Distribution Of Electron Concentration Research Articles

Quantum efficiency enhancement of reflective GaAs photocathodes with exponential-doping structure generating a favorable built-in electric field

The rapid development of GaAs photocathodes has led to an increased focus on the attainment of high quantum efficiency. Three types of exponential-doping structures with a high to low doping concentration distribution from the interior to the surface are proposed for reflective GaAs emission layers. These three structures generate different built-in electric fields that facilitate photoelectron emission. The one-dimensional continuity equations for the increasing, constant, and decreasing types of built-in electric fields are derived, respectively. The electron concentration distribution and quantum efficiency varying with the wavelength are solved numerically by the finite difference method. The simulation results indicate that the quantum efficiency of the GaAs photocathode with the increasing type of built-in electric field is superior to that with the constant built-in electric field, while the GaAs photocathode with the decreasing type of built-in electric field shows the worst performance. Then, the designed GaAs photocathodes with the increasing and constant types of built-in electric fields are grown by metal-organic chemical vapor deposition and activated by cesium-oxygen alternating deposition. The measured spectral response curves show that the quantum efficiency of the GaAs photocathode with the increasing type of built-in electric field is higher in the whole band than that with the constant type of built-in electric field. In addition, the exponential-doping structure generating the increasing type of built-in electric field is beneficial for improving the surface potential barrier and increasing the surface electron escape probability.

Different built-in electric fields for transmission-mode GaAs photocathodes through doping engineering: Design and modeling

In order to enhance the emission performance of transmission-mode GaAs photocathodes used in optoelectronic field, different exponential doping structures are designed for the GaAs emission layer to generate different types of built-in electric fields. These built-in electric fields include the constant, increasing and decreasing types, which can help photoelectron transport toward the emission surface. By solving the one-dimensional continuity equation using the finite difference method, the electron concentration distribution and quantum efficiency of the three types of exponential doping GaAs photocathodes are derived. Meanwhile, the light absorption distributions in the emission layer are simulated by the finite difference time domain method. By comparing the optical absorption distribution and the electron concentration distribution, it is concluded that, among the three types of exponential doping structures, the exponential doping structure generating the increasing built-in electric field has the most sufficient long-wave light absorption capacity and the strongest photoelectron transport ability, thereby achieving the highest quantum efficiency. This theoretical work can help understand the mechanism of improving the photoemission performance of GaAs photocathodes through doping engineering.

A semi-analytical and semi-numerical method for solving plasma instability of nonuniform two-dimensional electron gas

The plasma instability of two-dimensional electron gas (2DEG) is a crucial physical mechanism for generating terahertz radiation in field-effect transistors, especially in high electron mobility transistors (HEMTs). In this paper, we have proposed a new semi-analytical and semi-numerical method to deal with oscillation problems of any nonuniform 2DEG plasma, especially considering the steady-state distribution, which can be calculated and analyzed more quickly than only using numerical calculation. By constructing a wave equation, using the auxiliary function and Wentzel–Kramers–Brillouin approximation method, the wave vector of the plasma wave is obtained. On this basis, combined with the Dyakonov–Shur instability's boundary conditions, the oscillation frequency, the wave amplitude increment, and their correction caused by the nonuniformity can be obtained by numerical calculation. Furthermore, the analytical solution is obtained under reasonable approximate conditions for the linear distribution of electron concentration. It is proved that the electron concentration gradient in the channel will not only attenuate the wave increment but also decrease the plasmonic frequency in the case of linear distribution. Moreover, we get the reasons for the above conclusions through theoretical derivation. We also investigate the effects of various device parameters on attenuation, such as gate length, electron mobility, and voltage, which may explain the difference between the actual and theoretical values in HEMTs and provide new guidance for device design.

Spatial Distribution of Electron Concentration in a DC Glow Discharge Supported by a Hollow Cathode

Spatial Distribution of Electron Concentration in a DC Glow Discharge Supported by a Hollow Cathode

Two-dimensional modeling of diamond growth by microwave plasma activated chemical vapor deposition: Effects of pressure, absorbed power and the beneficial role of nitrogen on diamond growth

A two-dimensional (2D(r, z)) self-consistent model developed and validated in previous studies of diamond deposition processes in microwave (MW) plasma activated chemical vapor deposition reactors is applied in a systematic study of ways in which the gas pressure (over the range p = 75–350 Torr) and absorbed power (P = 1–3 kW) affect the plasma parameters, species distributions and diamond deposition processes from a gas mixture (1%CH4/H2 with 60 ppm added N2) at substrate temperatures (Ts = 1073 and 1323 K) and diameters (ds = 32–100 mm). A more limited set of process conditions for a 0.006%N2/4%CH4/H2 gas mixture and Ts = 1038–1153 K are investigated also. The study traces variations in the global distributions of electron concentration, electron and gas temperature, absorbed power density, key species concentrations and the hydrocarbon interconversion reactions, with particular focus on the radial profiles of CH3 radical and H atom concentrations just above the substrate and on predicted diamond growth rates, G(r). The results and the trends revealed when varying p and P should help guide future optimizations of deposition regimes. The absorbed power density is shown to exhibit quite steep gradients in both the radial (r) and axial (z) directions. This, and the finding of significant power absorption beyond the glowing plasma region, limits the utility of the oft-quoted ‘averaged power density’ as a parameter. The modeling also highlights the essential 2D character of the hydrocarbon interconversion and diffusion transfer processes, which challenges the applicability of any 1D modeling of such MW plasmas. Trace additions of N2 to a MW plasma activated CH4/H2 gas mixture have negligible effect on the plasma parameters or chemistry yet are known to boost the diamond growth rate. A semi-empirical expression for G(r) is developed further to explicitly include the effects of added N2 and a new mechanistic picture presented to account for the observed N-induced enhancements in G. This picture invokes stable moieties such as that formed by CH2 insertion into a CN dimer bond on the 2 × 1 reconstructed (100) diamond surface as ‘anchor’ sites that enable shorter CH2 surface migration lengths and more step-edges for irreversible incorporation of such migrating groups on the growing diamond surface.

Electrical properties of a high-precision hexagonal spiral silicon drift detector

With the deepening and expansion of semiconductor technology and research, in order to continuously optimize the structure and performance of semiconductor detectors, a high-precision hexagonal spiral silicon drift detector (SDD) is proposed in this paper. In order to obtain a more accurate spiral ring structure, this paper goes beyond the first-order formula in the Taylor expansion for calculating the radius of the spiral ring. Based on the first-order formula, the second-order formula for calculating the radius of the spiral ring is further developed and derived. The point coordinates are obtained by combining the radius, angle, and ring spacing change formula to obtain a more accurate spiral ring structure. The actual number of turns is more accurate than that obtained from first-order approximation, which better solves the problem of accurate calculation of the number of spiral rings and the structure of the spiral SDD in the existing technology, that is, the accurate calculation of the radius of the spiral ring. In order to verify the abovementioned theory, we model this new structure and use Technology Computer-Aided Design to system simulate and study its electrical properties, including potential distribution, electric field distribution, and electron concentration distribution. According to the simulation results, compared with the first-order formula, the second-order formula has better electrical properties; more uniform distribution of potential, electric field, and electron concentration; and a clearer electron drift channel.

Study of electrical properties and detection mechanism of a practical novel 3D-Spherical Electrode Detector

Among the 3D electrode Si detectors for high energy particle and X-ray detection, the traditional 3D-Trench electrode Si detector is a semiconductor detector that is widely used and discussed. Aiming at removing the shortcomings of the traditional 3D-Trench electrode Si detectors such as uneven electric field distribution, asymmetric electric potential, and the existence of some dead zone, we propose a new type 3D-Spherical Electrode Detectors and carry out extensive and systematic studies of their physical properties. We simulated detector electric field, electric potential, electron concentration distribution, full depletion voltage, leakage current, capacitance, the incident particle induced transient current and the weighting field. We systematically studied and analyzed detector’s electrical characteristics. By comparing with the traditional 3D-Trench electrode Si detectors, the new detector structure has more uniform electric field and potential, and less depletion voltage, leakage current and capacitance.

Regulation of the electron concentration distribution in TiO2/BaTiO3 photodetector

TiO2 film ultraviolet photodetectors (UVPDs) always suffered from high dark current and low on-off ratio, which have been studied by many works. To improve the performance of the UVPDs based on TiO2, we fabricated a UVPD based on TiO2 with self-polarized BaTiO3 (SPBTO) through a spin-coating method. Compared with the dark current (1.12 nA) and on-off ratio (1.15 × 103) of the single TiO2 film UVPD, that of TiO2-SPBTO film UVPD are 1.27 × 10−1 nA and 1.17 × 104, respectively. The result shows that TiO2-SPBTO has better photoelectric detection capabilities, which may be due to the modulating the electronic distribution of TiO2 films by SPBTO film. To explore the mechanism of the device, we analyzed the carrier distribution within the heterojunction by finite element analysis, which demonstrated that the SPBTO film modulates the electron concentration distribution of TiO2 film, resulting in a lower dark current and a higher on-off ratio. This work provides a potential candidate for high-performance UVPDs.

A simulation result of trapped charge in PPD CIS induced by total ionizing dose effect

This paper describes a method for simulating the total ionizing dose (TID) radiation effect of a PPD CMOS image sensor (CIS) using TCAD. This paper takes the CIS pixel unit as the research object, modeling the device structure and combining it with a gamma radiation model to simulate the distribution of radiation-induced charge generation rate, electron concentration, and potential at each interface in the entire region before and after irradiation with TID equal to 5, 20, 50, 80, 100, 200, and 300 krad(Si). At the same time, the curves of the above parameters changed with the increase of TID radiation have been simulated. Furthermore, the influence of different TIDs on the parameters mentioned above at each interface has been investigated based on the two main damage mechanisms of the TID radiation effect: radiation-induced oxide trapped charge and radiation-induced interface trapped charge. It was also discovered that as the TID increases, the depletion region of the pinned photodiode (PPD) becomes broader, causing the dark current to increase.

Expansion of the Coverage of Ionosondes for Vertical Ionospheric Sounding by the Application of Satellite Navigation System

A possibility of the ionosonde coverage expansion due to applying navigation satellite systems for supplying with the reliable short-wave radio communication acting along extended radio paths including multihop radio paths. It is shown on the basis of processing experimental data that the functionality increase of the vertical sounding ionosonde can be provided using the hardware-software ionospheric sounding complex that uses data of navigation satellites. The application of this complex in the place, where the vertical sounding ionosonde is located, allows to obtain the detailed picture of the electron concentration distribution not only far from the ionosonde but also inside the sounding region itself, along the trajectories that all points located under the ionosphere follow and that cross the responsibility zone of the vertical sounding station. It is shown that it is possible to create the dense network of the ionospheric control on the basis of vertical sounding stations distributed over a territory and hardware-software complexes.

Nonlinear finite element analysis of electromechanical behaviors in a piezoelectric semiconductor beam

In this paper, a one-dimensional beam model with extensional, flexure and shearing deformations for a piezoelectric semiconductor fiber is established based on the first-order beam theory and nonlinear electric current constitutive relation. To solve such a nonlinear beam model under transverse and axial forces, a finite element method is proposed together with an iterative process. Throughout numerical case studies, effects of the external forces and initial electron concentration are investigated on electromechanical fields. It is shown that the applied forces have a great influence on the distribution of electron concentration, which can freely regulate the electron transport in piezoelectric semiconductors.

Effects of an attached functionally graded layer on the electromechanical behaviors of piezoelectric semiconductor fibers

In this paper, we propose a specific two-layer model consisting of a functionally graded (FG) layer and a piezoelectric semiconductor (PS) layer. Based on the macroscopic theory of PS materials, the effects brought about by the attached FG layer on the piezotronic behaviors of homogeneous n-type PS fibers and PN junctions are investigated. The semi-analytical solutions of the electromechanical fields are obtained by expanding the displacement and carrier concentration variation into power series. Results show that the antisymmetry of the potential and electron concentration distributions in homogeneous n-type PS fibers is destroyed due to the material inhomogeneity of the attached FG layer. In addition, by creating jump discontinuities in the material properties of the FG layer, potential barriers/wells can be produced in the middle of the fiber. Similarly, the potential barrier configuration near the interface of a homogeneous PS PN junction can also be manipulated in this way, which offers a new choice for the design of PN junction based devices.

Analytical Models for External Cavity Lasers Using Tunable Etalons

Recently, external-cavity tunable lasers (ECTLs) have been widely used in WDM systems for their excellent characteristics, such as high side-mode suppression ratio (SMSR), low relative intensity noise (RIN) and narrow line width. In this paper, we propose an analytical model for external-cavity lasers using tunable etalons and elaborate on the selection of key parameters in the design scheme, which is rarely discussed in detail in previous researches. By numerically solving the model based on the rate equation and the transmission matrix, we analyzed the effects of different end-facet reflectivity on the SMSR, PI curves, electron and photon concentration distributions. In addition, By choosing appropriate physical parameters, we theoretically demonstrate an ECTL with a single wavelength tuning range of 1570.5–1603.6 nm (186.95–190.9 THz), and a power efficiency of 0.673 W/A, as well as an SMSR exceeds 50 dB.

Numerical Simulation of n‐MoSe2/p‐Si Solar Cells by AFORS‐HET

AbstractFor a thorough understanding of n‐MoSe2/p‐Si heterojunction solar cells, the effect of the MoSe2 band gap, the back electrode work function, the density of interface states, as well as the introduction of the interface layer on the performance of MoSe2/Si heterojunction solar cells is systematically investigated via AFORS‐HET simulation software. It is shown that a higher MoSe2 band gap, a higher back electrode work function, a lower density of interface states, and an introduction of a thin intrinsic hydrogenated amorphous silicon as an interface layer, are favorable for the achievement of high‐performance MoSe2/Si heterojunction solar cells. Through the simulation optimization, a photovoltaic conversion efficiency of 12.31% with an open‐circuit voltage of 0.61 V, a short‐circuit current density of 26.47 mA cm−2, as well as a fill factor of 75.4%, can be obtained for the n‐MoSe2/p‐Si heterojunction solar cells. To interpret the obtained simulation results, the authors have carefully analyzed the current–voltage curves, energy band diagrams, electron and hole concentration distributions, electron and hole recombination rates, etc. This study indicates that silicon‐based heterojunction solar cells with MoSe2 as an active layer are of great significance in the development of high‐efficiency photovoltaic devices.

On the compact modelling of Si nanowire and Si nanosheet MOSFETs

In this paper, three-dimensional technology computer aided design simulations are used to show that the electron concentration, current density, and electric field distribution from the interface at the lateral channels and from the top channel to the centre of the silicon wire, in nanowire and nanosheet structures, are practically same. This characteristic makes it possible to consider that the total channel width for these structures is equal to the perimeter of the transistor sheet, allowing to extend of the application of the symmetric doped double-gate model (SDDGM) model to nanowires and nanosheets metal-oxide-semiconductor field effect transistors, with no need to include new parameters. The model SDDGM is validated for this application using several measured and simulated structures of nanowires and nanosheets transistors, with different aspect ratios of fin width and fin height, showing very good agreement between measured or simulated characteristics and modelled. SDDGM is encoded in Verilog-A language and implemented in the SmartSPICE circuit simulator.

Design and simulation of silicon detector cells with spiral ring electrode structures

Traditional pixel detectors are neatly arranged rectangular or square electrodes that cover almost the entire surface. Large electrode areas lead to high capacitance, and capacitance is the primary factor affecting detector noise. A large noise signal will reduce detector detection performance and the system S/N ratio. We have designed a spiral ring electrode silicon detector cell with n-type silicon bulk and systematically studied its electrical properties, including electric potential distribution, electric field distribution, electron concentration distribution, leakage current, full depletion voltage, and detector capacitance. It has been verified that the spiral ring electrode silicon detector has a smaller capacitance than the traditional detector and a smaller depletion voltage compared with the previously designed structures such as the inner circle broom-shaped structure. At the same time, high position resolution pixel detectors can be made by the array of these detector cells.

Effects of edge and interior stresses on electrical behaviors of piezoelectric semiconductor films

We study electrical behaviors of a thin piezoelectric semiconductor film under edge stress or interior local stress. The theory of piezoelectricity with coupling to the diffusion-drift theory of semiconductors is used. It is reduced to a two-dimensional theory for the extension of thin piezoelectric semiconductor films. The two-dimensional equations are solved using COMSOL, a numerical analysis software. An n-type MoS2 monolayer is analyzed in detail. The electric potential and electron concentration distributions under edge stresses are calculated. The effects of the stress amplitude, the doping level of electrons and the in-plane material orientation are examined. The current-voltage relations with three edge electrodes under local stresses are also presented. It is shown that local stresses produce local potential barriers and wells which prohibit or allow currents depending on the stress level.

Theoretical bases of hypothetical sphere-electrode detectors and practical near-sphere-electrode (semisphere-electrode and near-semisphere-electrode) detectors

In this work, one-dimensional (1D) modeling and simulations of the electric and weighting fields will be systematically carried out for hypothetical sphere-electrode detectors. Exact 1D solutions are obtained for electric and weighting fields in these detectors. Although it is impossible to realize hypothetical sphere-electrode detectors with current technologies, the theoretical work and results on them are good bases for their offspring—practical semi-sphere-electrodes (semisph-electrode) and near-semisph-electrode detectors. The great reduction in full depletion voltage and small capacitance in semisph-electrode and near-semisph-electrode detectors make them very good for applications in photon sciences (x-ray) and safeguard and homeland security (hard x-ray and gamma-ray). A practical novel structure of the three-dimensional (3D) trench electrode detector, namely a novel semisph-electrode silicon detector has been proposed as a practical example. The novel detector has a semispherical shell electrode created in a cuboidal unit cell. It will overcome the traditional 3D-electrode detector deficiencies in electrical performance such as the asymmetrical electric field distribution. Through a computer-aided design tool, 3D modeling and electrical characteristics, simulation of a semispherical electrode silicon detector has been obtained, such as electric field distributions, electron concentration distribution, and full depletion voltage

First principle calculation of the effect of Cr, Ti content on the properties of VMoNbTaWMx (M = Cr, Ti) refractory high entropy alloy

The influence of Cr and Ti content on the phase structure, elastic constants, elastic properties and anisotropy of VMoNbTaWMx (M = Cr, Ti) refractory high entropy alloy was studied by the first principle method in this paper. Based on the plane wave pseudo-potential and density functional theory, the structure model of solid solution was established by virtual crystal approximation (VCA). By calculating the atom size difference (δ), valence electron concentration (VEC) and atom distribution (λ), it was indicated that the VMoNbTaWMx (M = Cr, Ti, 0 < x < 2) maintain a single BCC solid solution structure. The calculation results showed that with the increase of Cr content, the strength of the alloy increased, the plasticity presented a certain fluctuation, and the anisotropy first decreases and then increases. The anisotropy of VMoNbTaWCr was the lowest, VMoNbTaWCr1.75 had the highest plasticity and VMoNbTaWCr2 had the highest strength. With the increase of Ti element, the strength of the alloy decreased, the plasticity first decreased and then increased, and the anisotropy decreased first and then increased. VMoNbTaW had the best strength and plasticity and VMoNbTaWTi1.75 had the lowest anisotropy.

Monte Carlo simulation of RF breakdown in oxygen – the role of attachment

Breakdown in oxygen, in external radio-frequency (RF) electric field is analyzed by employing a Monte Carlo simulation (MCS). Results were obtained for 13.56 MHz and distance between electrodes of 15 mm. Physical background of an oxygen RF breakdown is explained by observing time-resolved spatial distributions of electron concentration, mean energy, elastic scattering rate, ionization rate and attachment. The role of attachment is investigated in cases when these processes are included and when they are not. Especially influence of the attachment is highlighted by comparing oxygen and argon breakdown-voltage curves and spatial profiles. The electron losses induced by attachment extend the motion of the electron prebreakdown swarm much closer to electrodes to achieve a greater production; hence, spatial profiles at high values of the product pd where p is the pressure and d is the gap between electrodes, become more similar to those at the minimum of the breakdown curve. The most striking difference between the breakdown curves in argon and in oxygen is in the high increase of the breakdown voltage for high pd in oxygen.