Velocity Overshoot Research Articles

Criteria of negative wake generation behind a cylinder

It was demonstrated by simulation in our previous study that both the normal stress and its gradient are responsible for the negative wake generation (overshoot in the axial velocity) and streamline shifting. Extensional properties of the fluids dominate the generation of the negative wake, while other factors strengthen or weaken the formation of velocity overshoot. In this study, the criteria for the negative wake generation are discussed in detail for various fluid models, including the PTT, the FENE-CR, the FENE-P, and the Giesekus models. With the FENE-CR fluid, it is easier to generate negative wake than with the FENE-P fluid. This confirms that the constant shear viscosity FENE-CR fluid enhances the velocity overshoot, and that the shear-thinning viscosity FENE-P fluid delays the negative wake generation. The Giesekus fluid has a similar behaviour to the PTT fluid with regarding to the critical conditions of negative wake generation when appropriate fluid parameters are selected. The mechanism of wall proximity in enhancing the negative wake generation is also demonstrated with the analysis for the first time.

Velocity overshoot in ultrathin double-gate silicon-on-insulator transistors

Concerning electron velocity overshoot in ultrathin double-gate silicon-on-insulator transistors, as a function of the silicon thickness and the electron sheet density, it is proved that for very small silicon thicknesses (smaller than 5 nm), velocity overshoot behavior is dominated by the average conduction effective mass; that is, the lower the average conduction effective mass, the higher the velocity overshoot peak. Thus, the velocity overshoot peak increases as the silicon thickness decreases, unlike low-field electron mobility, which diminishes abruptly at silicon thicknesses below 5 nm. This fact enables further reduction in the device channel length, in contrast to what might be supposed from the low-field mobility behavior.

A comprehensive study of velocity overshoot effects in double gate silicon on insulator transistors

A comprehensive study of velocity overshoot in double gate silicon on insulator (DGSOI) transistors has been undertaken. Monte Carlo simulations were performed to clarify the dependence of velocity overshoot effects on the low field mobility, channel inversion charge and silicon layer thickness. The relationships and dependences between the energy- and momentum-relaxation times were also investigated. In general, two opposite effects influence electron transport in DGSOI inversion layers as the silicon thickness is reduced: a reduction in the conduction effective mass and a phonon scattering increase. However, we show that electron mobility is mainly determined by the increase in the phonon scattering rate as the silicon thickness is reduced, i.e., the lower the silicon thickness the lower the electron mobility, while the velocity overshoot effects for ultrathin DGSOI inversion layers are dominated by the reduction of the average conduction effective mass, i.e., the lower the silicon thickness the higher the velocity overshoot peak.

Design, analysis, and synthesis of generalized single step single solve and optimal algorithms for structural dynamics

AbstractThe primary objectives of the present exposition are to: (i) provide a generalized unified mathematical framework and setting leading to the unique design of computational algorithms for structural dynamic problems encompassing the broad scope of linear multi‐step (LMS) methods and within the limitation of the Dahlquist barrier theorem (Reference [3], G. Dahlquist, BIT 1963; 3: 27), and also leading to new designs of numerically dissipative methods with optimal algorithmic attributes that cannot be obtained employing existing frameworks in the literature, (ii) provide a meaningful characterization of various numerical dissipative/non‐dissipative time integration algorithms both new and existing in the literature based on the overshoot behavior of algorithms leading to the notion of algorithms by design, (iii) provide design guidelines on selection of algorithms for structural dynamic analysis within the scope of LMS methods. For structural dynamics problems, first the so‐called linear multi‐step methods (LMS) are proven to be spectrally identical to a newly developed family of generalized single step single solve (GSSSS) algorithms. The design, synthesis and analysis of the unified framework of computational algorithms based on the overshooting behavior, and additional algorithmic properties such as second‐order accuracy, and unconditional stability with numerical dissipative features yields three sub‐classes of practical computational algorithms: (i) zero‐order displacement and velocity overshoot (U0‐V0) algorithms; (ii) zero‐order displacement and first‐order velocity overshoot (U0‐V1) algorithms; and (iii) first‐order displacement and zero‐order velocity overshoot (U1‐V0) algorithms (the remainder involving high‐orders of overshooting behavior are not considered to be competitive from practical considerations). Within each sub‐class of algorithms, further distinction is made between the design leading to optimal numerical dissipative and dispersive algorithms, the continuous acceleration algorithms and the discontinuous acceleration algorithms that are subsets, and correspond to the designed placement of the spurious root at the low‐frequency limit or the high‐frequency limit, respectively. The conclusion and design guidelines demonstrating that the U0‐V1 algorithms are only suitable for given initial velocity problems, the U1‐V0 algorithms are only suitable for given initial displacement problems, and the U0‐V0 algorithms are ideal for either or both cases of given initial displacement and initial velocity problems are finally drawn. For the first time, the design leading to optimal algorithms in the context of a generalized single step single solve framework and within the limitation of the Dahlquist barrier that maintains second‐order accuracy and unconditional stability with/without numerically dissipative features is described for structural dynamics computations; thereby, providing closure to the class of LMS methods. Copyright © 2003 John Wiley & Sons, Ltd.

A‐scalability and an integrated computational technology and framework for non‐linear structural dynamics. Part 2: Implementation aspects and parallel performance results

AbstractAn integrated framework and computational technology is described that addresses the issues to foster absolute scalability (A‐scalability) of the entire transient duration of the simulations of implicit non‐linear structural dynamics of large scale practical applications on a large number of parallel processors. Whereas the theoretical developments and parallel formulations were presented in Part 1, the implementation, validation and parallel performance assessments and results are presented here in Part 2 of the paper. Relatively simple numerical examples involving large deformation and elastic and elastoplastic non‐linear dynamic behaviour are first presented via the proposed framework for demonstrating the comparative accuracy of methods in comparison to available experimental results and/or results available in the literature. For practical geometrically complex meshes, the A‐scalability of non‐linear implicit dynamic computations is then illustrated by employing scalable optimal dissipative zero‐order displacement and velocity overshoot behaviour time operators which are a subset of the generalized framework in conjunction with numerically scalable spatial domain decomposition methods and scalable graph partitioning techniques. The constant run times of the entire simulation of ‘fixed‐memory‐use‐per‐processor’ scaling of complex finite element mesh geometries is demonstrated for large scale problems and large processor counts on at least 1024 processors. Copyright © 2003 John Wiley & Sons, Ltd.

Monte carlo simulations of the bandwidth of InAlAs avalanche photodiodes

We present a Monte Carlo simulation of the bandwidth of an InAlAs avalanche photodiode with an undepleted absorber. The carrier velocities are simulated in the charge layer and the multiplication region. It is shown that the velocity overshoot effect is not as significant as simpler models have suggested. At high electric field intensity, the electron effective saturation velocity is only slightly higher when impact ionization is significant, compared with when impact ionization is absent. The simulated 3 dB bandwidth is consistent with experiments for gains up to 50.

A Theoretical Comparison of Strained-Si-on-Insulator Metal–Oxide–Semiconductor Field-Effect Transistors and Conventional Si-on-Insulator Metal–Oxide–Semiconductor Field-Effect Transistors Using a Drift-Diffusion-Based Simulator

Strained-Si-on-insulator (strained-SOI) metal–oxide–semiconductor field-effect transistors (MOSFETs) were compared to conventional SOI MOSFETs in terms of short-channel effects and delay time using a drift-diffusion-based device simulator. In the case of 90 nm gate devices, threshold voltage Vth for strained-SOI nMOSFETs decreased with decreasing conduction-band edge discontinuity, while Vth dependence on conduction-band edge and valence-band edge discontinuities for strained-SOI pMOSFETs were complicated. An analytical expression for Vth in strained-SOI pMOSFETs could explain the simulated results. The Vth shift for strained-SOI nMOSFETs with shortening the gate length was slightly smaller than that for conventional SOI nMOSFETs, while Vth shift for strained-SOI pMOSFETs was larger than that for conventional SOI nMOSFETs. The Vth shift for strained-SOI nMOSFETs with a strain-Si thickness tSi greater than the source and drain junction depth xj was smaller than that for strained-SOI nMOSFETs with tSi<xj. The strained-SOI MOSFETs were approximately 15% faster than conventional SOI MOSFETs when the saturation velocities were assumed to be 107 cm/s for both strained- and conventional SOI MOSFETs. Strained-SOI nMOSFETs are 1.9 times faster than conventional SOI nMOSFETs when the electron velocity overshoot is developed.

SELF-CONSISTENT MODELING OF MOSFET QUANTUM EFFECTS BY SOLVING THE SCHRÖDINGER AND BOLTZMANN SYSTEM OF EQUATIONS

A new method for investigating quantum confinement effects in MOSFETs is presented. The method is based on the numerical solution of the Schrödinger and Boltzmann system of equations. A quantum Boltzmann equation is developed by replacing the classical potential with a quantum potential. The technique naturally accounts for highly nonequilibrium effects including velocity overshoot. The subbands are calculated and then populated using nonequilibrium statistics. Modeling results show the electron concentration and electron current density peaks shifted away from the interface. Calculations show reduced channel concentration which leads to lower drain current for the quantum case. A quantum distribution function is obtained.

Direction-dependent band nonparabolicity effects on high-field transient electron transport in GaN

Time-resolved electroabsorption measurements on an AlGaN/GaN heterojunction p–i–n diode provide evidence of electron velocity overshoot at fields as low as ∼130 kV/cm for transport in the c-direction of wurtzite GaN. Theoretical Monte Carlo calculations employing a full band structure indicate that at fields below ∼300 kV/cm, this velocity overshoot is associated primarily with band nonparabolicity in the Γ valley related to a negative electron effective mass rather than intervalley transfer. Similar calculations of transport in the basal plane indicate that in this case, both a higher threshold field for velocity overshoot and a lower steady-state velocity at a given field are expected.

Band structure anisotropy effects on the hole transport transient in 4H–SiC

We study the role of band structure anisotropy on the hole transport in 4H–SiC during the transient regime. For the same strength of the applied electric field, the drift velocity overshoot of the hole is stronger and reaches steady state later when the field is applied perpendicular to the c-axis, than when the field is in the c-axis direction. In both cases, the time for the hole drift velocity and mean energy to reach steady state is under 50 fs, depending on the electric field strength, and are one order of magnitude shorter than the time for the electron drift velocity and mean energy to attain the steady state.

Two-dimensional hydrodynamic model including inertia effects in carrier momentum for power millimetre-wave semi-conductor devices

A two dimensional-hydrodynamic model is carried out to predict the performance of short gate length power field effect transistors based on III–V semi-conductor systems. The model is based on the conservation equations, deduced from the Boltzmann transport equation, solved in their whole form by taking temporal and spatial variations in carrier momentum into account. This model is well suited to get accurate predictions for power devices having various gate recess topologies. Results are performed for devices with different gate lengths and compared with those obtained from simplified models. For gate lengths shorter than 0.5 μm, a drop in the overshoot velocity phenomenon involves a better estimation of the characteristics of the device.

Unsteady free convection flow over an infinite vertical porous plate due to the combined effects of thermal and mass diffusion, magnetic field and Hall currents

The unsteady free convection flow over an infinite vertical porous plate, which moves with time-dependent velocity in an ambient fluid, has been studied. The effects of the magnetic field and Hall current are included in the analysis. The buoyancy forces arise due to both the thermal and mass diffusion. The partial differential equations governing the flow have been solved numerically using both the implicit finite difference scheme and the difference-differential method. For the steady case, analytical solutions have also been obtained. The effect of time variation on the skin friction, heat transfer and mass transfer is very significant. Suction increases the skin friction coefficient in the primary flow, and also the Nusselt and Sherwood numbers, but the skin friction coefficient in the secondary flow is reduced. The effect of injection is opposite to that of suction. The buoyancy force, injection and the Hall parameter induce an overshoot in the velocity profiles in the primary flow which changes the velocity gradient from a negative to a positive value, but the magnetic field and suction reduce this velocity overshoot.

Avalanche speed in thin avalanche photodiodes

The duration of the avalanche multiplication process in thin GaAs avalanche photodiodes is investigated using a full band Monte Carlo (FBMC) model. The results are compared with those of a simple random path length (RPL) model which makes the conventional assumptions of a displaced exponential for the ionization path length probability distribution function and that carriers always travel at their saturated drift velocities. We find that the avalanche duration calculated by the RPL model is almost twice of that predicted by the FBMC model, although the constant drift velocities used in the former model are estimated using the latter. The faster response predicted by FBMC model arises partly from the reduced dead space but mainly from the velocity overshoot of ionizing carriers. While the feedback multiplication processes forced by the effects of dead space extend the avalanche duration in short structures, the effects of velocity overshoot in the realistic model more than compensate, significantly improving multiplication bandwidth.

Negative wake in the uniform flow past a cylinder

The upstream/downstream streamline shift and the associated negative wake generation (streamwise velocity overshoot in the wake) in a viscoelastic flow past a cylinder are studied in this paper, for the Oldroyd-B, UCM, PTT, and FENE-CR fluids, using the Discrete Elastic Viscous Split Stress Vorticity (DEVSS-ω) scheme (Dou HS, Phan-Thien N (1999). The flow of an Oldroyd-B fluid past a cylinder in a channel: adaptive viscosity vorticity (DAVSS-ω) formulation. J Non-Newtonian Fluid Mech 87:47–73). The numerical algorithm is a parallelized unstructured Finite Volume Method (FVM), running under a distributed computing environment through the Parallel Virtual Machine (PVM) library. It is demonstrated that both the normal stress and its gradient are responsible for the negative wake generation and streamline shifting. Fluid extensional rheology plays an important role in the generation of the negative wake. The negative wake can occur in flows where the fluid extensional viscosity does not increase rapidly with strain rate. The formation of the negative wake does not depend on whether the streamlines undergo an upstream or a downstream shift. Shear-thinning viscosity weakens the velocity overshoot and while shear-thinning first normal stress coefficient enhances the velocity overshoot. Wall proximity is not necessary for the velocity overshoot; however, it enhances the strength of the negative wake. For the Oldroyd-B fluid, the ratio of the solvent viscosity to the zero-shear viscosity plays an important role in the streamline shift. In addition, mesh dependent behaviour of normal stresses along the centreline at high De in most cylinder/sphere simulations is due to the convection of normal stress from the cylinder to the wake, which results in the maximum of the normal stress being located off the centreline by a short distance at high De.

Electron energy distribution during high-field transport in AlN

The energy distribution of electrons transported through intrinsic AlN heteroepitaxial films grown on SiC was directly measured as a function of applied field and AlN film thickness. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. Transport through films thicker than 95 nm at an applied field between 200 and 350 kV/cm occurred as steady-state hot electron transport following a Maxwellian energy distribution with a characteristic carrier temperature. At higher fields (470 kV/cm), intervalley scattering was evidenced by a multicomponent energy distribution featuring a second peak at the energy position of the first satellite valley. Velocity overshoot was observed in films thinner than 95 nm and at fields greater than 550 kV/cm. In this case, a symmetric energy distribution centered at an energy above the conduction band minimum was measured, indicating that the drift component of the electron velocity was on the order of the “thermal” component. A transient transport length of less than 80 nm was deduced from these observations.

Numerical Evaluation Of The Influence Of Fuel Generation On The Geometry Of A Diffusion Flame: Implications To Micro-gravity Fire Safety

Improvement in the understanding of the structure of flames in micro-gravity is a critical issue for spacecraft fire safety. A numerical study of the influence of a gas diffusion flame on a flow of air over a flat plate in micro-gravity environment is presented in this paper. 3D DNS simulations with a mixture fraction model for combustion have been performed with an adaptation of the Fire Dynamics Simulator code (FDS) developed at NIST. A particular flow speed representative of ventilation in spacecrafts and a fuel injection speed comparable to those induced by pyrolysis is used to illustrate the effect of the sample geometry, the fuel injection and thermal expansion of the flow characteristics. This work is benchmarked with experiments conducted on board of the CNES Airbus-300 and with the numerous observations available in the literature. It was observed that a leading edge can cause velocity overshoots and that fuel injection can have a significant effect in propagating the pressure perturbations downstream. Nevertheless, the calculations show that thermal expansion affects the flow in a much more dramatic way, thus becomes the most important parameter to be considered when evaluating the precision of analytical models.

Experimental observation of electron velocity overshoot in AlN

The energy distribution of electrons transported through intrinsic AlN heteroepitaxial films grown on 6H-SiC was directly measured as a function of the applied electric field. Following the transport, electrons were extracted into vacuum through a semitransparent Au electrode and their energy distribution was measured using an electron spectrometer. Transport through 80-nm-thick layers indicated the onset of quasiballistic transport for fields greater than 510 kV/cm. This was evidenced by a symmetric energy distribution centered at energies above the conduction band minimum. Drifted Fermi–Dirac energy distribution was fitted to the measured energy distribution, with the energy scale referenced to the bottom of the AlN conduction band. The drift energy and the carrier temperature were obtained as fitting parameters. Overshoots as high as five times the saturation velocity were observed and a transient length of less than 80 nm was deduced. In addition, the velocity-field characteristic was derived from these observations.

Transport Transient of Electrons in Wurtzite InN: The Effect of the Band Structure Anisotropy

We present a study on the transport transient of electrons in wurtzite InN subjected to an external electric field applied in the Γ–A or in the Γ–M directions. The electron drift velocity overshoot is shown to change with the field direction, being stronger in the latter than in the former case. This behavior is explained on the basis of the energy dependence of the band flattening, which is weaker in the Γ–A than in the Γ–M direction, and gives rise to a heavier electron effective mass.

Experimental and Theoretical Studies of Transient Electron Velocity Overshoot in GaN

We employ an optically detected time-of-flight technique with femtosecond resolution that monitors the change in the electroabsorption due to charge transport in an AlGaN/GaN heterojunction p-i-n diode to measure the electron velocity overshoot in GaN at room temperature. It has been found that electron velocity overshoot occurs at electric fields as low as 105 kV/cm, with the peak transient velocity increasing with E up to ∼320 kV/cm, at which field a peak velocity of 7.25 x 10 7 cm/s is attained within the first 200 fs after photoexcitation. At higher fields, the increase in transit time with increasing field suggests the onset of negative differential resistance due to intervalley transfer. The existence of transient velocity overshoot at fields lower than the calculated peak steady-state velocity suggests that it occurs while the electrons are primarily in the r valley. Full zone Monte Carlo calculations imply that the overshoot is associated more with band nonparabolicity in the Γ valley than with intervalley transfer at fields less than 325 kV/cm, and, in conjunction with theoretical calculations employing a semiclassical transport model, confirm the importance of this nonparabolicity for the determination of the temporal shape of the transient velocity overshoot curves.

Simultaneous observation of electron and hole velocity overshoots in an Al0.3Ga0.7As-based p–i–n semiconductor nanostructure

We report experimental results on simultaneous measurement of electron as well as hole transient transport in an Al0.3Ga0.7As-based p–i–n semiconductor nanostructure by using picosecond/subpicosecond Raman spectroscopy. Electron and hole velocity overshoots are directly observed. It is demonstrated that at T=300 K, E=15 kV/cm, and electron-hole pair density n≅5×1017 cm−1, electron overshoots its steady-state value by a factor of about 7; whereas hole about 2.5. These experimental results are discussed and explained.