Negative Resistance Characteristics Research Articles

Modulating the electronic transport properties of zigzag phosphorene nanoribbons through the coupling with the chromium triiodide molecules

Based on density functional theory (DFT) and non-equilibrium Green's function (NEGF) method, we have investigated spin-dependent electronic transport properties of zigzag phosphorene nanoribbons (ZPNRs) coupled with chromium triiodide (CrI3) molecules. Three different adsorption models have been considered: CrI3 molecule is placed on ZPNR surface (C-type), and CrI3 molecule is lateral adsorbed on the edge of ZPNR with its molecule panel vertical (V-type) or parallel (P-type). We discovered that ZPNR's electrical transport clearly displays spin polarization, owing to the coupling between ZPNR and CrI3 molecules. And spin-polarization is tightly dependent on the absorbing manners. Moreover, the negative differential resistance (NDR) characteristics can be observed from current-voltage curves and modulated by absorbing molecules. Furthermore, we also discovered that CrI3 is vertically adsorbed on the surface of the ZPNRs has a better spin-polarization efficiency than the other two cases.

Record peak current density of over 1500 kA/cm2 in highly scaled AlN/GaN double-barrier resonant tunneling diodes on free-standing GaN substrates

This work demonstrates high-performance AlN/GaN double-barrier resonant tunneling diodes (RTDs) with high peak current density grown by plasma-assisted molecular beam epitaxy on c-plane free-standing GaN substrates, featuring stable and repeatable negative differential resistance (NDR) characteristics at room temperature. By scaling down the barrier thickness of AlN barrier and the lateral mesa size of collector, the record peak current density of 1551 kA/cm2 is achieved along with a peak-to-valley current ratio (PVCR) of 1.24, which is attributed to the reduced resonant tunneling time under thinner AlN barrier and the suppressed external incoherent valley current by reducing the dislocation number contained in the RTD device with the smaller size of collector. By statistically analyzing the NDR performance of RTD devices with different thicknesses of AlN barrier, the average peak current density increases from 145.7 to 1215.1 kA/cm2, while the average PVCR decreases from 1.45 to 1.1, correspondingly, accompanying with a decreased peak voltage from 6.89 to 5.49 V, with downscaling the AlN barrier thickness from 1.5 to 1.25 nm. The peak current density obtained in this work is the highest value among all the reported nitride-based RTDs up until now while maintaining high PVCR value simultaneously, which illustrates that ultra-scaled RTD based on vertical quantum-well structure and lateral collector size is a valuable approach for the development of nitride-based RTDs with excellent NDR characteristics and reveals their great potential applications in high-frequency oscillation sources and high-speed switch circuits.

Effect of Interdiffusion and Crystallization on Threshold Switching Characteristics of Nb/Nb2O5/Pt Memristors.

The resistive switching response of two terminal metal/oxide/metal devices depends on the stoichiometry of the oxide film, and this is commonly controlled by using a reactive metal electrode to reduce the oxide layer. Here, we investigate compositional and structural changes induced in Nb/Nb2O5 bilayers by thermal annealing at temperatures in the range of 573-973 K and its effect on the volatile threshold switching characteristics of Nb/Nb2O5/Pt devices. Changes in the stoichiometry of the Nb and Nb2O5 films are determined by Rutherford backscattering spectrometry and energy-dispersive X-ray (EDX) mapping of sample cross sections, while the structure of the films is determined by X-ray diffraction, Raman spectroscopy, and transmission electron microscopy (TEM). Such analysis shows that the composition of the Nb and Nb2O5 layers is homogenized by interdiffusion at temperatures less than the crystallization temperature (i.e., >773 K) but that this effectively ceases once the films crystallize. This is explained by comparison with the predictions of a simple diffusion model which shows that the compositional changes are dominated by oxygen diffusion in the amorphous oxide, which is much faster than that in the crystalline phases. We further show that these compositional and structural changes have a significant effect on the electroforming and threshold switching characteristics of the devices, the most significant being a marked increase in their reliability and endurance after crystallization of the oxide films. Finally, we examine the effect of annealing on the quasistatic negative differential resistance characteristics and oscillator dynamics of devices and use a lumped element model to show that this is dominated by changes in the device capacitance resulting from interdiffusion.

Corrosion Potential Oscillation of Iron Electrodes in Nitric Acid

Although iron dissolves in diluted nitric acid solutions, it does not dissolve in concentrated solutions because of surface passivation. When a small amount of water is added to a concentrated solution, the dissolution and passivation of iron occur alternately, and the amount of gaseous products generated by nitric acid reduction varies in an oscillatory manner. During the corrosion process, the corrosion potential of iron oscillates spontaneously. In this study, we investigated the factors that cause oscillations in corrosion potential through electrochemical measurements using a three-electrode system and numerical simulations. The study revealed that an N-shaped negative differential resistance characteristic of iron oxidation plays a vital role in the oscillation of the corrosion potential and, simultaneously, the reduction of nitric acid results in oscillation. We considered metal corrosion has essential factors resulting in oscillatory instability. Thus, the corrosion potentials of various metals are expected to oscillate spontaneously under the appropriate conditions.

Spin transport properties of T-phase VSe2 2D materials based on eight-atom-ring line defects

Defects play a crucial role in modulating the properties of solid-state materials by inducing localized states that often benefit magnetic moments. Recently, direct experimental observations have shown the appearance of bunches of eight-atom-ring line defects in the T-phase VSe2 monolayer, a remarkable room-temperature two-dimensional magnetic metal. Using first-principles calculations, we have found that these line defects could enhance the magnetic moments, and half-metallicity (nearly 100% spin polarization) can be achieved in the structure with the densest defects. Furthermore, our analysis of the partial charge density distribution shows that a closer inter-defect distance between the d-states of vanadium atoms and p-states of selenium atoms increases the penalty of electrons occupying the spin-minority states. Moreover, we have discovered the high conductivity channel exhibits negative differential resistance characteristics within a bias range of 0.2 to 0.4 V (–0.4 to –0.2 V). Our findings broaden the scope of the applications of two-dimensional materials in spintronics and provide some theoretical guidance for the design of high-performance spintronic devices.

Non-equivalent nature of acetylenic bonds in typical square graphynes and intricate negative differential resistance characteristics

The role of acetylenic linkage in determining the exotic band structures of 4, 12, 2- and 4, 12, 4- graphynes is reported. The Dirac bands, as confirmed by both density functional theory and tight-binding calculations, are robust and stable over a wide range of hopping parameters between -hybridized carbon atoms. The shifting of the crossing points of the Dirac bands along the k-path of these two square graphynes is found to be in opposite direction with the hopping along with the acetylenic bond. A real space decimation scheme has also been adopted for understanding this interesting behavior of the band structure of these two graphynes. The condition for the appearance of a nodal ring in the band structure has been explored and critically tested by appropriate Boron-Nitrogen doping. Moreover, both the graphynes exhibit negative differential resistance in their current–voltage characteristics, with 4, 12, 2- graphynes showing superiority.

Photovoltaic and electrical investigation of In/WOx/CuPc/In heterojunctions with light intensity-dependent NDR behaviours

A peripherally tetrachalcone substituted copper(II) phthalocyanine was achieved by cyclotetramerization of the phthalonitrile in the presence of anhydrous CuCl2. The modified CuPc was thermally coated on WOx thin films from which a In/WOx/CuPc/In heterojunction was obtained. The surface of the thin films was investigated using scanning electron microscopy (SEM), where energy dispersive X-ray spectra (EDX) was used to confirm chemical structure. The crystal structures of the thin films were characterized using X-ray diffractometry (XRD). To determine the photovoltaic properties of the heterojunction, I–V plots were obtained under dark and various levels of illumination. It was seen that the heterojunction was responsive to the external light. Using the I–V plots, different diode parameters such as ideality factor, barrier height, series resistance, etc., were calculated. These plots also revealed that the In/WOx/CuPc/In heterojunction exhibits negative differential resistance (NDR) characteristics. Photodiode characteristics indicate that the heterojunction exhibits self-powered photodiode behaviours. It was seen that In/WOx/CuPc/In heterojunctions are sensitive to the external light that photovoltaic characteristics alter when the light intensity alters. However, no clear relation between the light intensity and photovoltaic characteristics were evidenced.

Robust fuzzy stabilization control for the traction converters in high-speed train

High-speed railways have quickly gained popularity as an environmentally beneficial mode of transportation. In addition, converters and electric motors are becoming more widely used. However, power converters and motors can act as constant power loads when tightly controlled, making the source converters unstable. In this article, robust fuzzy control is proposed for stabilizing power electronic converters feeding a constant power load in a high-speed train. The negative incremental resistance characteristics of the trains are first investigated. Then, utilizing the Takagi–Sugeno fuzzy model and the parallel distributed compensation approach, a robust stabilizing controller is constructed to ensure asymptotic descent of the dynamic system state trajectories toward the equilibrium position. Moreover, using linear matrix inequalities, the intended control gain can be automatically computed. The critical adjustment time of the controller and total harmonic distortions of the line currents are selected to measure and compare the dynamic stability of the designed controller. The suggested technique not only maintains global stability in the face of considerable changes in the constant power loads, but it also offers a fast dynamic reaction time and precise tracking over a wide operating range. Finally, simulations and experiments are performed to validate the proposed approach.

Nitrogen-doped zinc oxide nanoribbons for potential resonant tunneling diode applications.

Armchair ZnONRs doped with nitrogen are investigated in the current manuscript for possible applications based on negative differential resistance (NDR). To conduct the theoretical research, we use density functional theory (DFT) in conjunction with the non-equilibrium Green's function (NEGF) formalism to carry out first principles computations. Pristine ZnONR (P-ZnONRs) is a semiconductor with a wide energy bandgap (Eg) of 2.53 eV. However, one edge N-doped ZnONRs (SN-ZnO) and both edge N-doped ZnONRs (DN-ZnO) are metallic. Partial density of states (PDOS) reveals that the metallicity is caused by the doped nitrogen atom. The transport characteristics analysis revealed the negative differential resistance (NDR) characteristics in the N-doped ZnONRs. The peak-to-valley current ratios (PVCR) are computed and measured to be 4.58 × 1021 and 1.83 × 1022 for SN-ZnO and DN-ZnO, respectively. The obtained findings suggest the significant potential of armchair ZnONRs for NDR-based applications such as switches, rectifiers, oscillators, memory devices, etc.

A virtual adaptive RC damper control method to suppress voltage oscillation in DC microgrid

In DC Microgrid tightly regulated load converters operate as a constant power load (CPL). CPLs have negative incremental resistance characteristics, which leads to voltage oscillations and instability in DC microgrid systems. As a result, a virtual adaptive resistance–capacitance (VARC) damper control mechanism has been developed in this study to overcome the instability impact generated by the CPL. To satisfy the need for system stability, a technique to insert a virtual R-C connected in parallel with the capacitor (located on the feeder-side converter) is proposed. The proposed active damping control is self-adaptive for variations in load current. The proposed method allows for a straightforward setting of the control parameters, in which the dynamic performance of the load converter was unaltered. Stability is provided by an inherent and accurate adaptive control technique, as well as by strong resilience during sudden changes in input voltage and CPL plug-and-play. Both of these factors contribute to high levels of robustness. Therefore, practical restrictions such as weight reduction and low power dissipation, which are essential for onboard DC MGs, are maintained. The simulation results are carried out and the effectiveness of the proposed technique is tested by considering various operating conditions. Furthermore, the proposed technique is compared with the existing methods and it is found to be superior under various operating conditions.

Time-domain signal analysis of dielectric response of nonlinear SDBD thruster in near space

The nonlinear time domain response signals of surface dielectric barrier discharge (SDBD) are the basis for studying the energy loss of SDBD thruster. This paper is based on the construction of a double layer lumped parameter circuit model in solid dielectric and discharge plasma regions; based on virtual instrument technology, a test device for AC dielectric characteristics of a nonlinear SDBD thruster was built to obtain the time domain signals of AC voltage and response current. According to the characteristic spectral curve of the voltage and the response current signal in the time domain, the response current is decomposed into resistive current (conduction current) and capacitive current (relaxation current) by an appropriate algorithm. The experimental results show that the response current in the discharge plasma region is mainly the capacitive current, and the peak capacitive current is about 2.9 times of the peak resistance current; Relaxation planned loss is the main form of energy loss in the discharge plasma region. Due to space charge limiting current effect and space charge saturation effect, the static and dynamic resistance of discharge plasma region have platform area and negative resistance characteristics respectively; Both static and dynamic capacitance have negative capacitor characteristics.

Design of the Threshold-Controllable Memristor Emulator Based on NDR Characteristics.

Due to the high manufacturing cost of memristors, an equivalent emulator has been employed as one of the mainstream approaches of memristor research. A threshold-type memristor emulator based on negative differential resistance (NDR) characteristics is proposed, with the core part being the R-HBT network composed of transistors. The advantage of the NDR-based memristor emulator is the controllable threshold, where the state of the memristor can be changed by setting the control voltage, which makes the memristor circuit design more flexible. The operation frequency of the memristor emulator is about 250 kHz. The experimental results prove the feasibility and correctness of the threshold-controllable memristor emulator circuit.

A Double-PLLs-Based Impedance Reshaping Method for Extending Stability Range of Grid-Following Inverter Under Weak Grid

Conventional vector current control scheme has been widely used in grid-connected inverter systems. However, it suffers from severe power limitation issues under weak grids. This article reveals that the coupling effect between the power delivery and the voltage at the point of common coupling causes the static power limit, which only depends on physical parameters of the system. When the stability of the control scheme is considered, negative resistance characteristics of the phase-locked loop (PLL) mainly result in the small-signal stability limit, which is called the dynamic power limit in this article. Because the small-signal stability limit can be improved by using different control methods, it attracts lots of research attention. In this article, a simplified <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">d</i> – <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</i> small-signal impedance model is introduced to show the destabilizing factors. Then, an initial small-signal impedance reshaping method is proposed to counteract the major destabilizing factor caused by the PLL. Based on the initial impedance reshaping method, an improved control scheme is proposed. Through exhaustive mathematical analysis, it is proved that the dynamic power limit can be extended almost to the static power limit by using this proposed method. Finally, simulation and experimental results verify the effectiveness of the proposed method.

Gate- versus defect-induced voltage drop and negative differential resistance in vertical graphene heterostructures

To enable the computer-aided design of vertically stacked two-dimensional (2D) van der Waals (vdW) heterostructure devices, we here introduce a non-equilibrium first-principles simulation method based on the multi-space constrained-search density functional formalism. Applying it to graphene/few-layer hBN/graphene field-effect transistors, we show that the negative differential resistance (NDR) characteristics can be produced not only from the gating-induced mismatch between two graphene Dirac cones in energy-momentum space but from the bias-dependent energetic shift of defect levels. Specifically, for a carbon atom substituted for a nitrogen atom (CN) within inner hBN layers, the increase of bias voltage is found to induce a self-consistent electron filling of in-gap CN states, which in turn changes voltage drop profiles and produces symmetric NDR characteristics. With the CN placed on outer hBN layers, however, the pinning of CN states to nearby graphene significantly modifies device characteristics, demonstrating the critical impact of atomic details for 2D vdW devices.

Emergence of multiple negative differential transconductance from a WSe2 double lateral homojunction platform

Recent advances in two-dimensional (2D) van der Waals heterostructures have re-spurred the investigation of non-linear negative differential carrier transport transistors as a potential ingredient for the upcoming digital-driven hyperconnected era. Although notable findings employing various 2D heterojunctions have been recently demonstrated, the performance metrics are insufficient to fulfill requirements for practical device applications, and fundamental difficulties associated with multiple vertical stacking processes in device fabrication must be resolved. Here, we report the realization of multiple negative differential transconductance (NDT) and resistance (NDR) characteristics obtained from a single p-i-n WSe2 double lateral homojunction (DLHJ) transistor. Forming the DLHJ through the selective surface charge transfer doping of a homogeneous bilayer WSe2 film, double NDT characteristics that excel those expected from individual junction components and have high peak-to-valley current ratios (PVCRs) of 36.6 and 12.9 are achieved. Performing combined experimental characterizations and first-principle calculations, obtained exceptional NDT performance at room temperature is determined to originate from the modulation of trap-assisted tunneling and nonlinear 2D band shifts in response to the gate control. Furthermore, the quaternary logic operation based on the triple NDR with the largest PVCR of 137 is achieved by the correlated biasing of the WSe2 DLHJ device.

Input-Series-Output-Parallel DC Transformer Impedance Modeling and Phase Reshaping for Rapid Stabilization of MVDC Distribution Systems

This paper focuses on the instability problem of the medium-voltage DC (MVDC) distribution system and proposes an impedance phase reshaping (IPR) method. To obtain the load impedance model of the MVDC distribution system, the input impedance of the input-series-output-parallel (ISOP) DC transformer (DCT) is derived by the generalized average modeling (GAM). Based on the obtained model, the traditional ISOP DCT controller optimization (IDCO) approach is discussed and the IPR method is developed. An impedance phase controller is introduced based on the original control method. According to the optimized impedance stability criterion, the parameters of the impedance phase controller are determined. Compared with the IDCO approach, the proposed method weakens the negative resistance characteristic of the load impedance at the resonant frequency. Therefore, the MV bus voltage oscillation is rapidly mitigated. Besides, the dynamic performance of the system using the IPR method can be classified as good. The simulation results show that the mathematical model is correct, and the proposed method is effective for the rapid stabilization of MVDC distribution systems.

Understanding composite negative differential resistance in niobium oxide memristors

Volatile memristors, or threshold switching devices, exhibit a diverse range of negative differential resistance (NDR) characteristics under current-controlled operation and understanding the origin of these responses is of great importance for exploring their potential as nano-scale oscillators for neuromorphic computing. Here we use a previously developed two-zone, parallel memristor model to undertake a systematic analysis of NDR modes in two-terminal metal-oxide-metal devices. The model assumes that the non-uniform current distribution associated with filamentary conduction can be represented by a high current density core and a lower current-density shell where the core is assumed to have a memristive response due to Poole-Frenkel conduction and the shell is represented by either a fixed resistor or a second memristive region. A detailed analysis of the electrical circuits is undertaken using a lumped-element thermal model of the core-shell structure, and is shown to reproduce continuous and discontinuous NDR responses, as well as more complex compound behaviour. Finally, an interesting double-window oscillation behaviour is predicted and experimentally verified for a device with compound NDR behaviour. These results clearly identify the origin of different NDR responses and provide a strong basis for designing devices with complex NDR characteristics.

Modulation of electrical performance of zigzag edged tetra-penta-octagonal graphene nanoribbons based devices via boundary passivations

Recently, an sp2 hybridized planar two-dimensional graphene-based carbon allotrope composed of tetra-penta-octagonal rings has attracted considerable interest. The electronic structures and transport properties of zigzag-edged tetra-penta-octagonal-graphene nanoribbons (TPO-ZGNRs) modified via boundary passivation were investigated using density functional theory and the non-equilibrium Green’s function. TPO-ZGNRs were passivated with H, 2H, O, OH, and Cl on the edge to produce TPO-ZGNRs-X (X=H, 2H, O, OH, and Cl). TPO-ZGNRs-H exhibits metallic properties regardless of the nanoribbon width and exists in the non-magnetic state. Although TPO-ZGNRs-OH and TPO-ZGNRs-Cl remain metallic, TPO-ZGNRs-2H and TPO-ZGNRs-O are semiconductors. In addition, the current–voltage curves of three TPO-ZGNRs-X (X=H, OH, and Cl) homojunction device models contain a negative differential resistance region. Notably, the heterojunction device model in which TPO-ZGNRs are passivated with H and O atoms at different edge regions has negative differential resistance and rectification characteristics. This study is expected to facilitate the development of new carbon materials.

Room temperature asymmetric negative differential resistance characteristics of AlGaN/GaN resonant tunneling diodes grown by metal-organic chemical vapor deposition

A large area AlGaN/GaN resonant tunneling diode (RTD) grown by metal–organic chemical vapor deposition (MOCVD) is manufactured. The device adopts air-bridge structure suitable for the terahertz (THz) band, and a commercially available high-frequency GaN technology is applied in the process of device fabrication. Negative differential resistance (NDR) characteristics of the device are observed under the bipolar bias voltage at room temperature, which exhibits clear anti-symmetry in the peak-to-valley current ratio (PVCR), peak voltage and peak current density. The work systematically investigates how the built-in electric field introduced by polarization effects modulates the resonant tunneling transport utilizing TCAD tool Silvaco-ATLAS, and analyzes the enhancement of the resonant tunneling under negative bias voltage of the RTD with low-aluminum content AlGaN as barriers. The AlGaN/GaN RTD with conventional double-barrier single (DBS) quantum well (QW) structure shows reproducible NDR characteristics under the bipolar bias voltage at room temperature, which facilitates the design and extensive study of complicated device structures.

1039 kA/cm2 peak tunneling current density in GaN-based resonant tunneling diode with a peak-to-valley current ratio of 1.23 at room temperature on sapphire substrate

In this Letter, we present the excellent negative differential resistance (NDR) characteristics of AlN/GaN double-barrier resonant tunneling diodes (RTDs) in which the active layers are grown by molecular beam epitaxy on thick GaN-on-sapphire templates manufactured by metal-organic chemical vapor deposition. Here, indium flux is introduced as surfactant to reduce the interface roughness and improve the sharpness of heterointerface during epitaxial growth of AlN/GaN/AlN quantum well. The processed device with a top collector diameter of 1 μm size demonstrates a record peak current density of 1039 kA/cm2 while simultaneously featuring a peak-to-valley current ratio of 1.23 at room temperature, excellent achievements among all the reported GaN-based RTDs on any substrates. In addition, no degradation of device performance together with free of hysteresis is observed for the 1000 times consecutive up-to-down voltage sweeps under forward bias. These remarkable achievements are attributed to the marked improvement in heterointerface quality of AlN/GaN/AlN double-barrier quantum well by adopting indium as surfactant during epitaxial growth as clarified by transmission electron microscopy analysis, which dramatically suppresses the interface roughness scattering and elements interdiffusion, thus significantly improving the NDR signatures in current–voltage curves. The obtained results in this work illustrate that indium-surfactant added epitaxy technique turns out to be a promising approach for the modulation of vertical electron resonant tunneling in III-nitride heterostructures and realization of highly reproducible and reliable AlN/GaN double-barrier RTDs, in favor of implementation for future high-power solid-state electronics operating in terahertz spectra.