项目的满意度，又注重评价项目的重视度，比较全面地评价地方国有企业科技进步的程度，从而确定企业的等级．数值例表明，据此法评判的结果与实际吻合．     

使用辨识得到的运算电感模型估计同步发电机d轴等值电路参数

提出了一种利用辨识得到的运算电感模型确定同步发电机d轴等值电路参数的方法。由于辨识得到的运算电感模型的参数通常带有误差,直接根据解析公式求取的等值电路参数值可能远离其真值。为获得较为准确的等值电路参数值,首先将用解析公式计算得到的等值电路参数值作为初值,然后基于发电机的混合状态模型,用预测误差法对等值电路参数进行估计。数值仿真的结果验证了本文所提方法的有效性。     

TLM modelling of diffusion,drift and recombination of charge carriers in semiconductors

The paper provides an extension to the one-dimensional TLM diffusion model. The extended diffusion node presented here models the exact transport equation with diffusion drift and recombination of charge carriers in semiconductors. A general algorithm for providing a numerical solution to the transport phenomena is also presented here. The analytical solution for infinitely long semiconductors is compared with the TLM numerical results.