Negative Differential Resistance Region Research Articles

Harmonic Reflection Amplifier for Widespread Backscatter Internet-of-Things

The highly power-efficient backscattering communication scheme holds solid potentials to lead the Internet of Things technologies. However, the asymmetric radio frontend architectures that ensure cheap and pervasive communicating things and tags at one end of the link, overburden the other end of the link (“reader”) with complicated and expensive hardware. This turns out to prevent the integration of the technology in the wide spectrum of consumer electronic devices (e.g. RF smartphone platforms). By separating TX and RX frequency bands, the harmonic backscattering scheme drastically loosens the technological constraints toward integration. We propose here the use of a single tunnel diode to build a harmonic reflection amplifier (HRA), using the nonlinear behavior of the negative differential resistance region to both generate harmonics and amplify the signals. The circuit also behaves like a common backscatter, with the same-frequency backscattering modulation for operation with legacy readers. Operating below 150 mV, the HRA drains 144-μW dc power with a measured positive conversion gain of 18 dB.

Computer Simulation of the Electric Transport Properties of the FeSe Monolayer

Abstract The paper deals with the model research of electric transport characteristics of stressed and non-stressed FeSe monolayers. Transmission spectra, current-voltage characteristic (CVC) and differential conductivity spectra of two-dimensional FeSe nanostructure have been calculated within the framework of the density functional theory and non-equilibrium Green’s functions (DFT + NEGF). It has been shown that the electrophysical properties depend on the geometry of the sample, the substrate, and the lattice constant. On CVC of non-stressed sample in the range from −1.2 V to −1 and from 1.2 V to 1.4 V, a region of negative differential resistance (NDR) has been observed. NDR is at both signs of the applied voltage due to the symmetry of the nanostructure. d2I/dV2 is used to determine the nature of the electron-phonon interaction and the features of quasiparticle tunnelling in stressed and non-stressed samples. The results obtained can be useful for calculating new elements of 2D nanoelectronics.

Estimation of Kinetic Parameters of Additives By Semi-Analytical Solution

The electrodeposition of Cu and other metals is an essential step in the interconnect fabrication, 3-D integration, and various packaging technologies. Such electrodeposition processes are enabled by a group of organic additives. Furthermore, strong agitation is often used to expedite the electrodeposition rate when the processing time becomes critical for large features, which however introduces non-uniform mass transport within a single feature and between different features. A numerical description of how organic additives behave in conjunction with external flow at various feature geometry and dimensions is a challenging task, but much needed for chemistry and process development. Among the various requirements, the additive behavior or their electrochemical impacts on Cu deposition needs to be precisely described.This talk reports an effort to obtain the numerical parameters that describe the additive behaviors during Cu deposition on rotating disc electrodes (RDE). A one-dimensional semi-analytical model is developed for Cu electrodeposition from a two-component additive system. The steady-state mass transport differential equations of each chemical species, including diffusion and convection, are solved analytically at different potentials, while the boundary conditions at the RDE surface are coupled with each other through the surface coverages of adsorbed species. The steady state of such surface coverage results from a dynamic equilibrium between three co-existing processes, adsorption, desorption and incorporation (consumption), namely, Adsorption Rate – Desorption Rate – Consumption Rate = 0 A set of solutions, including the concentration profile of each species in electrolyte as well as the surface coverage of each adsorbate, can be obtained at a fixed potential. The equations are first solved considering only the suppressor for a range of over-potential. It was found that, for a specific range of voltages and kinetic parameters, multiple non-trivial solutions of surface coverage exist. In other words, multiple steady states exist at a same applied potential. As shown in Figure 1, this translates to a S-shaped negative differential resistance (NDR) feature in cyclic voltammogram (CV), consistent with experimental observation reported in literature. The numerically calculated NDR region was found sensitive to the additive concentration and therefore extremely useful in determining the electrochemical parameters using multiple experimental datasets with different concentrations. The swarm optimization algorithm used for parameter fitting and the correlation between multiple numerical solutions with the NDR region will be discussed in details in the talk. Figure 1

Influence of water on electrical and mechanical properties of self-assembled and self-healing PEM films

Self-healing coatings as thin films have been widely exploited in the last decade due to the conformational ability to adhere practically at any surface, bringing multiple regenerations in damaged areas. Poly(ethylene imine) (PEI) and poly(acrylic acid) (PAA) are the most used polyelectrolytes to grow self-healing layer-by-layer films; however, few studies have been dedicated to evaluate the role of water on the electrical and mechanical properties of the films. Multilayered (PEI/PAA) structures were easily self-assembled on various substrates, allowing homogeneous and semi-transparent coverages. We associate the exponential and linear growth regimes of (PEI/PAA) with changes in the electrical characteristics acquired after each step of the film formation. We demonstrated the central role of trapped water on the space charge accumulation at polymer/electrode interface, due to the presence of negative differential resistance regions, and the transition of the charge transport to a space charge regime after a critical potential. We have also investigated changes in the mechanical properties, showing the influence of trapped water in the film elastic modulus, wear resistance, and resistance to plastic deformation.

Homogeneous quantum cascade lasers operating as terahertz frequency combs over their entire operational regime

AbstractWe report a homogeneous quantum cascade laser (QCL) emitting at terahertz (THz) frequencies, with a total spectral emission of about 0.6 THz, centered around 3.3 THz, a current density dynamic range Jdr = 1.53, and a continuous wave output power of 7 mW. The analysis of the intermode beatnote unveils that the devised laser operates as an optical frequency comb (FC) synthesizer over the whole laser operational regime, with up to 36 optically active laser modes delivering ∼200 µW of optical power per optical mode, a power level unreached so far in any THz QCL FC. A stable and narrow single beatnote, reaching a minimum linewidth of about 500 Hz, is observed over a current density range of 240 A/cm2 and even across the negative differential resistance region. We further prove that the QCL FC can be injection locked with moderate radio frequency power at the intermode beatnote frequency, covering a locking range of 1.2 MHz. The demonstration of stable FC operation, in a QCL, over the full current density dynamic range, and without any external dispersion compensation mechanism, makes our proposed homogenous THz QCL an ideal tool for metrological applications requiring mode-hop electrical tunability and a tight control of the frequency and phase jitter.

Modeling current transport in boron-doped diamond at high electric fields including self-heating effect

In this work, current multiplication at high electric field in epitaxial boron-doped diamond with high acceptor concentration is analyzed, including self-heating effect and impurity impact ionization. Quasi-static current-voltage (I-V) characteristics were measured using a transmission-line pulse setup with 100 ns pulse duration on samples with two Ohmic titanium/gold electrodes. Unambiguous exponential and super-exponential behaviors are observed in the I-V curves along with, in some cases, negative differential resistance. The self-heating effect is analyzed using transient interferometric mapping of the thermal energy distribution between electrodes with a ns time scale. Measured I-V characteristics are modeled by finite element method and by considering boron acceptor ionization due to self-heating effect and impurity impact ionization. Simulated I-V characteristics, in particular the appearance of the negative differential resistance region attributed to self-heating, are in good agreement with experimental data.

Experiment and simulation of the characteristics and mechanisms of self-oscillations in parallel-plate glow discharges

This paper presents experimental results of the characteristics appearance and the mechanism of self-oscillation in a parallel-plate glow discharge with argon as the discharge gas at 266 Pa. The cathode and anode are molybdenum plates with a diameter of 3 cm. The distance between the electrodes is 1 cm. Discharge average voltage and current vary between 300 V and 500 μA, respectively. Light emission from the electrode gap is measured by a charge coupled device camera and argon spectra are recorded of emission lines at 650–800 nm. Results show that self-oscillations exist in the negative differential resistance region in the static V–I curve. An obvious negative differential resistance also can be observed in the dynamic of the V–I curve for the self-oscillations. The appearance and disappearance of the self-oscillating phenomenon are companied with an obvious change in all of the discharge current, discharge images, and electron excited temperature. It can be determined that the self-oscillation is a mode transition between low-current stage of Townsend discharge and high-current stage of glow discharge. The frequency range of self-oscillations starts with 0.4 k Hz up to 24 k Hz in dependence of the average discharge current and the external capacitors. When the self-oscillation disappears, the luminescence shows a significant radial contraction, the electron excitation temperature rise obviously. The frequency of oscillations exhibits a substantial linear increase with the average discharge current and decreases with the increase in the external capacitance from 0 pF up to 250 pF. A resistance–capacitance circuit model is used to simulate the self-oscillation discharge in this experimental glow discharge. Results show that the waveforms simulated by this circuit model are well consistent with those obtained experimentally at different average discharge currents and external parallel capacitances. The equivalent plasma resistance decreases with an increase in the average discharge current. The theory of charge and discharge of a capacitor provides good insights into the characteristics and mechanism of self-oscillations in parallel-plate discharges.

Drift\u2013diffusion simulation of S-shaped current\u2013voltage relations for organic semiconductor devices

We present an electrothermal drift–diffusion model for organic semiconductor devices with Gauss–Fermi statistics and positive temperature feedback for the charge carrier mobilities. We apply temperature-dependent Ohmic contact boundary conditions for the electrostatic potential and discretize the system by a finite volume based generalized Scharfetter–Gummel scheme. Using path-following techniques, we demonstrate that the model exhibits S-shaped current–voltage curves with regions of negative differential resistance, which were only recently observed experimentally.

Carrier-Transport Processes in n+-GaAs/n0-GaAs/n+-GaAs Isotype Heterostructures with a Thin Wide-Gap AlGaAs Barrier

Experimental studies of n+-GaAs/n0-GaAs/n+-GaAs isotype heterostructures with a 100-nm-thick wide-gap N0-AlGaAs barrier situated in the n0-GaAs region are carried out. It is shown that the current–voltage characteristic of the structures under study has a negative-differential-resistance (NDR) portion, with the transition to this region occurring with a time delay that may reach tens of nanoseconds. It is found that operation in the NDR region is associated with the onset of impact-ionization process. Numerical analysis in terms of the energy-balance model demonstrated that the transition to the NDR region is associated with the formation of an electric field domain that covers a part of the lightly doped region between the thin wide-gap N0-AlGaAs barrier and the heavily doped n+-GaAs layer and with the onset of impact ionization at the interface with the heavily doped n+-GaAs layer. A comparative analysis of the experimental data and the modeling results showed that, for the current–voltage characteristics of the heterostructures under study to be correctly described, the model should take into account the less pronounced ability of a heterojunction to restrict carrier transport in the barrier layer.

Superior growth, yield, repeatability, and switching performance in GaN-based resonant tunneling diodes

We report the direct measurement of record fast switching speeds in GaN/AlN resonant tunneling diodes (RTDs). The devices, grown by plasma-assisted molecular-beam epitaxy, displayed three repeatable negative differential resistance (NDR) regions below a bias of +6 V. A room temperature peak-to-valley current ratio (PVCR) > 2 was observed, which represents a marked improvement over recent reports. Measurements carried out on hundreds of devices, of varying sizes, revealed a yield of ∼90%. Repeatability measurements consisting of 3000 sweeps resulted in a standard deviation, relative to the mean, of < 0.1%. Temperature dependent measurements combined with non-equilibrium Green's function based quantum transport simulations suggest the presence of both three-dimensional (3D) and two-dimensional (2D) emitters, giving rise to three NDR regions. Finally, a valley current density vs perimeter-to-area-ratio study indicates the presence of a surface leakage current mechanism, which reduces the PVCR.

Photon-Driven Broadband Emission and Frequency Comb RF Injection Locking in THz Quantum Cascade Lasers

We present homogeneous quantum cascade lasers (QCLs) emitting around 3 THz which display bandwidths up to 950 GHz with a single stable beatnote. Devices are spontaneously operating in a harmonic comb state, and when in a dense mode regime they can be injection locked at the cavity roundtrip frequency with very small RF powers down to -55 dBm. When operated in the electrically unstable region of negative differential resistance, the device displays ultra-broadband operation exceeding 1.83 THz (Δf/f=50%) with high phase noise, exhibiting self-sustained, periodic voltage oscillations. The low CW threshold (115 A·cm-2) and broadband comb operation (Δf/f=25%) make these sources extremely appealing for on-chip frequency comb applications.

Исследования процессов транспорта носителей заряда в изотипных гетероструктурах типа n-=SUP=-+-=/SUP=--GaAs/n-=SUP=-0-=/SUP=--GaAs/n-=SUP=-+-=/SUP=--GaAs с тонким широкозонным барьером AlGaAs

Abstract Experimental studies of n ^+-GaAs/ n ^0-GaAs/ n ^+-GaAs isotype heterostructures with a 100-nm-thick wide-gap N ^0-AlGaAs barrier situated in the n ^0-GaAs region are carried out. It is shown that the current–voltage characteristic of the structures under study has a negative-differential-resistance (NDR) portion, with the transition to this region occurring with a time delay that may reach tens of nanoseconds. It is found that operation in the NDR region is associated with the onset of impact-ionization process. Numerical analysis in terms of the energy-balance model demonstrated that the transition to the NDR region is associated with the formation of an electric field domain that covers a part of the lightly doped region between the thin wide-gap N ^0-AlGaAs barrier and the heavily doped n ^+-GaAs layer and with the onset of impact ionization at the interface with the heavily doped n ^+-GaAs layer. A comparative analysis of the experimental data and the modeling results showed that, for the current–voltage characteristics of the heterostructures under study to be correctly described, the model should take into account the less pronounced ability of a heterojunction to restrict carrier transport in the barrier layer.

Investigation of Switching Time in GaN/AlN Resonant Tunneling Diodes by Experiments and P-SPICE Models

The experimental and simulated switching behavior across the negative differential resistance (NDR) region of GaN/AlN double-barrier resonant tunneling diodes (RTDs) is presented. The shortest 10%–90% experimental switching time was ~55 ps. The experimental results are also studied with P-SPICE circuit models, which show that the relatively low peak-to-valley current ratio (~1.5), relatively high specific contact resistance ( $\geq \textsf {1}\times \textsf {10}^{-\textsf {6}}\,\,\Omega $ -cm2), and relatively large specific capacitance limit the switching time.

Current–Voltage Characterization of Transparent ITO/ZnO:B/ZnO:(Al + In)/Ag Schottky Diodes Prepared with Multilayer Films by Sol–Gel Deposition

We fabricated a ZnO-based Schottky diode via the deposition of a ZnO film co-doped with Al + In (4 at.%) on a boron-doped ZnO film (8 at.%). Each film was prepared by layering coatings (2, 3, 4, and 5 layers) by sol–gel deposition. The finished diode consists of the combination of seven layers (each layer with a thickness of around 90 nm). The total thickness of the diode is around 700 nm. The films were previously studied and structurally, optically and electrically characterized. Additionally, for comparative purposes, we fabricated and characterized un-doped ZnO films. The energy bandgap values of the un-doped films, mono-doped films, and co-doped films were 3.30 eV, 3.32 eV, and 3.34 eV, respectively. X-ray diffraction did not show traces of different phases from hexagonal Wurtzite-type ZnO. The electrical resistivity values obtained were 386, 4.44 × 104, and 3.37 Ω-cm, respectively. The junction diodes were built by depositing layers of the high-resistivity material (ZnO:B) on ITO conductor substrates, followed by the deposition of layers of the low-resistivity material (ZnO:Al + In) on the same substrate. The I–V characteristics of these diodes were analyzed in terms of the number of the deposited layers (or the different thickness of the films). The results show a Schottky-type behavior in the dark and under light (spot lamp of 160 W), which is controlled by the thickness of the resistive layer. From the I–V curves, the characteristic parameters including barrier height, ideality factor, and series resistance were calculated. From the transconductance (gm=dI/dV), it was possible to identify the presence of all the layer–layer interfaces. Depending on the thickness of the resistive ZnO:B film, we found a region of negative differential resistance and a region of visible light detection.

The operation of THz quantum cascade laser in the region of negative differential resistance

We investigate the light-current-voltage characteristics and emission spectra of 2.3 THz quantum cascade laser operating in the negative differential resistance (NDR) region. It was shown that the formation of electric field domains (EFDs) leads to a large number of discontinuities on the current-voltage and the total optical power on current characteristics. Measurements of emission spectra at different current (before the NDR region and in the NDR region) shows that the formation of EFDs results in decrease of the output intensity, but does not influence on Fabry-Perot multi-mode structure of THz QCL. The developed theoretical model predicts the formation of EFDs in the NDR region and qualitatively explain the experimental results.

Accurate Small-Signal Equivalent Circuit Modeling of Resonant Tunneling Diodes to 110 GHz

This article presents a novel, on-wafer deembedding technique for the accurate small-signal equivalent circuit modeling of resonant tunneling diodes (RTDs). The approach is applicable to stabilized RTDs, and so enables the modeling of the negative differential resistance (NDR) region of the device’s current–voltage ( $I$ – $V$ ) characteristics. Furthermore, a novel quasi-analytical procedure to determine all the equivalent circuit elements from the deembedded S-parameter data is developed. Extraction results of a $10\,\,\mu \text{m}\,\,\times 10\,\,\mu \text{m}$ stabilized, low-current density RTD at different bias points show excellent fits between modeled and measured S-parameters up to 110 GHz.

Pattern Formation With Locally Active S-Type NbOx Memristors

The main focus of this paper is the evolution of complex behavior in a system of coupled nonlinear memristor circuits depending on the applied coupling conditions. Thereby, the parameter space for the local activity and the edge-of-chaos domain will be determined to enable the emergence of the pattern formation in locally coupled cells according to Chua’s principle. Each cell includes a Niobium oxide-based memristor, which may feature a locally active behavior once it is suitably biased on the negative differential resistance region of its DC current-voltage characteristic. It will be shown that there exists a domain of parameters under which each uncoupled cell may become locally active around a stable bias state. More specifically, under these conditions, the coupled cells are on the edge-of-chaos, and can support the static and dynamic pattern formation. The emergence of such complex spatio-temporal behavior in homogeneous structures is a prerequisite for information processing. The theoretical results are confirmed by measurements as well as by the numerical simulations of the accurate device and circuit models.

Improvement of NbOx-based threshold switching devices by implementing multilayer stacks

In this work the I–V characteristics of a niobium oxide-based threshold switching device were optimized to match the requirements for its application in neuromorphic circuits. Those neuromorphic circuits rely on coupled oscillators utilizing the volatile resistive switching effect of the memristor. A large voltage extension of the negative differential resistance region of the threshold switch enables enhanced signal amplification, and, furthermore, can lead to a better tolerance to device variability. A symmetric switching behavior as well as a high device stability for the operation in both voltage polarities is mandatory to allow the integration in circuits that utilize the connection of several threshold switching devices operated in different polarities. These properties are similarly important for the adoption of the threshold switches as selector devices in bipolar resistive memory arrays. Furthermore, a low forming voltage is desirable because it leads to a better control during the forming step. To meet all those requirements the application of multilayer stacks consisting of niobium and niobium oxide layers is proposed and their optimization is investigated in detail.

Multiple slopes in the negative differential resistance region of NbOx-based threshold switches

Niobium oxide devices exhibit threshold switching behavior which enables their use as selectors in memory arrays or as locally active devices for neuromorphic computing. Among the basic dynamical phenomena appearing in non-linear circuits, the oscillations generated in a relaxation oscillator, which is making use of the negative differential resistance (NDR) effect of a threshold switching device, are of special significance for the design of neuromorphic electronic systems. Here, the necessary requirements for the emergence of oscillations of this kind in a simple relaxation oscillator circuit and their influence on the shape of the measured quasi-static I– characteristic of the threshold switch are examined. In the corresponding experiments multiple NDR regions were found to appear in the quasi-static I– characteristic of the threshold switch concurrently with the occurrence of oscillations. The observed ‘multiple NDR phenomenon’ is therefore merely a measurement artefact due to the averaging effect associated to the operating principles of the source measure unit (SMU) utilized to measure the device current and voltage. In this work, we analyzed how the emergence of oscillatory behavior in the relaxation oscillator depends upon the device layer stack composition. The probability of the appearance of oscillations within a large current range can be increased by decreasing the oxygen content in the sub-stoichiometric bottom layer of a niobium oxide bi-layer stack. It is shown that this trend is caused by the resulting decrease in the value of the product between thermal capacitance and thermal resistance of the threshold switching device. Furthermore, the changed stack composition reduces the variability and changes the forming voltage, which goes hand in hand with a change of the threshold voltage.

Specific Features of Carrier Transport in n+–n0–n+ Structures with a GaAs/AlGaAs Heterojunction at Ultrahigh Current Densities

The current–voltage characteristics of n+-GaAs/n0-GaAs/N0-AlGaAs/N+-AlGaAs/n+-GaAs isotype heterostructures and n+-GaAs/n0-GaAs/n+-GaAs homostructures are studied. It is shown that, for a heterostructure under reverse bias providing the injection of electrons from n0-GaAs into N0-AlGaAs, the maximum operating voltage reaches a value of 48 V at a thickness of the N0-AlGaAs layer of 1.0 μm, and the current–voltage characteristic has no region of negative differential resistance. The operation of a homostructure is accompanied by a transition to the negative-differential-resistance region at a voltage of 10 V. Theoretical analysis in terms of the energy-balance model demonstrated that the reverse-biased isotype heterostructure has no negative-differential-resistance region because, in this case, the field domain does not collapse in contrast to what occurs in homostructures.