Charge Density Model Research Articles

Analytical modeling of threshold voltage and on-resistance in multi-barrier E-mode MISHEMT with gate-recess and field-plates

This work presents an analytical model for threshold voltage and On-resistance of multi barrier Metal–Insulator–Semiconductor High-Electron-Mobility-Transistor (MISHEMT) with gate-recess and field-plates. The device featuring a high two-dimensional electron-gas (2DEG) density in the channel region. The primary objectives of this device are to achieve a high threshold voltage (Vth) and enhance electron mobility with specific low ON-resistance (Ron_sp) by mitigating the degradation effects arising from scattering and interface-charges. Also, a physics based analytical model for Vth and 2DEG charge density at upper and lower channels is presented. This model is validated by comparing with TCAD numerical simulations and are well matched. The proposed MISHEMT demonstrates improved electron mobility in the lower channel of 1260 cm2/V.s, Vth of ∼2.6 V and Ron_sp is minimized by 33 % in contrast with a conventional MISHEMT. Additionally, the proposed MISHEMT becomes a promising device for achieving both high threshold voltage and mobility which are required for power semiconductor devices.

Investigation of Upward Leader Development Characteristics by Two Kinds of Charge Density Models in Wind Turbines

ABSTRACTThe upward discharge is a significant factor threatening the operation of wind farms. This paper investigates the inception and development of upward leaders of wind turbines' tip, where blade rotation prevents charge accumulation, using charge density models from laboratory discharges and artificially triggered lightning. The results indicate that larger return stroke currents create higher spatial potentials, and slower downward leader speeds allow for longer development time, favoring the development of upward leaders. As wind turbine sizes increase, the rotation of the blade tips resembles artificially triggered lightning, and the long air gap discharge model may underestimate the development of upward leaders. Calculations from the artificially triggered lightning model reveal improved upward leader formation when further laterally away from the downward leader within a certain distance, elucidating why multiple upward leaders are frequently observed around turbines. The striking distance and attractive radius are largely determined by the development of upward leaders. The absence of corona charge shielding at blade tips means upward leader inception is independent of downward leader velocity, emphasizing downward leaders' significant influence on turbines. Thus, the formula for estimating a wind turbine's lightning incidence requires integrating the return stroke current and velocity of the downward leader.

A Novel high-voltage DMG Fe-doped AlGaN/AlN/GaN HEMTs with sheet charge density model

In this work, two-dimensional electron gas of the dual material gate Fe doped AlGaN/AlN/GaN High Electron Mobility Transistors (HEMTs) device, sheet charge density model is demonstrated. The model is constructed by considering the electron charge in the barrier layer and the quasi triangular quantum well. This sheet charge density is calculated by Fermi-Dirac statistics by the formation of various sub energy bands and it is valid for all bias voltages. Higher carrier densities are produced and the introduction of the Fe-doped AlN layer reduces the electrons penetration to pass through the AlGaN barrier layer. The model's transcoductance variation is basically constant over an extensive variety of gate source voltages, thereby rendering it perfect regarding high-performance amplifications. The developed analytical model is demonstrated and the simulation results are validated with TCAD simulation. A good agreement is achieved with both analytical and simulation results for all of the voltages employed and device geometries.

Characteristics of Surface Charge Accumulation on Spacers and Its Influencing Factors

Charge accumulation usually happens on the surface of spacers under DC operation, which is susceptible to inducing surface flashover. In order to explore the surface charge accumulation mechanisms and the influences of dielectric conductivity, gas ion mobility, and temperature field on the surface charges, a time-varying charge density model at the gas–solid interface of spacers was established. The results of the simulation show that the discontinuity of the current density between the spacer bulk side and the gas ion flow is the fundamental reason for the charge accumulation on the spacer surface. Additionally, the value of current density fluxes at the interface continues to decrease with the change of the electric field, and the progress of charge transfer gradually stabilizes. Moreover, the dielectric conductivity directly affects the charge accumulation process, and there is a critical conductivity in which the effect of charge conduction in dielectrics counteracts that of gas-phase charge deposition, theoretically. When the conductivity is higher than the critical conductivity, the solid-side charge conduction is the main source of the surface charge accumulation, while the gas-phase charge deposition on the gas side plays a dominant role when the conductivity is lower than the critical conductivity. The charge accumulation is not significantly affected by gas ion mobility when the temperature is evenly distributed. However, under the temperature field with gradient distribution, the current density fluxes at the interface change, causing the polarity reverse of the accumulated charge. In the high-temperature region, the volume current density surges simultaneously with the conductivity, leading to a higher density of surface charge accumulation. Lastly, the design of spacers needs to keep the current densities on both sides of the interface as similar as possible in order to avoid excessive charge gathering in localized areas.

Modulation Mechanism of Transition Elements Fe and Mn on the Interface Bonding Performance of Bronze/Diamond Composites

The modulation mechanism of iron (Fe) and manganese (Mn) in transition‐metal elements on the interface bonding and mechanical properties of bronze (Cu3Sn)‐based/diamond composites is investigated through first‐principles calculations. Transition‐elements‐doping scenarios are investigated employing six‐layer slab models. It is revealed that the doping of Fe or Mn can make the Cu3Sn/diamond interface more stable, which effectively improves the wettability of the Cu3Sn/diamond interface based on the calculation results and analysis of interface energy, differential charge density model, and density of states. However, co‐doping with both Fe and Mn weakens the wettability of the Cu3Sn/diamond interface. Finally, wettability tests and microstructure characterizations demonstrate that the doping of Fe and Mn represents an effective approach to controlling the interface bonding performance of bronze/diamond composites.

In data we trust: X-ray diffraction experiments for charge density investigations.

A short commentary is given on the paper by Vosegaard et al. [Acta Cryst. (2023), 79, https://doi.org/10.1107000/S205252062300776X], which compares charge density models derived from four datasets measured on conventional diffractometers with the results of a high-quality dataset from SPring-8.

Deciphering the driving forces in crystal packing by analysis of electrostatic energies and contact enrichment ratios.

Hirshfeld surface analysis is a widely used tool for identifying the types of intermolecular contacts that contribute most significantly to crystal packing stabilization. One useful metric for analyzing these contacts is the contact enrichment descriptor, which indicates the types of contacts that are over- or under-represented. In this statistical study, enrichment ratios were combined with electrostatic energy (Eelec) data for a variety of compound families. To compute the electrostatic interaction energy between atoms, charge density models from the ELMAM2 database of multipolar atoms were used. As expected, strong hydrogen bonds such as O/N-H...N and O/N-H...O typically display large enrichment values and have the most negative (i.e. favorable) electrostatic energies. Conversely, contacts that are repulsive from an electrostatic perspective are usually the most under-represented. Analyzing the enrichment ratio and electrostatic energy indicators was shown to help identify which favorable contacts are the most competitive with each other. For weaker interactions, such as hydrophobic contacts, the behavior is less clear cut and can depend on other factors such as the chemical content of the molecule. The anticorrelation between contact enrichment and Eelec is generally lost for weaker contacts. However, we observed that C...C contacts are often enriched in crystal structures containing heterocycles, despite the low electrostatic attraction. For molecules with only weak hydrogen bond donors/acceptors and hydrophobic groups, the correlation between contact enrichment and Eelec is still evident for the strongest of these interactions. However, there are some exceptions where the most favorable contacts from an electrostatic perspective are not the most over-represented. This can occur in cases where the shape of the molecule is complex or elongated, favoring dispersion forces and shape complementarity in the packing.

Revisited relativistic Dirac-Hartree-Fock X-ray scattering factors. II. Chemically relevant cations and selected monovalent anions for atoms with Z = 3-112.

The previously described approach for determination of the relativistic atomic X-ray scattering factors (XRSFs) at the Dirac-Hartree-Fock level [Olukayode et al. (2023). Acta Cryst. A79, 59-79] has been used to evaluate the XRSFs for a total of 318 species including all chemically relevant cations [Greenwood & Earnshaw (1997). Chemistry of the Elements], six monovalent anions (O-, F-, Cl-, Br-, I-, At-), the ns1np3 excited (valence) states of carbon and silicon, and several exotic cations (Db5+, Sg6+, Bh7+, Hs8+ and Cn2+) for which the chemical compounds have been recently identified, thus significantly extending the coverage relative to all the earlier studies. Unlike the data currently recommended by the International Union of Crystallography (IUCr) [Maslen et al. (2006). International Tables for Crystallography, Vol. C, Section 6.1.1, pp. 554-589], which originate from different levels of theory including the non-relativistic Hartree-Fock and correlated methods, as well as the relativistic Dirac-Slater calculations, the re-determined XRSFs come from a uniform treatment of all species within the same relativistic B-spline Dirac-Hartree-Fock approach [Zatsarinny & Froese Fischer (2016). Comput. Phys. Comm. 202, 287-303] that includes the Breit interaction correction and the Fermi nuclear charge density model. While it was not possible to compare the quality of the generated wavefunctions with that from the previous studies due to a lack (to the best of our knowledge) of such data in the literature, a careful comparison of the total electronic energies and the estimated atomic ionization energies with experimental and theoretical values from other studies instils confidence in the quality of the calculations. A combination of the B-spline approach and a fine radial grid allowed for a precise determination of the XRSFs for each species in the entire 0 ≤ sin θ/λ ≤ 6 Å-1 range, thus avoiding the necessity for extrapolation in the 2 ≤ sin θ/λ ≤ 6 Å-1 interval which, as was shown in the first study, may lead to inconsistencies. In contrast to the Rez et al. work [Acta Cryst. (1994), A50, 481-497], no additional approximations were introduced when calculating wavefunctions for the anions. The conventional and extended expansions were employed to produce interpolating functions for each species in both the 0 ≤ sin θ/λ ≤ 2 Å-1 and 2 ≤ sin θ/λ ≤ 6 Å-1 intervals, with the extended expansions offering a significantly better accuracy at a minimal computational overhead. The combined results of this and the previous study may be used to update the XRSFs for neutral atoms and ions listed in Vol. C of the 2006 edition of International Tables for Crystallography.

Revisiting theN(N + 1)/2‐sites‐type Gaussian charge model for permutationally invariant polynomial fitting of global molecular tensor surfaces

AbstractThe exact expressions for the dipole, quadrupole, and octupoles of a collection ofNpoint charges involve summations of corresponding tensors over theNsites weighted by their charge magnitudes. When the point charges are atoms (in a molecule) theN‐site formula is an approximation, and one must integrate over the electron density to recover the exact multipoles. In the present work we revisit theN(N + 1)/2‐site point charge density model of Hall (Chem. Phys. Lett.6, 501, 1973) for the purpose of fitting ab initio derived multipole moment hypersurfaces using permutationally invariant polynomials (PIP). We examine new approaches in PIP‐fitting procedures for the dipole, quadrupole, octupole moments, and polarizability tensor surfaces (DMS, QMS, OMS and PTS, respectively) for a non‐polar CCl4and a polar CHCl3and show that compared to the primitiveN‐site model theN(N + 1)/2‐site model appreciably improves the relative RMSE of the DMS and does much more substantially so, by an order of magnitude, for the corresponding ones of QMS and OMS. Training datasets are obtained by sampling potential energies up to 18 000 cm−1above the global minima, generated by molecular dynamics simulations at the DFT B3LYP/aug‐cc‐pVDZ level of theory.

Electromagnetic field produced in high-energy small collision systems within charge density models of nucleons * *Supported by the NSFC (12075094, 11535005)

Recent experiments show that , an observable designed to detect the chiral magnetic effect (CME), in small collision systems ( ) is similar to that in heavy ion collisions ( ). This introduces a challenge to the existence of the CME because it is believed that no azimuthal correlation exists between the orientation of the magnetic field ( ) and participant plane ( ) in small collision systems. In this work, we introduce three charge density models to describe the inner charge distributions of protons and neutrons and calculate the electric and magnetic fields produced in small collisions at both RHIC and LHC energies. Our results show that the contribution of the single projectile proton is the main contributor to the magnetic field after averaging over all participants. The azimuthal correlation between and is small but not vanished. Additionally, owing to the large fluctuation in field strength, the magnetic-field contribution to may be large.

Сопоставление численных моделей динамики электрически заряженных газовзвесей с массовой и поверхностной плотностями зарядов для различных дисперсностей частиц

The purpose of the study was to mathematically model the dynamics of inhomogeneous electrically charged media, such as that of gas suspensions, i.e., solid particles suspended in a gas. The mathematical model implemented a continuum approach to modeling the dynamics of inhomogeneous media, which implies taking into account intercomponent heat transfer and momentum exchange. The carrier medium was described as a viscous, compressible, heat-conducting gas, the equations of the model were supplemented with initial and boundary conditions, and the system of the equations was integrated by an explicit finite-difference method. To obtain a monotonic grid function, a nonlinear scheme for correcting the numerical solution was used. The mathematical model was supplemented with the Poisson equation describing the electric field, which is formed by electrically charged disperse inclusions. The Poisson equation was integrated by finite-difference methods on a gas-dynamic grid. The flow of a gas suspension caused by the motion of dispersed particles under the action of the Coulomb force was studied numerically. Flows of gas suspensions with surface and mass densities of electric charge were modeled. For the surface charge density model, the Coulomb force acting on the unit mass of the gas suspension increases with a decrease in the dispersion of particles. For the mass charge density, the dispersion of particles does not affect the specific Coulomb force acting on the particles. The intensity of the gas suspension flow increases with decreasing particle size, both for the mass and surface models of charge density. For the surface charge density model, as the particle size decreases, the intensity of the gas pressure drop in the emerging gas suspension flow increases more than when modeling the gas suspension dynamics with a mass distribution of the electric charge density

Analytical modeling of I–V characteristics using 2D Poisson equations in AlN/β-Ga2O3 HEMT

In this paper, physics-based analytical models using two-dimensional (2D) Poisson equations for surface potential, channel potential, electric field, and drain current in AlN/β-Ga2O3 high electron mobility transistor (HEMT) is presented. The analytical expression of different quantities is achieved based on full depletion approximation of the AlN barrier layer and polarization charge induced unified two-dimensional electron gas (2DEG) charge density model. For the validation of the developed model, results are compared with 2D numerical simulation results, and a good consistency is found between the two. The drain current model is also validated with experimental results of a similar dimension device. The developed model can be a good reference for different β-Ga2O3-based HEMTs.

A Fractal Model for Effective Excess Charge Density in Variably Saturated Fractured Rocks

AbstractEstimating hydraulic properties and monitoring water flow in fractured rocks using self‐potential observations essentially relies on our ability to model streaming potential. One of the most promising approaches for modeling electrokinetic couplings is based on the macroscopic quantification of the excess charge which is effectively transported by pore water flow. In this study, we derive a fractal model to predict the effective excess charge density for fully and partially water saturated fractured media. Fractures are conceptualized as parallel plates with a fractal pattern described by the Sierpinski carpet. From the calculation of the excess charge in a single fracture and a flux averaging upscaling procedure, we obtain closed‐form expressions for the effective excess charge density. This new analytical model explicitly depends on the fracture water ionic concentration, interface properties, water saturation, porosity and permeability. Model predictions under saturated conditions are compared to published data measured in laboratory during hydraulic fracturing. The model development also shows the independence of the excess charge from the pore shapes: when expressed in terms of hydrological parameters, one can find an expression identical to recently published models for sedimentary porous media using capillary tubes. These results extend the validity of the existing models to fractured rocks and highlight the importance of hydraulic parameters for an accurate modeling of electrokinetic couplings.

Analytical model for 2DEG charge density in β-(Al x Ga1−x )2O3/Ga2O3 HFET

In this paper, a physics-based analytical model for two-dimensional electron gas (2DEG) charge density (n s) and pinch-off voltage (V off) in -(Al x Ga1−x )2O3/Ga2O3 heterostructure field effect transistor (HFET) is reported. The modeling approach includes solving the Poisson–Schrödinger equation in -(Al x Ga1−x )O3 barrier layer followed by using standard Fermi–Dirac statistics equations for calculating 2DEG charge density in the potential well. The developed model is then used to analyze the effect of Al composition (x) and barrier thickness (d) of -(Al x Ga1−x )2O3 layer on 2DEG charge density. The obtained parameters from our model for charge density and pinch-off voltages are validated with the experimental results in the literature demonstrating a good accuracy. It was observed that the 2DEG charge density increases with Al composition and Al x Ga1−x O barrier layer thickness, but at the cost of increased pinch off voltage. A trade-off between both is necessary to achieve parametric optimization of -(Al x Ga1−x )2O3/Ga2O3 HFET. This model will be useful to study the various parametric constraints for device optimization in terms of 2DEG charge density and pinch off voltage.

Further Validation of Quantum Crystallography Approaches.

Quantum crystallography is a fast-developing multidisciplinary area of crystallography. In this work, we analyse the influence of different charge density models (i.e., the multipole model (MM), Hirshfeld atom refinement (HAR), and the transferable aspherical atom model (TAAM)), modelling of the thermal motion of hydrogen atoms (anisotropic, isotropic, and with the aid of SHADE or NoMoRe), and the type of radiation used (Mo Kα and Cu Kα) on the final results. To achieve this aim, we performed a series of refinements against X-ray diffraction data for three model compounds and compared their final structures, geometries, shapes of ADPs, and charge density distributions. Our results were also supported by theoretical calculations that enabled comparisons of the lattice energies of these structures. It appears that geometrical parameters are better described (closer to the neutron values) when HAR is used; however, bonds to H atoms more closely match neutron values after MM or TAAM refinement. Our analysis shows the superiority of the NoMoRe method in the description of H-atom ADPs. Moreover, the shapes of the ADPs of H atoms, as well as their electron density distributions, were better described with low-resolution Cu Kα data in comparison to low-resolution Mo Kα data.

Predicting the frequency-dependent effective excess charge density: A new upscaling approach for seismoelectric modeling

The seismoelectric method is based on the capacity of seismic waves to generate measurable modifications of the electrical field in porous media. Even though it combines the advantages of seismic and geoelectrical methods, it remains largely underused in hydrogeophysics. Its signal results from an electrokinetic coupling that can be modeled using either the coupling coefficient or the effective excess charge density. The traditional approach is based on the frequency-dependent coupling coefficient, which relates the pressure drop with the change in the electrical potential. A more recent approach consists of describing the excess charge that is effectively dragged by water flowing within the pores. We have developed a new model for the frequency-dependent effective excess charge density. For this, we make use of a mechanistic upscaling of the electrokinetic coupling in a capillary. This novel approach introduces inertial effects arising within the pore space in the flux-averaging procedure to explain the frequency dependence of the effective excess charge density. The presented model is successfully compared to previous models and published data. This new frequency-dependent upscaling approach has the potential of fundamentally improving our current understanding of the seismoelectrical signal in more complex environments, such as partially saturated and fractured media.

Analytical Model for Two-Dimensional Electron Gas Charge Density in Recessed-Gate GaN High-Electron-Mobility Transistors

A physics-based analytical model for GaN high-electron-mobility transistors (HEMTs) with non-recessed- and recessed-gate structure is presented. Based on this model, the two-dimensional electron gas density (2DEG) and thereby the on-state resistance and breakdown voltage can be controlled by varying the barrier layer thickness and Al mole fraction in non-recessed depletion-mode GaN HEMTs. The analytical model indicates that the 2DEG charge density in the channel increases from 2.4 × 1012 cm−2 to 1.8 × 1013 cm−2 when increasing the Al mole fraction from x = 0.1 to 0.4 for an experimental non-recessed-gate GaN HEMT. In the recessed-gate GaN HEMT, in addition to these parameters, the recess height can also control the 2DEG to achieve high-performance power electronic devices. The model also calculates the critical recess height for which a normally-ON GaN switch becomes normally-OFF. This model shows good agreement with reported experimental results and promises to become a useful tool for advanced design of GaN HEMTS.

Electrostatic force of dust deposition originating from contact between particles and photovoltaic glass* *Project supported by the National Natural Science Foundation of China (Grant Nos. 12064034 and 11562017), the Leading Talents Program of Science and Technology Innovation in Ningxia Hui Autonomous Region, China (Grant No. 2020GKLRLX08), the CAS Light of West China Program (Grant No. XAB2017AW03), and the Key Research

Charged photovoltaic glass produces an electrostatic field. The electrostatic field exerts an electrostatic force on dust particles, thus making more dust particles deposited on the glass. In this paper, the contact electrification between the deposited dust particles and the photovoltaic glass is studied. Meanwhile, the surface charge density model of the photovoltaic glass and the electrostatic force of charged particles are analyzed. The results show that with the increasing of the particle impact speed and the inclination angle of the photovoltaic panel, the charges on particles increase to different degrees. Under a given condition, the electrostatic forces acting on the charged particles at different positions above the glass plate form a bell-shaped distribution at a macro level, and present a maximum value in the center of the plate. As the distance between the particle and the charged glass decreases, the electrostatic force exerted on the particle increases significantly and fluctuates greatly. However, its mean value is still higher than the force caused by gravity and the adhesion force, reported by some studies. Therefore, we suggest that photovoltaic glass panels used in the severe wind-sand environment should be made of an anti-static transparent material, which can lessen the dust particles accumulated on the panels.

A lattice Boltzmann analysis of the electro-thermo convection and heat transfer enhancement in a cold square enclosure with two heated cylindrical electrodes

In this paper, the electro-thermo convection (ETC) of a perfectly insulted liquid induced by two energized and heated circular cylinders in a cold square cavity is numerically discussed. The abundant flow motion phenomenen are comprehensive results of the buoyancy force and Coulomb force. A newly developed unified lattice Boltzmann (LB) model for flow field, electric potential, charge density and temperature field is applied to direct numerical simulations. Influences of Rayleigh number Ra, electric Rayleigh number RaE on heat transfer enhancement are studied under strong charge rejection regime. Based on the numerical results, it is found that the efficiency of heat transfer decreases with the rise of Ra, and the contributions made by external electric field for heat transfer enhancement can be ignored when Ra is sufficiently large. While the heat transfer capacity always enhances as the increase of RaE, especially when parameter RaE exceeds a critical threshold. Additionally, the effects of geometric parameter δ on heat transfer coefficient are also analyzed.

Dynamics for droplet-based electricity generators

The finding of power generation from droplets (Yin et al., 2014) [1], based on the moving boundary of electrical double layer, has triggered great research enthusiasm, and a breakthrough in instantaneous electric power density was achieved recently by a droplet-based electricity generator (DEG) (Xu et al., 2020) [2]. However, the dynamic mechanism for such droplet-based electricity generators remains elusive, impeding optimization of the DEGs for practical applications. Through comprehensive experiments, we developed a dynamic model of surface charge density that can explain the underlying mechanism for the DEGs. The spreading droplet in touch with the top electrode can be equivalently regarded as an additional part of the top electrode, and the change of droplet area causes the change of surface charge density of the top electrode, driving electrons to migrate between the two electrodes. The insight of the dynamic mechanism paves a way for optimal design and practical applications of the DEGs. • Direct correspondence between the droplet-induced voltage signal and area change rate dS/dt of the droplet is revealed. • A dynamic model based on experiments is proposed, which provides deep insights of the underlying mechanism for the DEGs. • Variable impacts on the droplet-based electricity generator are quantitatively demonstrated.