Negative Differential Conductance Effect Research Articles

Negative differential conductivity in the hopping transport model

AbstractThe effect of negative differential conductivity (NDC) in the hopping transport mode is studied by computer simulations. NDC is an important effect in a class of memory devices. Understanding the origin of NDC is vital to modeling these devices. Based on analytical arguments, it has been suggested by Shklovskii et al. that NDC can arise in the hopping model due to geometrical traps. Our computer simulations have confirmed this result. Another possibility is that the NDC is caused by Coulomb interaction between the charge carriers. This has been reported for special configurations of sites by Shin et al. We show that the NDC effect due to Coulomb interaction is not sensitive to the exact configuration of sites, and that it can appear also when the sites are placed randomly.

Resonant tunneling and negative transconductance in single barrier bilayer graphene structure

Using the nonequilibrium Green’s function method, the electronic transport in a gate-induced barrier bilayer graphene structure is investigated. Strong resonant effects are shown to result in high amplitude oscillation of conductance as a function of Fermi energy and barrier height. Beyond a small effect of negative differential conductance (with peak to valley ratio less than 2), strong oscillations of transconductance are achieved. The amplitude of such oscillations between positive and negative values may exceed 5 mS/μm. This effect might be helpful for further development of graphene-based nanoelectronics.

Exchange effects in spin-polarized transport through carbon nanotube quantum dots

We investigate linear and nonlinear transport across single-walled carbon nanotube quantum dots weakly coupled to spin-polarized leads. We consider tubes of finite length and small diameter, where not only forward scattering contributions of the Coulomb potential, but also short-ranged processes play an important role. In particular, they induce exchange effects leading for electron fillings 4n+2 either to a non-degenerate groundstate of spin S=0 or to a triplet groundstate. In the linear regime we present analytical results for the conductance - for both the S=0 and the triplet groundstate - and demonstrate that an external magnetic field is crucial to reveal the spin nature of the groundstates. In the nonlinear regime we show stability diagrams that clearly distinguish between the different groundstates. We observe a negative differential conductance (NDC) effect in the S=0 groundstate for antiparallel lead magnetization. In presence of an external magnetic field spin blockade effects can be detected, again leading to NDC effects for both groundstates.

Investigation of Bipolar Resistance Switching Properties in SrTiO<sub>3 </sub>Thin Films with Symmetric Electrodes Structure

A noncrystalline SrTiO3 (STO) thin film was grown on Pt/Ti/SiO2/Si substrates by pulsed laser deposition. The Pt thin film was deposited on STO film as top electrode, forming a symmetric structure (Pt/STO/Pt) to exclude the influence of the electrical difference in STO-electrode interfaces. It was found that a stable bipolar resistive switching was obtained in this structure. In a current sweep, an obvious S type negative differential conductance (NDC) phenomenon was found in both polarities. And in a voltage sweep, no NDC was observed due to the current control mechanism of S type NDC effect. The possible mechanism of this effect is discussed preliminarily.

Negative differential conductivity of positrons in gases

This paper reports on a new series of calculations of positron transport properties based on current experimental cross section data. It is found that negative differential conductivity (NDC) occurs in the bulk drift velocity W but not the flux drift velocity w. The origin of the phenomenon lies in the “reactive” nature of positron collisions associated with positronium Ps formation, and is quite different in origin to the better known NDC effect in w arising from certain combinations of inelastic–elastic cross sections. Moreover, while the Ps formation process is qualitatively similar (at least from a kinetic theory perspective) to electron attachment, it is characterized by a cross section several orders of magnitude larger and hence the “reactive” NDC effect is correspondingly more pronounced. In this paper we test both established conditions for NDC, and develop new criteria, using simple mathematics and physical arguments where possible.

Negative differential conductivity in n +-SnO 2:F/nano-porous TiO 2/InOHS/Au heterojunction

The effect of the negative differential conductivity (NDC) in the n +-SnO 2:F/nano-porous TiO 2/InOHS/Au heterojunction is observed at room temperature. It was shown the current–voltage curve with the NDC region, which appears on a positive branch after a negative voltage pulse, has been applied to the heterojunction. Differences in voltage and current between the extreme points, and position of the NDC region are established as a function of the preliminary pulse parameters. The NDC disappears after the following positive voltage biases or the supplementary UV/vis illumination. The possibility of assigning of the NDC region position with the negative pulse parameters and UV illumination is discussed. Tunnel current and the NDC effect in the n +-SnO 2:F/nano-porous TiO 2/InOHS/Au heterojunction is considered in a model of semiconductor (metal)–insulator–semiconductor with tunneling through deep electronic traps in por-TiO 2 with activation energy 175–375 mV. This effect could be useful for creating solid-state devices: memory cells and voltage light-controlled tunnel diodes.

Effect of Negative Differential Conductance in Carbon Nanotubes

Measurements of transport at high electric fields in metallic single-walled carbon nanotubes (CNTs) have shown either saturation of the current or a region of negative differential conductance (NDC) characterized by the current, after reaching a maximum, decreasing with further increase in voltage. We point out that both types of behavior are characteristic of NDC, but the NDC is masked in samples showing current saturation due to generation of space charge, leading to a nonuniform electric field. We derive the relation between the carrier concentration, the electric field at which the drift velocity peaks, and the length of the sample that is required for the NDC to be manifest as saturation.

Magnetophonon oscillations in the negative differential conductance of dilute nitrideGaAs1−xNxsubmicron diodes

We study the nonlinear electron dynamics that occurs in the dilute nitride alloy Ga(AsN) when, in high applied electric fields, conduction electrons gain sufficient energy to approach the energy of the localized N level. This process leads to a strong negative differential conductance (NDC) effect. In a quantizing magnetic field, we observe that the threshold electric field for NDC is resonantly modulated when electrons can relax energy by inter-Landau-level transitions involving the emission of a longitudinal optical phonon. This unusual manifestation of magnetophonon resonance reveals that the mechanism giving rise to NDC is an inherently fast $(<{10}^{\ensuremath{-}12}\phantom{\rule{0.3em}{0ex}}\mathrm{s})$ process of relevance for terahertz electronics.

Negative differential conductance observed in electron field emission from band gap modulated amorphous-carbon nanolayers

Amorphous-carbon (a-C)-based quantum confined structures were synthesized by pulsed laser deposition. In these structures, electrons are confined in a few nanometer thick sp2 rich a-C layer, which is bound by the vacuum barrier and a 3nm thick sp3 rich a-C base layer. In these structures anomalous field emission properties, including negative differential conductance and repeatable switching effects, are observed when compared to control samples. These properties will be discussed in terms of resonant tunneling and are of great interest in the generation and amplification of high-frequency signals for vacuum microelectronics and fast switching devices.

Diode effect in transport through a quantum dot coupled to non‐collinearly polarized ferromagnetic leads

Electron tunneling through a spin-split discrete level of an interacting quantum dot coupled to two ferromagnetic electrodes with non-collinear magnetizations is investigated theoretically by means of the nonequilibrium Green function approach. It is shown that the spin splitting of the dot level due to an external magnetic field leads to qualitatively new effects. In particular, numerical results show that the negative differential conductance and diode effect may occur in symmetrical junctions with non-collinear magnetizations and for large enough magnetic polarization of the leads. It is also found that in asymmetrical junctions with one electrode being half-metallic, the spin splitting gives rise to an enhancement of the diode-like features in transport characteristics. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Designable electron transport features in one-dimensional arrays of metallic nanoparticles: Monte Carlo study of the relation between shape and transport

We study the current and shot noise in a linear array of metallic nanoparticles taking explicitly into consideration their discrete electronic spectra. Phonon assisted tunneling and dissipative effects on single nanoparticles are incorporated as well. The capacitance matrix which determines the classical Coulomb interaction within the capacitance model is calculated numerically from a realistic geometry. A Monte Carlo algorithm which self-adapts to the size of the system allows us to simulate the single-electron transport properties within a semiclassical framework. We present several effects that are related to the geometry and the one-electron level spacing like e.g. a negative differential conductance (NDC) effect. Consequently these effects are designable by the choice of the size and arrangement of the nanoparticles.

Electron transport and effective ionization coefficients in C2F6, C2F6–Ar and C2F6–N2 mixtures

The electron drift velocity, the longitudinal diffusion coefficient and the effective ionization coefficients in C2F6 and its mixtures with Ar and N2 have been studied in a pulsed Townsend experiment. The density-reduced electric field strength E/N, was varied widely in the range 0.1–500 Td (1 Td = 10−17 V cm2). In all cases, negative differential conductivity (NDC) regions were found in the plots of the electron drift velocity as a function of E/N. Our drift velocity values were found to be in good agreement with previous work on C2F6 and its mixtures. In some cases, the E/N range was increased. The longitudinal diffusion coefficient also displays the NDC effects; these are more pronounced as the C2F6 content in the mixture decreases. The effective ionization coefficient for C2F6 was measured over a wider range of E/N than that covered by previous studies. Our effective ionization coefficients for C2F6 and those derived previously disagree strongly. We are not aware of any previously published data on effective ionization coefficients for the mixtures herein studied. The limiting field strength for C2F6–N2 turned out to be nearly a factor of two higher than that for C2F6–Ar, for C2F6 concentrations being less than 10% in both mixtures.

Negative differential conductance in a benzene-molecular device

We predict a negative differential conductance effect in the nonlinear transport through a molecular device containing a weakly coupled benzene ring. An effective interacting Hamiltonian for the π electrons (derived from an electronic structure calculation) serves as input for the transport calculation.

Spatially periodic structures in electron swarms: ionization, NDC effects and multi-term analysis

In reference (Robson R E, Li B and White R D 2000 J. Phys. B: At. Mol. Opt. Phys. 33 507), we revisited the Franck–Hertz experiment, and gave solutions of the Boltzmann equation describing the spatially-resolved relaxation profiles of a non-hydrodynamic swarm of electrons streaming at a steady rate from a plane source into mercury vapour. In this paper, we extend this study to other cases and develop a formalism for both ions and electrons and consider situations where both conservative and non-conservative collisions may take place. As in Robson et al (2000), we employ a `two-temperature' Burnett function representation of operators in velocity space in the Boltzmann equation. Configuration space is represented by a finite mesh of points and a finite difference technique is developed accordingly. Boundary conditions are specified for the general problem and techniques for solving the resulting large system of algebraic equations are discussed. The importance of a `multi-term' analysis and the existence of negative differential conductivity (NDC) under non-hydrodynamic conditions is displayed by considering electrons in methane. The explicit effect of ionization on the spatial relaxation profiles is considered along with a study on the importance of treating ionization as a true non-conservative process as opposed to another inelastic process. The spatial relaxation profiles are compared with predictions from eigenvalue theory.

Ambipolar Schottky barrier silicon-on-insulator metal–oxide–semiconductor transistors

A novel Schottky barrier silicon-on-insulator metal–oxide–semiconductor field-effect transistor featuring metallic source/drain and an electrical drain junction has been fabricated and characterized. The formation of electrical drain junction, or the field-induced drain (FID), is controlled by a metal field-plate overlying the passivation oxide. Excellent ambipolar operation is demonstrated on the device. Specifically, on/off current ratios up to 10 8 and 10 7 are observed for p- and n-channel operations, respectively. Meanwhile, the off-state leakage current shows very weak dependence on the voltage difference between the main-gate and the drain, highlighting the effectiveness of FID. Finally, negative differential conductance effect is observed for n-channel operation, which is identified as due to dynamic hot electron trapping during device characterization.

Space lateral transfer and negative differential conductance regimes in quantum waveguide junctions

Nonlinear conduction properties of multiport quantum waveguide-based devices are theoretically investigated. A two dimensional finite element solver for the time independent Schrödinger equation combined with Poisson equation has been developed. It handles arbitrary geometrical designs and potential profiles in the device active zone. Starting from transmission spectra calculated out of equilibrium, current–voltage characteristics of a quantum branch line directional coupler are derived as a function of the chemical potential of the injection region and of the applied bias between the input and output terminals of the structure. Under the approximation of ballistic transport, a detailed analysis of mono- and multimode propagation regimes shows that pronounced negative differential conductance effects combined with real space lateral transfers can be obtained. The robustness of the current–voltage characteristics as a function of temperature is also addressed.

S-type negative differential conductivity and voltage switching due to the avalanche in semiconductor heterostructures

A large S-type negative differential conductivity effect in reversed-biased heterostructure hot-electron diodes was found experimentally and explained theoretically. It was demonstrated that this effect originates both from an enhanced band-to-band impact ionization in the lightly doped -GaAs cladding region and from an accompanying electric field redistribution throughout the device.

Negative differential conductivity of electrons in electron-beam sustained discharge in pure xenon

The mechanism of negative differential conductivity (NDC) in an electron-beam sustained gas discharge of pure Xe is investigated by modeling. The drift velocity of electrons in Xe has been calculated using a self-consistent kinetic model. Model calculations involves the self-consistent solution of the Boltzmann equation for electron energy distribution function in the two-term approximation and a complete set of rate equations for the excited and ionic species in the discharge plasma. It is shown that the main factor responsible for the observation of the NDC effect is Coulomb interaction between electrons. Regions in experimentally achievable parameter space where NDC can be observed are determined.

Electron transfer between two coupled quantum wells in a resonant tunneling diode structure

Resonant tunneling effects between the quantum well of a double barrier heterostructure and the accumulation layer of the emitter region are investigated. Numerous singularities in the current-voltage characteristic of a GaAs Al 0.4Ga 0.6As diode, such as a negative differential conductance effect with a current contrast of 15:1, have been found and analyzed by means of self-consistent calculations of conduction band profile and tunneling probabilities. At relatively low bias, charge transfer can be interpreted as direct or scattering-assisted transitions with optical phonon emission, by considering the anti-crossing of the ground states or the energy conservation rules. At higher voltages, very fast tunneling processes at the time scale of relaxation mechanisms (through the second level of the accumulation layer) were demonstrated. Such a hot electron transition with direct evidence of negative differential conductance effect involves a non expected build-up of the resonance as a function of the symmetry of confining potentials in terms of transmission probabilities.

RESONANT TUNNELING IN DOUBLE BARRIER SEMICONDUCTOR NANO-STRUCTURES

The principles of operation of a double barrier resonant tunneling diode (DBRTD) giving rise to negative differential conductivity effect are first reviewed. Next, the physics of resonant tunneling based on (i) the time-independent conventional approach and (ii) the time-dependent quantum measurement approach, as applied to a DBRTD, is discussed. Expressions for the resonant tunneling current densities through the barriers are then derived on the ideas of quantum measurement. Through the well the current, however, flows by the conventional mechanism. The three current density magnitudes are found to be identical under resonant conditions. Finally, an expression for the resonant tunneling current density due to a group of incident electrons is derived.