Calculated Current-voltage Characteristics Research Articles

Frequency-phase-locking mechanism inside DC SQUIDs and the analytical expression of current-voltage characteristics

Direct-current superconducting quantum interference devices (dc SQUIDs) are ultra-sensitive flux-to-voltage convertors widely applied for biomagnetism and geophysics; they are a kind of magnetic-field-effect transistors (MFETs) with flux-modulated current-voltage characteristics. Compared to semiconductor FETs, dc SQUIDs still lack analytical expressions to interpret their inner formation mechanisms of the current-voltage characteristics. This article presents a frequency-phase-locking (FPL) model to derive the analytical expression of dc SQUIDs and reveal how the current-voltage characteristics are shaped by the circuit parameters between two Josephson junctions. The application of the analytical expression in the calculations of current-voltage characteristics is demonstrated; the results are compared with the numerical simulations. It is shown that a dc-SQUID is an FPL system inside and works as a current-modified nonlinear resistor in readout circuits; its current-voltage characteristics are the projections of three impedances of the network between Josephson currents. Those understandings enable electronic engineers to evaluate the design of dc-SQUID circuits directly through three network impedances extracted from the layout.

Hydrogenated cove-edge aluminum nitride nanoribbons for ultrascaled resonant tunneling diode applications: a computational DFT study

While synthesizing quasi-one-dimensional nanoribbons, there is a finite probability that edges have cove-edge defects. This paper focuses on the structural, electronic, and transport properties of cove-edge aluminum nitride nanoribbons (AlNNR) using density functional theory and the non-equilibrium Green’s function (NEGF) method. The cove-edge AlNNRs are thermodynamically stable and exhibit metallic behavior. Interestingly, the calculated current–voltage characteristics of the cove-edge AlNNR-based nanodevices show negative differential resistance (NDR). The H-AlN-Cove nanodevice exhibits high peak-to-valley current ratio (PVCR) of the order of 107. The calculated PVCR of the H-AlN-Cove nanodevice is 106 times higher than that of the silicene nanoribbon (SiNR) and graphene nanoribbon (GNR), and 104 times higher than that of the phosphorene nanoribbon (PNR) and arsenene nanoribbons (ANR)-based devices respectively. The NDR feature with high PVCR provides a prospect for the cove-edge AlNNR in nanodevice applications.

Gap distance dependence on field emission at the nanogap between silicon cleavage surfaces

In this study, we developed a model of field emission at a vacuum nanogap composed of semiconductor electrodes and conducted numerical calculations to evaluate its field emission characteristics. The current at the silicon nanogap was also measured, and the results were compared with the calculated results. Large-area vacuum nanogaps are expected to be used for highly efficient room-temperature thermionic power generation owing to their good thermal insulation, and their electrical conduction of tunneling current is sufficient for practical use. Because vacuum nanogaps should be parallel and smooth with a large area, we proposed and examined the use of cleavage to fabricate nanogaps in single-crystal materials. A nanogap with a nominal area of 26.5 μm2 was fabricated in a microelectromechanical device to evaluate its properties, and the model was used for the analysis. The calculated current–voltage characteristics of the semiconductor nanogap of both n-type and p-type silicon were affected by the bandgap and the presence of carriers, i.e., electrons and holes. Moreover, the Fowler–Nordheim plot was confirmed to be valid for semiconductor nanogaps based on the calculated results. We proposed an effective work function that compensates for the discrepancy between semiconductor nanogaps and the Fowler–Nordheim theory and enables a more accurate analysis. Field-emission currents flowing through a 1.1 nm nanogap were measured and matched with the calculated results by adjusting the field enhancement factor and emission area. The model developed and confirmed experimentally in this work will be helpful in examining field emissions at semiconductor nanogaps.

Stochastic behavior of an interface-based memristive device

A large number of simulation models have been proposed over the years to mimic the electrical behavior of memristive devices. The models are based either on sophisticated mathematical formulations that do not account for physical and chemical processes responsible for the actual switching dynamics or on multi-physical spatially resolved approaches that include the inherent stochastic behavior of real-world memristive devices but are computationally very expensive. In contrast to the available models, we present a computationally inexpensive and robust spatially 1D model for simulating interface-type memristive devices. The model efficiently incorporates the stochastic behavior observed in experiments and can be easily transferred to circuit simulation frameworks. The ion transport, responsible for the resistive switching behavior, is modeled using the kinetic cloud-in-a-cell scheme. The calculated current–voltage characteristics obtained using the proposed model show excellent agreement with the experimental findings.

АВТОМАТИЗИРОВАННЫЙ РАСЧЕТ СТАТИЧЕСКИХ ХАРАКТЕРИСТИК КРЕМНИЕВЫХ БЫСТРОВОССТАНАВЛИВАЮЩИХСЯ ДИОДОВ ДЛЯ ПРИМЕНЕНИЯ В УСТРОЙСТВАХ СИЛОВОЙ ЭЛЕКТРОНИКИ

Calculating, analysing and verifying current-voltage characteristics (CVC) of a silicon fast-recovery diode (FRD) are performed using the methods of instrumental-technological modelling. The article determines the planar structure configuration of a semiconductor device crystal, the levels of doping and doses of irradiation with high-energy particles, at which the required values of operating and maximum forward currents and reverse (breakdown) voltages are achieved. The main tasks to be solved in the article are to determine the optimal design and technological parameters of the fast-recovery diode (FRD) structure and verifying the calculated current-voltage characteristics (CVC) values with the experimental data taken from the SKKE310F12 fast recovery diode at various ambient temperatures. The novelty of the work is represented by the obtained verified instrumental-technological model of FRD, which is possible for industrial implementation in the form of a semiconductor device at the production base of domestic microelectronics enterprises. The results of the research can be used in producing the element base of power electronics for pulsed power converters, namely discrete FRD, FRD structures as part of power modules based on MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and IGBT transistors.

Electro-physical properties of GaAs nanoscopic clusters

The electric current through the Cu-GaAs-Cu nanoclusters and their quasi-bandgap widths (HOMO-LUMO differences) are calculated. The ab initio electron propagator theory (Green-Keldysh functions formalism) is implemented for the calculations (that has been never done before for Cu-As-Cu nanoclusters). The calculated current-voltage characteristics exhibit non-linear character with magnitudes of current reaching several pAs. The calculated quasi-bandgap width (HOMO-LUMO difference) of nanocluster exhibits solid-like behavior with the increase of nanocluster's size. The obtained dependencies and characteristics will be useful for semiconductor quantum-dots manufactures and practitioners in the field of semiconductor nanoelectronics and optoelectronics, in particular, for developers of EM radiation sources and detectors with frequency-tuning ability.

Mathematical model of current-voltage characteristic of steady-state aluminum anodization

The developed mathematical model satisfactorily describes the dependence of anodic potential on the current density of steady-state aluminum anodization in the region of average current densities where its main assumptions are fulfilled: the absence of merging of adjacent pores and the absence of overheating of oxide relative to the electrolyte. The model involves several parameters required for calculations. For anodization of aluminum, the values of these parameters are obtained by the analysis of experimental results published by various authors. Calculated current-voltage characteristics agree well with published experimental data. Using the model equations and appropriate parameters, one can calculate not only the dependence of anodic potential on the current density, but also main relationships between the characteristics of anodization: the barrier layer thickness, potential, porosity, spherical radius of pore bottom and spherical radius of cavity in the metal.

Sharp Negative Differential Resistance from Vibrational Mode Softening in Molecular Junctions.

We unravel the critical role of vibrational mode softening in single-molecule electronic devices at high bias. Our theoretical analysis is carried out with a minimal model for molecular junctions, with mode softening arising due to quadratic electron-vibration couplings, and by developing a mean-field approach. We discover that the negative sign of the quadratic electron-vibration coupling coefficient can realize, at high voltage, a sharp negative differential resistance (NDR) effect with a large peak-to-valley ratio. Calculated current-voltage characteristics, obtained based on physical parameters for a nitro-substituted oligo(phenylene ethynylene) junction, agree very well with the measurements. Our results establish that vibrational mode softening is a crucial effect at high voltage, underlying NDR, a substantial diode effect, and the breakdown of current-carrying molecular junctions.

A microstructure-oriented mathematical model of a direct internal reforming solid oxide fuel cell

A standard fuel cell requires hydrogen for operation; however, to achieve near-term applications fuel cells need to use the existing infrastructure and already available fuels such as methane. In this respect, solid oxide fuel cells are promising candidates for distributed stationary power generation. However, feeding methane directly into the anode is liable to carbon deposition at low temperatures or to cause thermal stress due to the endothermic steam-methane reforming reaction at high temperatures. Because of these drawbacks, direct chemical conversion of methane on the Ni-based anode has not been realized in practical applications so far. Therefore, numerical modeling plays a vital role to optimize the anode and thereby assure the safe operating condition of the direct reforming process. This paper presents experimental and numerical studies on the direct internal reforming solid oxide fuel cell. A microstructure-orientated mathematical model of micro-scale transport phenomena, chemical and electrochemical reactions in an anode-supported solid oxide fuel cell is developed. The model divides a cell-assembly into six computational subdomains to bypass typical numerical artifacts and assure a realistic charge-transfer distribution in the electrodes. The proposed novel mathematical model positively passed verification with the comparison of the calculated current-voltage characteristics with the experimental ones.

Microscopic Tunneling Model of Nb–AlN–NbN Josephson Flux-Flow Oscillator

Since the very first experimental realization of a Josephson flux-flow oscillator (FFO), its theoretical description has been limited by the phenomenological perturbed sine-Gordon equation (PSGE). While PSGE can qualitatively describe the topological excitations in Josephson junctions that are sine-Gordon solitons or fluxons, it is unable to capture essential physical phenomena of a realistic system such as the coupling between tunnel currents and electromagnetic radiation. Furthermore, PSGE neglects any dependence on energy gaps of superconductors and makes no distinction between symmetric and asymmetric junctions: those made of two identical or two different superconducting materials. It was not until recently when it became possible to calculate properties of FFO by taking into account information about energy gaps of superconductors (Gulevich et al. in Phys Rev B 96:024215, 2017). Such approach is based on the microscopic tunneling theory and has been shown to describe essential features of symmetric Nb–AlO\(_\mathrm{x}\)–Nb junctions. Here, we extend this approach to asymmetric Nb–AlN–NbN junctions and compare the calculated current–voltage characteristics to our experimental results.

Accurate Modeling of Gate-Induced Drain Leakage for III-V Mosfets

As the channel length of the metal oxide semiconductor field-effect transistors (MOSFESTs) enters into the nanoscale regime, quantum mechanical effects such as carrier confinement, interference effects and tunneling become dominant. Therefore, accurate and efficient device simulation tools based on a quantum mechanical formalism are necessary to interpret experimental results. Among several approaches, the Non-Equilibrium Green’s Function (NEGF) method provides most rigorous framework for treating quantum transport in nanoscale devices. However, a full NEGF treatment can be highly demanding and in some cases almost impossible to implement due to numerical burden. As a concrete example, let us examine Fig. 1. The schematic shows the well-known band diagram for gate induced drain leakage. When the gate voltage is lowered at high drain voltage, a tunneling current can flow from the bulk to the drain through band-to-band tunneling. There are two energy regions of interest: (i)E1; the two ends of which are cut off by the band edges and (ii)E2; which is the region underneath the valence band in the body region but above E1. In a ballistic scenario, the region E2 is devoid of electrons and all the band-to-band current flows in the region E1. This is also what is calculated in NEGF. However, in reality, electron-electron interaction quickly fills up E2 according to the Fermi statistics, and so a tunneling current should also flow in this region.Especially, for direct and low band gap materials such as III-V, this current can be a significant part of the total tunneling current. The problem is that it is numerically extremely challenging, if not impossible, to include electron electron interactions in the NEGF formalism, and therefore to account for the current that flows in E2. To account for this problem, in this work, we have taken a two-pronged approach. First, we show that a Wentzel-Kramers-Brillouin (WKB) approximation calculated using the imaginary part of the full band-structure provides a very close agreement with NEGF results (typically within a few percent). This is especially accurate at low current levels where the Laplace solution of the potential provides a very good estimate for the self-consistent potential of the problem. We then combine this with a mode-space calculation of the currentvoltage characteristics. The mode-space approach is particularly valid when the device is uniform along the transport direction as it is the case for double gate or most of the FinFET structures. The mode space approach demands much smaller numerical burden as each mode is solved independently. This also means that in a standard approach, it cannot account for band-to-band tunneling. Our strategy is to combine the WKB approximation with the mode space approach so that the ordinary current is calculated using the mode space while the band-to-band tunneling current is calculated using WKB. We first show that this combined strategy provides current-voltage characteristics that show excellent agreement with full, real-space, NEGF calculations, including the band-to-band tunneling in the E1 region. We then add the additional current that flows in E2 by assuming a Fermi distribution of the electrons in that region. Our work thus provides a very fast, yet accurate approach to calculate currentvoltage characteristics in direct bandgap semiconductors using the NEGF method. Figure 1

Josephson flux-flow oscillator: The microscopic tunneling approach

We elaborate a theoretical description of large Josephson junctions which is based on the Werthamer's microscopic tunneling theory. The model naturally incorporates coupling of electromagnetic radiation to the tunnel currents and, therefore, is particularly suitable for description of the self-coupling effect in Josephson junction. In our numerical calculations we treat the arising integro-differential equation, which describes temporal evolution of the superconducting phase difference coupled to the electromagnetic field, by the Odintsov-Semenov-Zorin algorithm. This allows us to avoid evaluation of the time integrals at each time step while taking into account all the memory effects. To validate the obtained microscopic model of large Josephson junction we focus our attention on the Josephson flux flow oscillator. The proposed microscopic model of flux flow oscillator does not involve the phenomenological damping parameter, rather, the damping is taken into account naturally in the tunnel current amplitudes calculated at a given temperature. The theoretically calculated current-voltage characteristics is compared to our experimental results obtained for a set of fabricated flux flow oscillators of different lengths. Our theoretical calculation agrees well with the obtained experimental results, and, to our knowledge, is the first where theoretical description of Josephson flux flow oscillator is brought beyond the perturbed sine-Gordon equation.

Electron Transport Properties of Two-Dimensional Si1P1 Molecular Junctions**Supported by the National Natural Science Foundation of China under Grant No 11374033.

We focus on two new 2D materials, i.e., monolayer and bilayer silicon phosphides (Si1P1). Based on the elastic-scattering Green’s function, the electronic-transport properties of two-dimensional monolayer and bilayer AuSi1P1-Au molecular junctions are studied. It is found that their bandgaps are narrow (0.16 eV for a monolayer molecular junction and 0.26 eV for a bilayer molecular junction). Moreover, the calculated current-voltage characteristics indicate that the monolayer molecular junction provides constant output current (20 nA) over a wide voltage range, and the bilayer molecular junction provides higher current (42 nA).

Simulation of electron transport in GaAs/AlAs superlattices with a small number of periods for the THz frequency range

The electron transport in superlattices based on GaAs/AlAs heterostructures with a small number of periods (6 periods) is calculated by the Monte Carlo method. These superlattices are used in terahertz diodes for the frequency stabilization of quantum cascade lasers in the range up to 4.7 THz. The band structure of superlattices with different numbers of AlAs monolayers is considered and their current–voltage characteristics are calculated. The calculated current–voltage characteristics are compared with the experimental data. The possibility of the efficient application of these superlattices in the THz frequency range is established both theoretically and experimentally.

Endohedral metallofullerenes, M@C60 (M = Ca, Na, Sr): selective adsorption and sensing of open-shell NOx gases

Based on density-functional theory and non-equilibrium Green's function calculations, we demonstrate that endohedral metallofullerenes (EMFs) are reactive to open-shell gases, and therefore have the potential application as selective open-shell gas sensors. The adsorption of eight gas species (CO, H2O, H2S, NO2, NO, SO2, O2 and NH3) on three EMFs (M@C60, M = Ca, Na and Sr) shows that the adsorption energies of the EMFs towards NO2 and NO are significantly higher than the closed-shell species. Moreover, the high selectivity appears relatively insensitive to the inserted metal atoms. The calculated current-voltage characteristics of gold-M@C60-gold structures (M = Ca, Na) show that the adsorption of NO2 leads to significant change in conductivity, suggesting a potential application as an EMF gas resistive sensing device.

Modeling of the polymer solar cell with a P3HT:PCBM active layer

In this paper we present a theoretical model for simulating the behavior of a polymer solar cell with a poly(3-hexylthiophene):1-(3-methoxycarbonyl) propyl-1-phenyl-[6, 6]-methanofullerene (P3HT:PCBM) active layer. Two different types of boundary conditions were considered, Dirichlet’s and mixed. For Dirichlet’s boundary conditions we have achieved an excellent agreement with the experiment. The influence of boundary conditions on the appearance of the s-shaped current-voltage characteristic (sometimes observed in experiments) has been investigated. When mixed boundary conditions are applied, calculated current-voltage characteristics are inevitably s-shaped. By altering the boundary carrier concentration, an s-shaped deformation in current-voltage characteristics is numerically simulated by using Dirichlet’s boundary conditions.

Magnetoresistance anomalies resulting from Stark resonances in semiconductor nanowires with a constriction

Magnetotransport properties of a semiconductor nanowire with a constriction have been studied within the Landauer–Büttiker formalism in the presence of the axially oriented magnetic field at low temperatures. The one-electron quantum states in the nanowire have been calculated within the adiabatic approximation which takes into account the three-dimensional structure of the nanowire and allows us to study the effect of the transverse quantum states on the electronic current. The calculated current-voltage characteristics exhibit well pronounced peaks that result from the enhancement of the electron transmission by the Stark resonant states formed in the triangular quantum well near the constriction. The effect of the Stark resonances is clearly manifested in the magnetoresistance as a function of the drain-source voltage. The calculated magnetoresistance exhibits two interesting features: (i) rapid jumps at certain voltages, caused by the enhancement of the electron transmission by the Stark resonances, (ii) changes of sign that stem from the magnetic-field induced changes of the current-voltage characteristics slope. The influence of the constriction parameters (radius, length, smoothness of the potential barrier, position of the constriction in the nanowire) on the electronic current has also been analyzed. Since the effective potential barrier created by the constriction in the nanowire is similar to that generated by the negatively charged gate surrounding the nanowire, the presented results can also be applied to the description of the magnetoresistance in the gated nanowires.

Generalization of the Townsend current-voltage characteristics of a corona discharge

The statistical treatment of the current-voltage characteristics of a corona discharge in gases and the generalization of them on the basis of the theory of similarity is considered. The dependence of the corona discharge current on the applied voltage is assumed to be two-parametric of the Townsend type. The parameters of the current-voltage characteristics are found using the least squares method on the basis of experimental data. The calculated current-voltage characteristics are reduced to a dimensionless form and are generalized in the form of the equation of the first quadrant bisector. The criteria and conditions of similarity of corona discharges are found. By the example of a positive corona discharge in helium at different pressures, there is generalized a number of experimental current-voltage characteristics using the proposed method.

Spin transistor operation driven by the Rashba spin-orbit coupling in the gated nanowire

A theoretical description has been proposed for the operation of the spin transistor in the gate-controlled InAs nanowire. The calculated current-voltage characteristics show that the electron current flowing from the source (spin injector) to the drain (spin detector) oscillates as a function of the gate voltage, which results from the precession of the electron spin caused by the Rashba spin-orbit interaction in the vicinity of the gate. We have studied the operation of the spin transistor under the following conditions: (A) the full spin polarization of electrons in the contacts, zero temperature, and the single conduction channel corresponding to the lowest-energy subband of the transverse motion and (B) the partial spin polarization of the electrons in the contacts, the room temperature, and the conduction via many transverse subbands taken into account. For case (A), the spin-polarized current can be switched on/off by the suitable tuning of the gate voltage, for case (B) the current also exhibits the pronounced oscillations but with no-zero minimal values. The computational results obtained for case (B) have been compared with the recent experimental data and a good agreement has been found.

Modeling the current-voltage characteristics of thin-film silicon solar cells based on photo-induced electron transfer processes

Power conversion efficiency of p-i-n type macrocrystalline silicon (µc-Si:H) solar cells has been analyzed in terms of sequential processes of photo-induced electron transfer. The effect of the excitonic state on the charged carrier generation has been studied compared to a conventional scheme in which only charged carriers are taken into account for the operation of the solar cells. A numerical model has been developed to calculate current-voltage characteristics of solar cells on the basis of two types of charged carrier generation processes (exciton process and charged carrier process). The light trapping effect due to a textured back surface reflector (BSR) was embedded in the numerical model by using the effective medium theory in combination with the matrix method in the field of the electromagnetic theory of light. As an application of this modeling, it was found that the reported data of the power conversion efficiency were not explained by the conventional charged carrier process model and that the combined model of the charged carrier process with the exciton process well explains the performance of the p-i-n type μc-Si:H solar cells. In this way, the typical power conversion efficiencies were estimated to be 10.5% for the device (i-layer thickness: 1.8 μm) with the BSR (period: 600 nm; height: 250 nm) and 8.6% for the device with the flat reflector under the condition that the fractions of the exciton process and charged carrier process were 60% and 40%, respectively.