Transport equations for electrons in two-valley semiconductors

Transport equations are derived for particles, momentum, and energy of electrons in a semiconductor with two distinct valleys in the conduction band, such as GaAs. Care is taken to state and discuss the assumptions which are made in the derivation. The collision processes are expressed in terms of relaxation times. The accuracy is improved by considering these to depend on the average kinetic energy rather than the electron temperature. Other transport equations used in the literature are discussed, and shown to be incomplete and inaccurate in many cases. In particular, the usual assumption that the mobility and diffusion constant depend locally on the electric field strength is shown to be incorrect. Rather, these quantities should be taken as functions of the local average velocity of electrons in the lower valley.     

Direct extraction and numerical simulation of the base andcollector delay times in double heterojunction bipolar transistors

A new method is presented to evaluate the base and collector transit times, τ_B and τ_C in heterojunction bipolar transistors (BBT's) from the phase and magnitude of the common-base current gain, α(ω), which itself was directly extracted from measured S-parameter data. The method is applied to InGaP/GaAs single and double HBT's. A smaller cutoff frequency in the latter device is attributed to τ_B and τ_C due to two effects: trapping of electrons in the conduction band triangular barrier existing at the base-collector (B-C) heterojunction and smaller saturation velocity of electrons in InGaP as compared to GaAs. Finally, a new B-C design of InGaP/GaAs DNBT's is proposed to partially compensate the transit time effects. Numerical simulation of the cutoff frequency demonstrates the superiority of the proposed structure for high-frequency applications     

ZnSe为发光层的蓝绿光薄膜电致发光器件的发光性能

采用具有电子加速能力的Si02代替传统夹层结构中的绝缘层,利用电子束蒸发的方法制备了结构为ITO/SiO2/ZnSe/SiO2/Al的电致发光器件,观察到了传统夹层结构所没有的ZnSe层电致发光.测量了器件的电致发光光谱及总发光强度随着驱动电压及驱动频率的变化关系.讨论了器件的发光机理,认为是初电子经过SiO2层加速,获得较高能量,然后碰撞ZnSe分子,将其价带的电子激发到导带,再跃迁回价带或缺陷能级与空穴复合发光.由于这种发光方式类似于阴极射线发光,只不过电子在固体中而不是真空中加速,所以称之为固态阴极射线发光.这种机理为实现蓝色电致发光提供了新的途径.     

Optical orientation of electrons in compensated semiconductors

The theory of the optical orientation of charge carriers in compensated III-V semiconductors and quantum wells for the case where electrons are excited to the conduction band from Mn-charged acceptor states is presented. It is shown that, in GaAs/AlGaAs quantum wells, the degree of the spin orientation of conduction-band electrons in this excitation scheme can be as high as 85%. This spin-orientation enhancement results from an increase in the heavy-hole contribution to the acceptor state in the vicinity of the defect center rather than from level splitting caused by quantum confinement. It is shown that the degree of circular polarization of the photoluminescence emitted upon the recombination of electrons thermalized at the bottom of the band with holes occupying the acceptor ground state in a quantum well can exceed 70%.     

On the electromagnetic pulse produced by nuclear explosions

The electromagnetic pulse (EMP) produced by the gamma rays from nuclear explosions is discussed. The gamma rays produce a current of Compton recoil electrons, and these electrons produce further ionization so that the air becomes conducting. The Compton current leads to the generation of electromagnetic fields according to Maxwell's equations. The conductivity tends to limit the magnitude of the fields. Approximate methods of solving the equations are described by considering time regimes in which various terms in the equations are negligible, e.g., either the conduction current or the displacement current can be dropped. Further advantage is obtained by replacing the transverse fields by outgoing and ingoing waves; outgoing waves are dominant at early times. Features of the solutions are described for nuclear bursts at the ground surface and at high altitude. The history of EMP is reviewed briefly.     

Interband scattering effects on secondary ionization coefficients in GaAs

The ionization rates of holes and electrons in GaAs were measured experimentally over a wide doping range covering field values from 2.2×10⁵ V/cm to 4.7×10⁵ V/cm. As opposed to most experimemental measurements in GaAs, no assumption of equal ionization rates of the two carriers has been made. By using the conventional theoretical relationship between carrier ionization coefficients and multiplication data, the effective α is observed to be larger than β in lightly doped diodes while β is larger than α in heavily doped diodes. Previous theories of ionization rates utilizing just the normal conduction and valence bands do not show any possibility of such a crossover. It is suggested that electron transitions to higher conduction bands, which effectively increase the equilibration time of the electron distribution function, offer a resolution of this difficulty. The dependence of the effective electron ionization rate on doping can be explained by the requirement that electrons must make an interband transition before reaching the ionization threshold energy. This interband transition time estimated by this experiment is of the order of 10^?13 sec and is comparable to the transit time of electrons in the avalanching region. The breakdown voltages extrapolated from the measured α and β are consistent with those observed experimentally.     

Effects of Two-Photon Absorption on Pump-Induced Refractive-Index Change in AlAsSb–InGaAs–AlAs Optical Waveguides

Injection of transverse-magnetic polarized 1.55- mum pump light pulses having an excessively high intensity into heavily-doped AlAsSb-InGaAs-AlAs coupled-double-quantum-well optical waveguides causes a large amount of electrons in the ground state of the subbands in the conduction band to transit to the excited state and results in bandgap shrinkage as well as a large amount of vacant sites in the ground state. Consequently, the simultaneously injected continuous-wave transverse-electric polarized 1.54- mum probe light is absorbed by the valence-band electrons through two-photon absorption (TPA) and transit to the vacant sites in the ground state of the subbands in the conduction band. As a result, the probe power is reduced instantaneously but recovers at a time constant in the order of 10^-10s. A theoretical evaluation of the effect of the above intersubband-pump-assisted TPA of the probe light shows that the pump-induced net refractive-index change in these devices is reduced by 50% when intersubband absorption and ISB-pump-assisted TPA of the probe light are of equal strength. Characterization of the TPA in these devices indicates that these devices have a TPA coefficient of 25 cm/GW.     

HgCdTe受主能态的非线性吸收光谱

本文介绍用可选支的连续CO_2激光器在低于禁带宽度的谱线范围内研究100K的Hg_(0.785)Cd_(0.215)Te线性与非线性吸收光谱,由受主能态到导带能态的直接跃迁机理成功地解释了实验现象.井由电子和空穴的速率方程导出饱和吸收的表达式.     

Tunneling emission of electrons from semiconductors’ valence bands in high electric fields

Tunneling emission currents of electrons from semiconductors to vacuum (needle-shaped GaAs photodetectors) and to a metal (silicon metal-insulator-semiconductor diodes with a tunneling-transparent insulator layer) are studied in high and ultrahigh electric fields. It is shown that, in semiconductors with the n-type conductivity, the major contribution to the emission current is made by the tunneling emission of electrons from the valence band of the semiconductor, rather than from the conduction band.     

New physical formulation of the thermionic emission current at theheterojunction interface

The thermionic emission current for electrons across the heterojunction interface is classically modeled as the difference between two opposing electron fluxes. A new, consistent, physical model, which includes the carrier degeneracy and nonideal behavior effects, is presented. It is shown that the emission current at the interface can be expressed in a simple closed-form formalism that gives the relationships among the average directional thermal velocity of electrons, the conduction band discontinuity, and the carrier activities at both sides of the interface. The conditions under which the thermionic emission occurs at the heterointerface are discussed     

Hole current in dual dielectric under positive gate voltage

n-channel dual-dielectric transistors with SiO2:Si3N4gate insulators were fabricated with and without boron implant in the channel. Under positive gate voltage stress, electrons can enter the insulator from the silicon, and holes can enter the silicon from the insulator. The electrons and holes were measured by the technique described by Ginovker et al.[1]. For oxides thicker than 30 Å, it is always observed that the silicon hole current is 3-4 orders of magnitude below the silicon electron current. The physical origin of this hole current is shown to be field-enhanced excitation of electrons from the valence band to the conduction band in the silicon prior to entering the insulator, and is not due to the holes from the insulator entering the silicon.     

Numerical Simulation of Bottom Oxide Thickness Effect on Charge Retention in SONOS Flash Memory Cells

In this paper, bottom-oxide thickness (T_bo) and program/erase stress effects on charge retention in SONOS Flash memory cells with FN programming are investigated. Utilizing a numerical analysis based on a multiple electron-trapping model to solve the Shockley-Read-Hall rate equations in nitride, we simulate the electron-retention behavior in a SONOS cell with T_bo from 1.8 to 5.0 nm. In our model, the nitride traps have a continuous energy distribution. A series of Frenkel-Poole (FP) excitation of trapped electrons to the conduction band and electron recapture into nitride traps feature the transitions between the conduction band and trap states. Conduction band electron tunneling via oxide traps created by high-voltage stress and trapped electron direct tunneling through the bottom oxide are included to describe various charge leakage paths. We measure the nitride-charge leakage current directly in a large-area device for comparison. This paper reveals that the charge-retention loss in a high-voltage stressed cell, with a thicker bottom oxide (5 nm), exhibits two stages. The charge-leakage current is limited by oxide trap-assisted tunneling in the first stage and, then, follows a 1/t time dependence due to the FP emission in the second stage. The transition time from the first stage to the second stage is related to oxide trap-assisted tunneling time but is prolonged by a factor     

Transition times between the extremum points of the current–voltage characteristic of a resonant tunneling diode with hysteresis

A numerical solution to the problem of transient processes in a resonant tunneling diode featuring a current–voltage characteristic with hysteresis is found for the first time in the context of a coherent model (based on the coupled Schrödinger and Poisson equations) taking into account the Fermi distribution of electrons. The transitions from the high-current to the low-current state and vice versa, which result from the existence of hysteresis and are of great practical importance for ultrafast switches based on resonant tunneling diodes, are studied in detail. It is shown that the transition times for such processes initiated by the application of a small voltage can significantly exceed the characteristic time ?/Γ (where G is the width of the resonance level). It is established for the first time that the transition time can be reduced and made as short as the characteristic time ?/Γ by applying a sufficiently high voltage. For the parameters of the resonant-tunnelingdiode structure considered in this study, the required voltage is about 0.01 V.     

多晶MAPbBr_(3)薄膜中束缚载流子热发射复合过程北大核心CSCD

有机-无机卤化铅钙钛矿(organic inorganic lead halide perovskite,OLHP)半导体材料内部的陷阱是影响OLHP的光电性能的重要因素。为了理解多晶的甲胺溴基钙钛矿((Methylammonium)PbBr_(3),MAPbBr_(3))薄膜中陷阱对光生载流子复合的影响,本文采用了时间分辨微波光电导(time resolved microwave conductivity,TRMC)技术探究了多晶MAPbBr_(3)薄膜的光生载流子复合动力学过程。实验测量结果表明多晶MAPbBr_(3)薄膜的载流子复合过程包括自由载流子复合与束缚载流子的热发射复合两部分。其中,与束缚载流子热发射复合相关的能级远离连续带,且对应的能级深度约为0.6 eV,分布宽度约为89.2 meV。本文同时利用变激发波长TRMC对比实验,分析浅束缚光生电子与导带光生电子复合过程的差异。相比于导带上的电子,实验结果表明浅束缚电子跃迁到深束缚能级的概率更大。     

Dynamic generation of spin-wave currents in hybrid structures

Spin transport through the interface in a semiconductor/ferromagnetic insulator hybrid structure is studied by the nonequilibrium statistical operator method under conditions of the spin Seebeck effect. The effective parameter approach in which each examined subsystem (conduction electrons, magnons, phonons) is characterized by its specific effective temperature is considered. The effect of the resonant (electric dipole) excitation of the spin electronic subsystem of conduction electrons on spin-wave current excitation in a ferromagnetic insulator is considered. The macroscopic equations describing the spin-wave current caused by both resonant excitation of the spin system of conduction electrons and the presence of a nonuniform temperature field in the ferromagnetic insulator are derived taking into account both the resonance-diffusion propagation of magnons and their relaxation processes. It is shown that spin-wave current excitation is also of resonant nature under the given conditions.     

Absorption coefficient for the intraband transitions in quantum dot materials

In this paper, we present calculations of the absorption coefficient for transitions between the bound states of quantum dots grown within a semiconductor and the extended states of the conduction band. For completeness, transitions among bound states are also presented. In the separation of variables, single band k·p model is used in which most elements may be expressed analytically. The analytical formulae are collected in the appendix of this paper. It is concluded that the transitions are strong enough to provide a quick path to the conduction band for electrons pumped from the valence to the intermediate band. Copyright © 2012 John Wiley & Sons, Ltd.     

A fast numerical algorithm for heterojunction structures

In this paper, the two-dimensional electron gas (2DEG) concentration is numerically calculated for single and double heterostructure band profiles by solving Schroödinger's and Poisson's equations self-consistently. An expected energy level is introduced to enhance the speed of calculation in obtaining quantized energy levels through the iteration process. Conventional AlGaAs/GaAs and AlGaAs/InGaAs/GaAs structures are selected to prove the validity of this calculation. Three different concentrations, namely, positively ionized donors, free electrons in the conduction band and 2DEG, are considered through the band profiles. Both the 2DEGs in the narrow bandgap and free electrons in the wide bandgap have been calculated and compared with the data available in the literature. Furthermore, positively ionized donors are also obtained and correlated with 2DEG and free electrons to predict the gate capacitance characteristics of the devices, and showed good agreement with the experimental data.     

氮化镓注镁(Mg:GaN)的光致发光

利用低压MOCVD在蓝宝石衬底上外延生长了GaN,用离子注入法掺入Mg杂质,退火后,进行光致发光测量,观察到显著的蓝光发射和黄带发射.光谱分析给出了与注入Mg离子相关的GaN禁带中能级的精细结构,其中: 间位Mg(Mgi)能级(导带下170meV)到替位Mg(MgGa)受主能级(价带上250meV)的跃迁产生了415nm发光峰; 该能级到价带上390meV能级的跃迁,以及带有紧邻N空位的替位Mg(MgGaVN)能级(导带下310meV)到 MgGa受主能级的跃迁,均产生了438nm发光峰.另外,退火使GaN晶格结构部分恢复,再现了黄带发射.     

Measurement of density of the gap states ina-Si:H by the normalization of photoconductivity

The spectra of the optical absorption coefficient in low absorption region are obtained by using a normalization procedure for the photoconductivity spectra. The results are explained in terms of the optical transition of electrons from localized states in the exponential valence band tail and in dangling bond states 1.0 eV below the conduction band edge to extended conduction band states. Then the density of the gap states below the Fermi levelE _F is obtained. From the investigation of recombination kinetics, the average density of the gap states over the range of ) and the density of the gap states above the Fermi levelE _F are obtained. These indicate that the width of the conduction band tail is smaller than that of the valence band tail.