Velocity-field Relationship Research Articles

Corrections to “Modeling of the Gate Bias- Dependent Velocity-Field Relationship and Physics-Based Current-Voltage Characteristics in AlGaN/GaN HFETs”

Corrections to “Modeling of the Gate Bias- Dependent Velocity-Field Relationship and Physics-Based Current-Voltage Characteristics in AlGaN/GaN HFETs”

Disorder-induced domain wall velocity shift at high fields in perpendicularly magnetized thin films

Domain wall dynamics in a perpendicularly magnetized system is studied by means of micromagnetic simulations in which disorder is introduced as a dispersion of both the easy-axis orientation and the anisotropy constant over regions reproducing a granular structure of the material. High field dynamics show a linear velocity-field relationship and an additional grain size dependent velocity shift, weakly dependent on both applied field and intrinsic Gilbert's damping parameter. We find the origin of this velocity shift in the nonhomogeneous in-plane effective field generated by the tilting of anisotropy easy axis introduced by disorder. We show that a one-dimensional analytical approach cannot predict the observed velocities and we augment it with the additional dissipation of energy arising from internal domain wall dynamics triggered by disorder. This way we prove that the main cause of higher velocity is the ability of the domain wall to irradiate energy into the domains, acquired with a precise feature of disorder.

Intrinsic carrier mobility extraction based on a new quasi-analytical model for graphene field-effect transistors

The most common method of mobility extraction for graphene field-effect transistors is proposed by Kim. Kim’s method assumes a constant mobility independent of carrier density and gets the mobility by fitting the transfer curves. However, carrier mobility changes with the carrier density, leading to the inaccuracy of Kim’s method. In our paper, a new and more accurate method is proposed to extract mobility by fitting the output curves at a constant gate voltage. The output curves are fitted using several kinds of current–voltage models. Besides the models in the literature, we present a modified model, which takes into account not only the quantum capacitance, contact resistance, but also a modified drift velocity-field relationship. Comparing with the other models, this new model can fit better with our experimental data. The dependence of carrier intrinsic mobility on carrier density is obtained based on this model.

Temperature- and Doping-Dependent Anisotropic Stationary Electron Velocity in Wurtzite GaN

The influences of temperature, dopant density, free-carrier density, and field orientation on the electron drift mobility and velocity-field relationship in wurtzite <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</i> -axis GaN are quantified by means of theoretical investigation. Electron velocity perpendicular to the growth plane is uniformly lower than that parallel to the growth plane for field strengths below 500 kV/cm, although anisotropy within the basal plane itself is found to be insignificant. The calculated low-field electron mobility is demonstrated to be consistent with recent Hall measurements over a range of dopant densities. Low-field mobility is enhanced under the influence of free-carrier densities above the background doping due to both increased screening of ionized impurities and a reduction in A1-LO phonon lifetime through the plasmon-phonon interaction.

A THz-range planar NDR device utilizing ballistic electron acceleration in GaN

A planar and ultra-short gallium nitride (GaN) diode structure is investigated as a potential Terahertz (THz) range negative differential resistance (NDR) diode. An empirical velocity-field relation, exhibiting a peak electron velocity as high as 7 × 10 7 cm/s, is employed to characterize the high-field transport in the simulations, accounting for ballistic electron acceleration and velocity reduction due to phonon build up. The resulting device operation is in accumulation-layer transit-time mode and large-signal circuit simulation results are reported along with discussions. Conversion efficiencies up to ∼3.4% at ∼1.5 THz are shown to be possible.

Nonlinear transport in quantum-cascade lasers: The role of electric-field domain formation for the laser characteristics

We report on terahertz quantum-cascade lasers exhibiting discontinuities in the continuous-wave (cw) current-voltage and output power characteristics, which are related to the presence of electric-field domains (EFDs). The investigated lasers are based on a GaAs/Al0.25Ga0.75As heterostructure emitting between 4.2 and 4.4 THz and operate in cw mode up to 50 K. The observed effects related to EFDs are similar to those occurring in weakly coupled superlattices and can be described by the same equations, using an effective drift velocity-field relation. A qualitative agreement between experiments and simulations has been found.

Motion of transverse domain walls in thin magnetic nanostripes under transverse magnetic fields

The motion of transverse magnetic domain walls (TDW) in thin magnetic nanostripes under transverse magnetic fields (TMF) is investigated. In the absence of axial fields, an approximate static TDW profile is obtained under a TMF with an arbitrary orientation. This profile becomes exact if the TMF is parallel or perpendicular to the stripe plane. Under nonzero axial fields, the TDW becomes asymmetric and twisted, and it moves along the wire axis with two different propagation modes, rigid-body mode and precession mode, depending on the strength of the axial field. The critical strength separating these two modes is called modified Walker limit HW′. The TMF dependence of HW′, the TDW velocity and maximum twisting angle at HW′ were investigated both numerically and analytically. Moreover, it is shown that an early proposed velocity-field relationship fits well to the average velocities of a TDW above HW′. These results should be important for future developments of magnetic nanodevices based on DW propagation.

SIMULATION AND EXPERIMENTAL RESULTS ON GaN BASED ULTRA-SHORT PLANAR NEGATIVE DIFFERENTIAL CONDUCTIVITY DIODES FOR THz POWER GENERATION

A GaN based negative differential conductivity diode utilizing transient ballistic transport effects is proposed and large-signal circuit simulations along with preliminary experimental results are presented. The diode is an n +- n - n + structure and transport is described by an empirical velocity-field relation which is derived directly from femtosecond pulse-probe measurements available in literature and incorporated into the simulations through curve fitting. Efficient THz generation is predicted as a result of ~2.8 peak-to-valley ratio. Pulsed current-voltage characteristics were measured and N -type dependence was observed.

Generation of even harmonics of sub-THz radiation in bulk GaAs in the presence of a static electric field

The static electric field effects on nonlinear carrier dynamics in low-doped GaAs bulk under the influence of an intense sub-terahertz field are studied by a three-dimensional multivalleys Monte Carlo simulation. The conversion efficiency is calculated by using the appropriate Maxwell equation for the propagation of an electromagnetic wave along a given direction in the medium. Production of odd and even harmonics due to the nonlinearity of the velocity-field relation is investigated.

Charge, current, and noise partitioning in MOSFET in the presence of mobility degradation

The Ward-Dutton (WD) partitioning scheme is used extensively to develop transient and high-frequency advanced compact models in MOSFET analysis. However, it remains an open question if this scheme can be used for field-dependent mobility that is enhanced in state-of-the-art submicrometer technologies. In this paper, after demonstrating that the well-known WD partitioning is indeed invalid for field-dependent mobility, the authors develop a very general partitioning strategy that can always be defined in small-signal analysis for any arbitrary velocity-field relationship. It has also been shown that for large-signal operation, the existence of a partitioning scheme can be determined by the solution of an integral equation

Scaling effects in AlGaN/GaN HEMTs: Comparison between Monte Carlo simulations and experimental data

AlGaN/GaN based HEMTs are showing an unexpected behaviour of the transport characteristic compared to the GaAs and InGaAs HEMT technology. A downscaling of the source-gate and gate-drain lengths of GaN HEMTs produced a maximum output current increase, which is not compatible with a framework where the electrons velocity is saturated. Monte Carlo simulations are able to reproduce this experimental behaviour showing how this phenomena is related to intrinsic velocity-field relation in GaN. We attribute the lack of overshooting phenomena to the high scattering rate of III–V nitrides and to the strong localisation of the high electric field region. We also have found that such suppression could be avoided with a proper scaling of the devices.

Monte Carlo simulation of the hole transport properties for wurtzite GaN

The simulation of the hole transport properties of wurtzite phase GaN are performed by the full band ensemble Monte Carlo approach. The band structure data of GaN used in the simulation are obtained by the empirical pseudopotential method. The scattering mechanisms included in the simulation are acoustic phonon scattering, optical phonon scattering, polar optical phonon scattering, piezoelectronic scattering, ionized impurity scattering, as well as the intervalley scattering. The dependence of the hole average drift velocity and average energy in three main symmetry directions for wurtzite GaN on the electric field are obtained. At room temperature, the velocity-field relation shows the non-saturated characteristics. The maximum drift velocity is about 6×106 cm s-1, and the maximum average energy is 0.12 eV in the range of the field of this work. These values are much smaller than the corresponding parameters of the electrons in GaN. Furthermore, the dependence of the diffusion mobility on the impurity density is reported.

A semi empirical approach for submicron GaN MESFET using an accurate velocity field relationship for high power applications

A semi empirical model has been proposed for sub-micron GaN MESFET's to calculate the I–V characteristics using an accurate velocity-field relationship obtained by fitting it with the Monte Carlo (MC) simulation. The results so obtained are compared with the experimental results to validate our model and are also compared with the results obtained from the simple saturation model to present the influence of electron drift velocity modeling on the device parameters. The model has been extended to predict the microwave parameters such as transconductance and output conductance of the device.

Analytic Model of I–V Characteristics of 4H-SiC MESFETs Based on Multiparameter Mobility Model

A physical analytic model is developed for behavior of electron transport, and for the output current-voltage (I-V ) characteristics in 4H-SiC metal-semiconductor field-effect transistors (MESFETs). In this model, different analytical expressions are used for the electron mobility as a function of electric field and for the velocity-field relationship. Compared with the empirical three-parameter model, the multiparameter mobility model proposed in this paper is comparatively more realistic, since it better reproduces the drift velocity-field characteristics obtained by Monte Carlo (MC) calculations. Thus, the resulting I-V characteristics are in excellent agreement with experimental data.

Chaotic dynamics in terahertz-driven semiconductors with negative effective mass

We report on a detailed theoretical study of current self-oscillations and chaotic dynamics in negative effective mass (NEM) ${p}^{+}{\mathrm{pp}}^{+}$ diodes driven by dc and ac electric fields with a terahertz (THz) frequency. An ``$N$-shaped'' velocity-field relation is yielded by using the nonparabolic balance-equation theory with a realistic treatment of carrier scatterings by impurity, acoustic phonon, and optic phonon. The dependence of the self-oscillating mode and its frequency on the dc bias, doping concentration, and lattice temperature is examined in detail. The THz-driven ${p}^{+}{\mathrm{pp}}^{+}$ NEM diodes can produce a cooperative nonlinear oscillatory mode which leads to very complicated chaotic dynamics with the dc bias, ac amplitude, and ac frequency as the controlling parameters. The transitions between the periodic and chaotic states are carefully studied by different chaos-detecting methods, such as phase portrait, Poincar\'e bifurcation diagram, power spectrum, and first return map. The resulting power spectrum bifurcation diagram displays a very complicated mosaic structure with a self-similar emergence of high-order mixing frequencies.

TRANSPORT IN EXCITED STATES OF SEMICONDUCTOR SUPERLATTICES

The electronic conduction processes in the minibands and excited states of semiconductor superlattices are reviewed and discussed. The paper includes an introduction on miniband conduction and hopping conduction between localized states. It focusses subsequently on interminiband transport, both in the context of theoretical approaches and of experimental observations of these effects. Rabi oscillations between interacting Wannier–Stark levels are considered in detail. The role of scattering, and the manifestation of multipeaked velocity-field relationships on carrier and electric field profiles are evidenced. The final part concerns transport above the superlattice barriers, and quasi-ballistic transport in excited superlattice minibands.

Simulation of negative-effective-mass terahertz oscillators

We present a model calculation of hole current oscillations in doped p+pp+ ballistic diodes using the nonparabolic balance-equation theory and a time-dependent drift-diffusion model. Such an oscillation originates from a negative effective mass (NEM) region in the hole dispersion relation. In the present balance-equation calculation, we consider the scatterings by hole-impurity, hole-acoustic phonon, hole-polar-phonon, and hole-nonpolar-phonon–hole interactions, and yield a “N-shape” velocity-field relation, which are quite different from the two-valley results for electrons in bulk GaAs. We provide a detailed analysis of the resulting oscillations as a function of the applied voltage, base length, base doping, and the dispersion relation. Typical frequencies for a 0.2 μm structure NEM oscillator are in the terahertz range. Qualitative agreement is obtained between the present calculations and the existing Boltzmann results.

Nonlinear space-charge-limited ion currents in solid helium

Expressions are obtained for steady-state and transient space-charge-limited currents for a nonlinear velocity-field relation of the form v(E) = (μ/γ) × sinh(γE). Our experimental results are analyzed with these expressions. The positive-ion diffusion coefficient is a tensor with two activation energies associated with motion along c axis and in the basal plane. For bcc3He the dependence of negative-ion velocity on field is highly nonlinear and temperature dependent. Results are compared with those of other authors and discussed in light of different models for ionic motion in solid helium. The diffusion coefficients for positive ions and isotopic impurities are compared and discussed.

PCIM: a physically based continuous short-channel IGFET model for circuit simulation

We present an accurate analytical IGFET model (PCIM), for short-channel devices down to sub-half micron channel lengths. The model is described by a single drain current equation, valid for both weak and strong inversion regions of device operation. The model contains a new velocity-field (/spl upsi/-/spl epsi/) relation for carriers in the channel region. Combining this relation with the channel length modulation expression, obtained using engineering approximations to the two-dimensional fields near the drain end in saturation, results in an accurate drain conductance equation. The value for the carrier saturated velocity extracted from the I-V data for different CMOS technologies is 7-8/spl times/10/sup 6/ cm/s for electrons and 5-6/spl times/10/sup 6/ cm/s for holes, consistent with the reported values. The model not only predicts accurate output conductance, which is important for analog design, but also accurately simulates intrinsic gate capacitances for short channel devices. Since the model is inherently continuous, device conductances and capacitances are smooth and continuous at the transition points. This continuity results in enhanced convergence properties of the circuit simulator SPICE. Because the model is physically based, the temperature dependence of device characteristics in the temperature range 0-120/spl deg/C can easily be predicted simply by taking the temperature dependence of the threshold voltage, carrier mobility and velocity saturation parameters.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Enhanced optical effect in a high-electron-mobility phototransistor device: two-dimensional modeling considering a realistic velocity-field relation

An enhanced optical effect is observed in a high electron mobility transistor device when the radiation is allowed to fall on the gaps of the source, gate, and drain of a n -AlGaAs/GaAs system. A 2-D model is presented considering a realistic velocity field relation of carriers in the n -AlGaAs layer. Due to the incident radiation, a photovoltage is developed across the heterojunction in addition to band-edge discontinuity between the n -AlGaAs and GaAs layers. This photovoltage drags the electrons from the n -AlGaAs layer into the heterointerface, thus significantly enhancing the sheet concentration of 2-D electron gas. The drain current and transconductance of the device increase considerably. Plots have been made for the I-V characteristics and the transconductance versus gate voltage. Also, a ratio of drain current under illumination and drain current under a dark condition has been plotted against the input optical power density, which indicates that the overall gain of the device is enhanced by a factor of more than 10 compared to its dark case.