Negative Differential Resistance Region Research Articles

Obtaining memristor elements based on non-noble materials

This study is aimed at creating memristor elements based on TiO2/TiOx layers with electrodes that do not contain noble and rare-earth metals, by vacuum deposition method. The characteristics of these elements analyzed by voltammetric methods show that the appearance of an N-type region of negative differential resistance on the current–voltage curve can be caused only by metal electrodes whose vacuum work function exceeds that of TiO2. The appearance of the N-type region on the current–voltage curve of a memristor element is possible only after electrically assisted vacuum forming. Mo–TiO2/TiOx–Ni/Cu structures, for which the ILR/IHR ratio reaches two orders of magnitude at a voltage of less than 4 V, have the most stable parameters.

Obtaining memristor elements based on non-noble materials

This study is aimed at creating memristor elements based on TiO2/TiOx layers with electrodes that do not contain noble and rare-earth metals, by vacuum deposition method. The characteristics of these elements analyzed by voltammetric methods show that the appearance of an N-type region of negative differential resistance on the current–voltage curve can be caused only by metal electrodes whose vacuum work function exceeds that of TiO2. The appearance of the N-type region on the current–voltage curve of a memristor element is possible only after electrically assisted vacuum forming. Mo–TiO2/TiOx–Ni/Cu structures, for which the ILR/IHR ratio reaches two orders of magnitude at a voltage of less than 4 V, have the most stable parameters.

Spin-selectable, region-tunable negative differential resistance in graphene double ferromagnetic barriers.

We propose a graphene device that can generate spin-dependent negative differential resistance (NDR). The device is composed of a sufficiently wide and short graphene sheet and two gated EuO strips deposited on top of it. This scheme avoids the graphene edge tailoring required by previous proposals. More importantly, we find a clear indication of a spin selectivity and a region tunability in the spin-dependent NDR: by changing the top gates of the device, NDR for spin up only, spin down only, or both spins (occurring sequentially) can be respectively realized; meanwhile, the central position of the NDR region in each case can be monotonously tuned over a wide range of bias voltage. These remarkable features are attributed to a gate controllability of the spin-dependent resonance levels in the device hence their deviations from the Fermi energy and the Dirac point in the source electrode respectively. They add a spin and a bias degree of freedom to the conventional NDR, which paves the way for designing a whole new class of NDR circuits.

Impact of Band-Tails on the Subthreshold Swing of III-V Tunnel Field-Effect Transistor

We present a simple model to evaluate the sharpness of the band edges for tunnel field-effect transistors (TFETs) by comparing the subthreshold swing and the conductance in the negative differential resistance region. This model is evaluated using experimental data from InAs/InGaAsSb/GaSb nanowire TFETs with the ability to reach a subthreshold swing well below the thermal limit. A device with the lowest subthreshold swing, 43 mV/decade at 0.1 V, exhibits also the sharpest band-edge decay parameter ${E}_{{0}}$ of 43.5 mV although in most cases the ${S} \ll {E}_{{0}}$ . The model explains the observed temperature dependence of the subthreshold swing.

Tunable Negative Differential Resistance in MISIM Tunnel Diodes Structure With Concentric Circular Electrodes Controlled by Designed Substrate Bias

The tunable negative differential resistance (NDR) characteristics of metal–insulator–semiconductor–insulator–metal (MISIM) tunnel diodes (TDs) structure with thin oxide were observed by biasing the substrate simultaneously with one of the TDs under designed voltage drops ( $\Delta \text{V}$ ). An inner circular electrode TD surrounded by a ring-shape electrode TD gate pattern was adopted. The NDR region can be adjusted by varying the $\Delta \text{V}$ between the inner TD and the body. Not only the peak-to-valley current ratio (PVCR) but also the voltage range where NDR occurs can be controlled. It is supposed that the injected electrons and the inversion charges increase and decrease with negative bias, respectively, for the MIS(p) TD and play an important role in this observation. Meanwhile, the asymmetry of inner gate and ring gate enhances the coupling efficiency at reverse bias region. In this paper, the PVCR can be tuned to more than two orders and the voltage of the NDR region can be shifted to larger than 1 V. The tunable NDR characteristic of MISIM provides a possible application of MIS TD.

Impact of strain on electronic and transport properties of 6 nm hydrogenated germanane nano-ribbon channel double gate field effect transistor

In this work, chair like fully hydrogenated germanane (CGeH) nano-ribbon 6 nm short channel double gate field effect transistor (DG-FET) has been modeled and the impact of strain on the I–V characteristics of CGeH channel has been examined. The bond lengths, binding and formation energies of various hydrogenated geometries of buckled germanane channel were calculated using local density approximation (LDA) with Perdew–Zunger (PZ) and generalized gradient approximation (GGA) with Perdew Burke Ernzerhof (PBE) parameterization. From four various geometries, chair like structure is found to be more stable compared to boat like obtuse, stiruup structure and table like structure. The bandgap versus width, bandgap versus strain characteristics and I–V characteristics had been analyzed at room temperature using density functional theory (DFT). Using self consistent calculation it was observed that the electronic properties of nano-ribbon is independent of length and band structure, but dependent on edge type, strain [Uni-axial (εxx), bi-axial (εxx = εyy)] and width of the ribbon. The strain engineered hydrogenated germanane (GeH) showed wide direct bandgap (2.3 eV) which could help to build low noise electronic devices that operates at high frequencies. The observed bi-axial compression has high impact on the device transport characteristics with peak to valley ratio (PVR) of 2.14 and 380% increase in peak current compared to pristine CGeH device. The observed strain in CGeH DG-FET could facilitate in designing novel multiple-logic memory devices due to multiple negative differential resistance (NDR) regions.

Enhancing power density of strained In0.8Ga0.2As/AlAs resonant tunneling diode for terahertz radiation by optimizing emitter spacer layer thickness

We demonstrate both theoretically and experimentally that the power density of resonant tunneling diode (RTD) can be enhanced by optimizing emitter spacer layer thickness, in addition to reducing barrier thickness. Compared to the widely used epitaxial structure with ultrathin emitter spacer layer thickness, appropriate increasing the thickness will increase the voltage drop in accumulation region, leading to larger voltage widths of negative differential resistance region. By measuring J-V characteristics, the specific contact resistivity, and the self-capacitance, we theoretically analyze the maximum output power of the fabricated RTDs. It shows that the optimized In0.8Ga0.2As/AlAs RTD with 20 nm emitter spacer thickness and 5 μm2 mesa area theoretically possesses the capability to reach 3.1 mW at 300 GHz and 1.8 mW at 600 GHz.

Tristate Memory Cells Using Double-Peaked Fin-Array III–V Tunnel Diodes Monolithically Grown on (001) Silicon Substrates

We demonstrate functional tristate memory cells using multipeaked GaAs/InGaAs fin-array tunnel diodes grown on exact (001) Si substrates. On-chip connection of single-peaked tunnel diode arrays produces I–V characteristics with multiple negative-differential resistance regions. We designed and fabricated two types of tristate memory cells. In one design, a double-peaked tunnel diode was used as the drive, and a reverse-biased single-peaked tunnel diode was used as the load. In the other design, the tristate memory cell was realized by the series connection of two forward-biased single-peaked tunnel diodes.

3D electrothermal simulations of organic LEDs showing negative differential resistance

Organic semiconductor devices show a pronounced interplay between temperature-activated conductivity and self-heating which in particular causes inhomogeneities in the brightness of large-area OLEDs at high power. We consider a 3D thermistor model based on partial differential equations for the electrothermal behavior of organic devices and introduce an extension to multiple layers with nonlinear conductivity laws, which also take the diode-like behavior in recombination zones into account. We present a numerical simulation study for a red OLED using a finite-volume approximation of this model. The appearance of S-shaped current–voltage characteristics with regions of negative differential resistance in a measured device can be quantitatively reproduced. Furthermore, this simulation study reveals a propagation of spatial zones of negative differential resistance in the electron and hole transport layers toward the contact.

(Invited) Formation of Conducting Filaments in TaOx-Based Resistive Switching Devices

Resistive switching phenomenon and resistive switching devices could form the basis for the next generation non-volatile solid state memories. Particularly promising are structures based on dielectric oxides such as TaOx and HfOx which have been demonstrated to large OFF/ON ratio, low energy dissipation per switching cycle, long endurance, and sub-nanosecond switching times. It is widely accepted that the first switching cycle, referred to as electroforming, results in formation of a conducting small diameter filament within the highly resistive matrix. This is due to the electric field which induces the transfer of oxygen ions across the interfaces and into the metal electrodes producing locally oxygen-deficient conducting oxide. Most of the structural changes induced during electroforming and switching have been reported in devices that experienced high current levels during switching cycle. The devices used in this study were 500×500 nm2 crossbar structures with 20 nm thick TiN electrodes and 100 nm of reactively sputtered amorphous TaOx. The devices were electroformed and electroformed/switched with the current compliance of 10-20 μA. The data have been gathered in several imaging modes of electron microscopy including bright field, High Angle Annular Dark Field (HAADF), Energy Dispersive X-ray Spectroscopy, and low loss and core regions of Electron Energy Loss Spectroscopy (EELS). The entire volume of the functional layer remained amorphous with no signs of crystallization. It contained only one area of bright contrast in HAADF images corresponding to the conductive filament with the diameter of approximately 50 nm. The maps produced by all techniques clearly indicate an increase of [Ta]/[O] ratio within the filament. In addition to overall change of the composition, the core of the filament shows a fine structure of the composition on the scale of 5-10 nm. We interpret the changes in distribution as due to thermodiffusion in the temperature gradient induced by current constriction when the device enters the negative differential resistance region of its I-V. The temperature distribution during this process has been estimated using a self-consistent finite element model.

Prototype of a bistable polariton field-effect transistor switch

Microcavity exciton polaritons are promising candidates to build a new generation of highly nonlinear and integrated optoelectronic devices. Such devices range from novel coherent light emitters to reconfigurable potential landscapes for electro-optical polariton-lattice based quantum simulators as well as building blocks of optical logic architectures. Especially for the latter, the strongly interacting nature of the light-matter hybrid particles has been used to facilitate fast and efficient switching of light by light, something which is very hard to achieve with weakly interacting photons. We demonstrate here that polariton transistor switches can be fully integrated in electro-optical schemes by implementing a one-dimensional polariton channel which is operated by an electrical gate rather than by a control laser beam. The operation of the device, which is the polariton equivalent to a field-effect transistor, relies on combining electro-optical potential landscape engineering with local exciton ionization to control the scattering dynamics underneath the gate. We furthermore demonstrate that our device has a region of negative differential resistance and features a completely new way to create bistable behavior.

Characterization of emitted light from travelling Gunn domains in Al0.08Ga0.92As alloy based Gunn devices

We report room temperature operation of light emitters based on Al0.08Ga0.92As Gunn devices fabricated in a simple bar geometry with wedged-shaped electrodes. High-speed I-V measurements reveal that, at the threshold of negative differential resistance region at around 3.8 kV/cm, current instabilities, i.e., Gunn oscillations, are created with a 3.8 ns period. Both edge and surface light emission are observed when the device is biased at an electric field of onset of the negative differential resistance (NDR) region at around 3.8 kV/cm and the intensity of the light exponentially increases at applied fields just above NDR threshold likewise in a conventional laser. The origin of the light emission, which has peak wavelength is around 816 nm corresponds to the band-gap energy of Al0.08Ga0.92As, is recombination of electrons and holes generated by impact ionisation process in travelling space charge domains, i.e., Gunn domains. We demonstrate that, with increasing applied field, the amplitude of Gunn domains increases which is a result of the enhanced generation of electrons and holes via impact ionisation. The intensity of the emitted light is observed to be dependent on applied electric field. At low electric fields, light intensity increases linearly then, when applied electric field reaches the onset of NDR region, increases exponentially. Besides, as applied field is increased, full width at half maximum (FWHM) of emitted light decreases to 56.5 nm from 62 nm, evolving into higher selective emission line in wavelength. The light emission from the device is determined to be independent of the polarity of the applied voltage. A comparison of surface emission and edge emission characteristics of the waveguided device are different from each other. Edge emission has higher electroluminescence intensity and better spectral purity than surface emission with well-defined longitudinal modes of Fabry-Pérot cavity, which indicates that, in such a device, lasing action arises from the recombination of excess carriers generated via impact ionisation in travelling Gunn domains. Besides, the edge emission peak of waveguided Al0.08Ga0.92As Gunn device at 4.1 kV/cm is split into two peaks with FWHM of 8 and 6 nm as well as neighbouring sharper minor peaks due to stimulated emission dominates by building-up photons in the cavity. Our results reveal that the proposed Gunn device can be a promising alternative to conventional diode lasers with its simpler design, only one type doped active region and voltage polarity-independent operation, but the duty cycle has to be chosen small enough to make the device operate at room temperature.

Unexpected behavior of transient current in thin PZT films caused by grain-boundary conduction

The behavior of the transient current at different preliminary polarizations has been studied in Pb(ZrTi)O3 (PZT) films with various grain structures. To affect the grain structure, PZT films were prepared by chemical solution deposition with a two-step crystallization process using combination of seed layers with a low Pb excess and the main layers with a 30 wt. % Pb excess. Some films were prepared with a fixed Pb excess in all the deposited layers. We found that the lead excess and the seed layer crystalline structure can affect the grain-boundary conduction which, in turn, influences the polarization dependence of the transient current and the appearance of current peaks which look like the so-called negative differential resistance region in the current-voltage curves. We show that the emergence of the current peaks in the PZT films depends on (i) whether the current flows inside the ferroelectric phase (grains) or outside, along grain boundaries and (ii) whether the applied bias direction is parallel or opposite to the polarization vector. A correlation between the grain-boundary conduction and current-polarization dependences is confirmed by the local current distribution measured by conductive atomic force microscopy. Possible mechanisms responsible for specific features of the transient current and appearance of the current peaks are discussed. The effect of grain-boundary conduction on the behavior of the current may be significant and should be taken into account in ferroelectric random access memory whose readout operation assumes registration of the magnitude of the polarization switching current under positive bias.

Resonant tunneling of charge carriers in InGaN/GaN superlattice

The result of studies of resonant tunnelling of charge carriers in InGaN/GaN unipolar structure is presented. Authors show that at temperatures below 150 K the multiple negative-differential resistance regions are observed on a reverse current-voltage characteristic which is typical for inhomogeneous distribution field for sample with 6 nm barrier thickness. At GaN barrier thickness 3 and 12 nm resonant tunneling were not observed.

Electro-Thermal Model of Threshold Switching in TaOx-Based Devices.

Pulsed and quasi-static current-voltage (I-V) characteristics of threshold switching in TiN/TaOx/TiN crossbar devices were measured as a function of stage temperature (200-495 K) and oxygen flow during the deposition of TaOx. A comparison of the pulsed and quasi-static characteristics in the high resistance part of the I-V revealed that Joule self-heating significantly affected the current and was a likely source of negative differential resistance (NDR) and thermal runaway. The experimental quasi-static I-V's were simulated using a finite element electro-thermal model that coupled current and heat flow and incorporated an external circuit with an appropriate load resistor. The simulation reproduced the experimental I-V including the OFF-state at low currents and the volatile NDR region. In the NDR region, the simulation predicted spontaneous current constriction forming a small-diameter hot conducting filament with a radius of 250 nm in a 6 μm diameter device.

Switching properties of quinquephenylene molecular device – A first-principles approach

The electronic properties of quinquephenylene molecular device are investigated through non-equilibrium Green’s function (NEGF) in combination with density functional theory (DFT) method. The density of charge along quinquephenylene molecular device is recorded in both the valence band and in the conduction band upon increasing the bias voltage. The findings of the present work clearly reveal that quinquephenylene molecular device can be used as switching device as well as it exhibits negative differential resistance region. Moreover, the electronic properties of quinquephenylene molecular device can be fine-tuned with the help of applied bias voltage and desired torsion angle of phenyl units.

Attachable and flexible aluminum oxide resistive non-volatile memory arrays fabricated on tape as the substrate

We fabricated 8 × 8 arrays of non-volatile resistive memory devices on commercially available Scotch® Magic™ tape as a flexible substrate. The memory devices consist of double active layers of Al2O3 with a structure of Au/Al2O3/Au/Al2O3/Al (50 nm/20 nm/20 nm/20 nm/50 nm) on attachable tape substrates. Because the memory devices were fabricated using only dry and low temperature processes, the tape substrate did not suffer from any physical or chemical damage during the fabrication. The fabricated memory devices were turned to the low resistance state at ∼3.5 V and turned to the high resistance state at ∼10 V with a negative differential resistance region after ∼5 V, showing typical unipolar non-volatile resistive memory behavior. The memory devices on the tape substrates exhibited reasonable electrical performances including a high ON/OFF ratio of 104, endurance over 200 cycles of reading/writing processes, and retention times of over 104 s in both the flat and bent configurations.

About bond model of S-type negative differential resistance in GaP LEDs

The bond models are presented that explain the S-type anomaly of GaP LEDs’ electrical characteristics at temperatures Т ≤ 120 K. A possible mechanism of negative differential resistance appearing in current-voltage characteristics is proposed, based on the features of the gallium phosphide complex band structure. The conductive zone absolute minimum in this crystal is near the Brillouin zone end. Due to the positive internal bond, controlled by the current, intervalley electron transfer occurs from the side valley to the higher one with the smaller effective electron mass. While the applied voltage is increased, electrons move from the lateral valley to the direct conductive zone bottom and an S-type negative differential resistance region appears.

RF Power Analysis on 5.8 GHz Low-Power Amplifier Using Resonant Tunneling Diodes

This letter reports the analysis of RF power characteristics in a microwave amplifier using resonant tunneling diodes (RTDs). The implemented IC shows a return loss of more than 11 dB with a low dc-power consumption of 0.42 mW and a power gain of 8.6 dB at 5.8 GHz. The maximum linear RF output power with a uniform gain of 8.6 dB is measured to be −25.4 dBm at the same frequency. The gain hump phenomenon is observed in an input power range from −32 dBm to −16 dBm, and is shown to arise from a sudden movement of the operating point from the negative differential resistance (NDR) region to the positive differential resistance (PDR) region, based on a large-signal load-line analysis together with a harmonic balance simulation.

A Nonlinear Circuit Simulation of Switching Process in Resonant-Tunneling Diodes

A large-signal circuit model is used to compute the switching time for double-barrier resonant-tunneling diodes. The model consists of linear circuit elements plus a nonlinear $I$ – $V$ characteristic. The linear elements include a series resistor, a capacitor, and an inductor. The capacitance considers the charge accumulation, depletion in spacer layers, as well as charging-discharging of the quantum-well (QW) region. The inductance accounts for the delay of the current with respect to the voltage across the QW during the abrupt switching transition through the negative differential resistance region. A second-order Runge–Kutta method is used to solve for the switching transient, and then fit to experimental data for a high-quality InGaAs/AlAs resonant tunneling diode (RTD) using the QW inductance as a fitting parameter. Excellent agreement is found for the 10%–90% switching time with a calculated capacitance of 98 fF and a fitted inductance of 1300 pH. This large-signal inductance is approximately $7\times $ greater than the small-signal inductance that has been successfully used to predict the fmax of RTDs such as the one tested here.