Current-controlled Negative Differential Resistance Research Articles

DC operating points of Mott neuristor circuits

In 1960 Hewitt Crane conceptualized neuristors as electronic logic networks that could mimic action potential generation by biological neurons. In 2012 Mott memristors were presented as nano-scale electronic devices for physically constructing neuristor networks. Both the original Mott memristor model and the simplified model presented in 2020 are stiff nonlinear first-order ordinary differential equations (ODEs) that describe the voltage controlled threshold switching and current controlled negative differential resistance, attributed to a Mott insulator-to-metal phase transition. By design, a Mott neuristor contains two identical Mott memristors plus linear resistors and capacitors (no inductors), which enables alternating threshold switching by the memristors to generate periodic (AC) output voltage spikes in response to a DC current or voltage input. This paper presents a steady-state analysis of dynamic neuristor models, including four-, five-, and six-state variables (each of which is a system of first-order ODEs involving the states) based on the original phenomenological model of the Mott memristor. Specifically, it reveals the unique stable operating points that occur for applied currents or voltages below the amounts needed to induce resistive switching. These DC operating points represent the physically meaningful “OFF” states for the circuit when the neuristor does not output voltage spikes. Jacques Hadamard's mathematical criteria for a well-posed physical model (existence, uniqueness, continuity, stability) motivated the search for these DC operating points (OFF states), which are needed for efficiently initializing neuristor models before conducting dynamic neuristor simulations. The key result of our analysis is the neuristor steady-state eq. (29) which is useful for understanding neuristor OFF states as well as the role of negative-differential resistance (NDR) in the operation of the Mott memristor model and neuristor circuits. In addition to the steady-state analysis, three numerical examples of dynamic neuristor circuit operation are also given and MATLAB code for the dynamic four-state model is available for interested readers.

Thermal transport in metal-NbOx-metal cross-point devices and its effect on threshold switching characteristics.

Volatile threshold switching and current-controlled negative differential resistance (NDR) in metal-oxide-metal (MOM) devices result from thermally driven conductivity changes induced by local Joule heating and are therefore influenced by the thermal properties of the device-structure. In this study, we investigate the effect of the metal electrodes on the threshold switching response of NbOx-based cross-point devices. The electroforming and switching characteristics are shown to be strongly influenced by the thickness and thermal conductivity of the top-electrode due to its effect on heat loss from the NbOx film. Specifically, we demonstrate a 40% reduction in threshold voltage and a 75% reduction in threshold power as the thickness of the top Au electrode is reduced from 125 nm to 25 nm, and a 24% reduction in threshold voltage and 64% reduction in threshold power when the Au electrode is replaced by a Pt electrode of the same thickness of NbOx film, due to its lower thermal conductivity. Lumped element and finite element modelling of the devices show that these improvements are due to a reduction in heat loss to the electrodes, which is dominated by lateral heat flow within the electrode. These results clearly demonstrate the importance of the electrodes in determining the electroforming and threshold switching characteristics of MOM cross point devices.

Thermal Conductivity of Amorphous NbOx Thin Films and Its Effect on Volatile Memristive Switching.

Metal-oxide-metal (MOM) devices based on niobium oxide exhibit threshold switching (or current-controlled negative differential resistance) due to thermally induced conductivity changes produced by Joule heating. A detailed understanding of the device characteristics therefore relies on an understanding of the thermal properties of the niobium oxide film and the MOM device structure. In this study, we use time-domain thermoreflectance to determine the thermal conductivity of amorphous NbOx films as a function of film composition and temperature. The thermal conductivity is shown to vary between 0.86 and 1.25 W·m-1·K-1 over the composition (x = 1.9 to 2.5) and temperature (293 to 453 K) ranges examined, and to increase with temperature for all compositions. The impact of these thermal conductivity variations on the quasistatic current-voltage (I-V) characteristics and oscillator dynamics of MOM devices is then investigated using a lumped-element circuit model. Understanding such effects is essential for engineering functional devices for nonvolatile memory and brain-inspired computing applications.

Engineering the Threshold Switching Response of Nb2O5-Based Memristors by Ti Doping

Two terminal metal-oxide-metal devices based on niobium oxide thin films exhibit a wide range of non-linear electrical characteristics that have applications in hardware-based neuromorphic computing. In this study, we compare the threshold-switching and current-controlled negative differential resistance (NDR) characteristics of cross-point devices fabricated from undoped Nb2O5 and Ti-doped Nb2O5 and show that doping offers an effective means of engineering the device response for particular applications. In particular, doping is shown to improve the device reliability and to provide a means of tuning the threshold and hold voltages, the hysteresis window, and the magnitude of the negative differential resistance. Based on temperature-dependent current-voltage characteristics and lumped-element modelling, these effects are attributed to doping-induced reductions in the device resistance and its rate of change with temperature (i.e., the effective thermal activation energy for conduction). Significantly, these studies also show that a critical activation energy is required for devices to exhibit NDR, with doping providing an effective means of engineering the current-voltage characteristics. These results afford an improved understanding of the physical mechanisms responsible for threshold switching and provide new insights for designing devices for specific applications.

Current-controlled negative differential resistance in small-polaron hopping system

Current-controlled negative differential resistance (CC-NDR) phenomenon attracts a lot of interest for fabricating the access devices of nonvolatile memory based on crossbar array architectures. However, simple, bipolar, two-terminal commercial devices that exhibit CC-NDR are currently lacking because a number of critical characteristics needed to be met for such application. Here, we report the CC-NDR observed in Mn1.56Co0.96Ni0.48O4 (MCNO)- a small-polaron hopping material. Our experimental data and simulation reveal that the CC-NDR arises from self-heating effect due to the nature of strong electron-phonon coupling in small-polaron hopping system. The reported CC-NDR exhibits adjustable threshold voltage from 10-3 to 102 V, on-state current from 105 to 108 A/cm2 and off-state current is as low as ∼10 A/cm2 depending on device dimensions, thermal isolation condition, environmental temperature and activation energy of material. Uniquely, unlike in NbO2, Nb2O5, TiO2, TaOx et. al. materials, the CC-NDR in MCNO is more stable and reliable, because it does not undergo any electroforming process. These traits make MCNO a very potential candidate for CC-NDR devices.

Negative differential resistance and quantum oscillations in FeSb2 with embedded antimony**Project supported by Guangdong Innovative and Entrepreneurial Research Team Program, China (Grant No. 2016ZT06D348), the National Natural Science Foundation of China (Grant No. 11874193), and the Shenzhen Fundamental Subject Research Program, China (Grant Nos. JCYJ20170817110751776 and JCYJ20170307105434022). The work at Brookhaven is supported by the US Department of Energy, Office

We present a systematical study on single crystalline FeSb2 using electrical transport and magnetic torque measurements at low temperatures. Nonlinear magnetic field dependence of Hall resistivity demonstrates a multi-carrier transport instinct of the electronic transport. Current-controlled negative differential resistance (CC-NDR) observed in current–voltage characteristics below ∼ 7 K is closely associated with the intrinsic transition ∼ 5 K of FeSb2, which is, however, mediated by extrinsic current-induced Joule heating effect. The antimony crystallized in a preferred orientation within the FeSb2 lattice in the high-temperature synthesis process leaves its fingerprint in the de Haas-Van Alphen (dHvA) oscillations, and results in the regular angular dependence of the oscillating frequencies. Nevertheless, possible existence of intrinsic non-trivial states cannot be completely ruled out. Our findings call for further theoretical and experimental studies to explore novel physics on flux-free grown FeSb2 crystals.

Designing Negative Differential Resistance Devices Based on Self‐Heating

AbstractThere are a number of important emerging applications that would benefit from simple circuit elements that exhibit tunable current‐controlled negative differential resistance (NDR). The advent of such devices would enable, for example, key components for threshold logic and neuromorphic computing such as threshold switches, periodic and chaotic oscillators, and small signal amplifiers. Circuit elements that provide NDR with modifiable electrical characteristics would also be useful for creating optimized “selectors” that enable addressing of individual memory cells in large resistance‐based memory arrays. Currently, there are no simple, bipolar, two‐terminal commercial devices that exhibit current‐controlled NDR. This paper demonstrates that current‐controlled NDR can, in principal, arise from any electrical conduction mechanism that depends superlinearly on temperature, and that in practice a broad spectrum of materials can be utilized to yield NDR. A model is presented that accurately predicts conditions under which NDR can be observed and guidelines are provided for choosing materials that result in desired electrical characteristics. This model accurately predicts the behavior of some existing structures and can be used to tailor future circuit elements for emerging applications. It may also explain the onset of a number of “soft breakdown” phenomena.

Temperature dependent frequency tuning of NbOx relaxation oscillators

This study investigates the temperature dependence of current-controlled negative differential resistance (CC-NDR) in Pt/NbOx/TiN devices and its effect on the dynamics of associated Pearson-Anson relaxation oscillators. The voltage range over which CC-NDR is observed decreases with increasing temperature such that no NDR is observed for temperatures above ∼380 K. Up to this temperature, relaxation oscillators exhibit voltage and temperature dependent oscillation frequencies in the range of 1 to 13 MHz. Significantly, the sensitivity of the frequency to temperature changes was found to be voltage-dependent, ranging from 39.6 kHz/K at a source voltage of 2 V to 110 kHz/K at a source voltage of 3 V, in the temperature range of 296–328 K. Such a behaviour provides insights into temperature tolerance and tuning variability for environmentally sensitive neuromorphic computing.

Chaotic dynamics in nanoscale NbO2 Mott memristors for analogue computing.

At present, machine learning systems use simplified neuron models that lack the rich nonlinear phenomena observed in biological systems, which display spatio-temporal cooperative dynamics. There is evidence that neurons operate in a regime called the edge of chaos that may be central to complexity, learning efficiency, adaptability and analogue (non-Boolean) computation in brains. Neural networks have exhibited enhanced computational complexity when operated at the edge of chaos, and networks of chaotic elements have been proposed for solving combinatorial or global optimization problems. Thus, a source of controllable chaotic behaviour that can be incorporated into a neural-inspired circuit may be an essential component of future computational systems. Such chaotic elements have been simulated using elaborate transistor circuits that simulate known equations of chaos, but an experimental realization of chaotic dynamics from a single scalable electronic device has been lacking. Here we describe niobium dioxide (NbO2) Mott memristors each less than 100 nanometres across that exhibit both a nonlinear-transport-driven current-controlled negative differential resistance and a Mott-transition-driven temperature-controlled negative differential resistance. Mott materials have a temperature-dependent metal-insulator transition that acts as an electronic switch, which introduces a history-dependent resistance into the device. We incorporate these memristors into a relaxation oscillator and observe a tunable range of periodic and chaotic self-oscillations. We show that the nonlinear current transport coupled with thermal fluctuations at the nanoscale generates chaotic oscillations. Such memristors could be useful in certain types of neural-inspired computation by introducing a pseudo-random signal that prevents global synchronization and could also assist in finding a global minimum during a constrained search. We specifically demonstrate that incorporating such memristors into the hardware of a Hopfield computing network can greatly improve the efficiency and accuracy of converging to a solution for computationally difficult problems.

Synaptic plasticity and oscillation at zinc tin oxide/silver oxide interfaces

Short-term plasticity, long-term potentiation, and pulse interval dependent plasticity learning/memory functions have been observed in junctions between amorphous zinc-tin-oxide and silver-oxide. The same junctions exhibited current-controlled negative differential resistance and when connected in an appropriate circuit, they behaved as relaxation oscillators. These oscillators produced voltage pulses suitable for device programming. Transmission electron microscopy, energy dispersive X-ray spectroscopy, and electrical measurements suggest that the characteristics of these junctions arise from Ag+/O− electromigration across a highly resistive interface layer. With memory/learning functions and programming spikes provided in a single device structure, arrays of similar devices could be used to form transistor-free neuromorphic circuits.

Threshold switching and electrical self-oscillation in niobium oxide films

Electrical self-sustained oscillations have been observed in a broad range of two-terminal systems and are of interest as possible building blocks for bio-inspired neuromorphic computing. In this work, we experimentally explore voltage-controlled oscillations in NbOx devices with a particular focus on understanding how the frequency and waveform are influenced by circuit parameters. We also introduce a finite element model of the device based on a Joule-heating induced insulator-metal transition. The electroformed device structure is represented by a cylindrical conductive channel (filament) comprised of NbO/NbO2 zones and surrounded by an Nb2O5−x matrix. The model is shown to reproduce the current-controlled negative differential resistance observed in measured current-voltage curves, and is combined with circuit elements to simulate the waveforms and dynamics of an isolated Pearson–Anson oscillator. Such modeling is shown to provide considerable insight into the relationship between the material response and device and circuit characteristics.

Memristor and selector devices fabricated from HfO2−xNx

Monoclinic HfO2−xNx has been incorporated into two-terminal devices exhibiting either memristor or selector operation depending on the controlled inclusion/suppression of mobile oxygen vacancies. In HfO2 memristors containing oxygen vacancies, gradual conductance modulation, short-term plasticity, and long-term potentiation were observed using appropriate voltage-spike stimulation, suggesting suitability for artificial neural networks. Passivation of oxygen vacancies, confirmed by X-ray absorption spectroscopy, was achieved in HfO2−xNx films by the addition of nitrogen during growth. Selector devices formed on these films exhibited threshold switching and current controlled negative differential resistance consistent with thermally driven insulator to metal transitions.

Low-temperature nonlinear effects in the conductivity of lightly doped cuprates La2−xSrxCuO4 in antiferromagnetic state

Low-temperature conductivity of antiferromagnetic cuprates La2−xSrxCuO4 prepared by solid-phase synthesis was investigated. The concentration of strontium in the samples was 0.01, 0.005 and 0.001. In the temperature range T < 100 K for all the samples the conduction mechanism corresponded to 3D variable-range hopping. For T > TN (where TN is the Neel temperature) a transition to the metallic type of conductivity was observed. Nonlinear effects in low-temperature conductivity, magnetoresistivity, as well as current-controlled negative differential resistance were found. It was established that the nonlinear behavior of conductivity intensified with decreasing the strontium concentration. For temperatures T < 10 K, the effect of positive magnetoresistance was observed. It is suggested that this effect can be attributed to the presence of a new low-temperature magnetic phase (spin density waves).

Sub-100 fJ and sub-nanosecond thermally driven threshold switching in niobium oxide crosspoint nanodevices

We built and measured the dynamical current versus time behavior of nanoscale niobium oxide crosspoint devices which exhibited threshold switching (current-controlled negative differential resistance). The switching speeds of 110 × 110 nm2 devices were found to be ΔtON = 700 ps and ΔtOFF = 2.3 ns while the switching energies were of the order of 100 fJ. We derived a new dynamical model based on the Joule heating rate of a thermally driven insulator-to-metal phase transition that accurately reproduced the experimental results, and employed the model to estimate the switching time and energy scaling behavior of such devices down to the 10 nm scale. These results indicate that threshold switches could be of practical interest in hybrid CMOS nanoelectronic circuits.

Engineering nonlinearity into memristors for passive crossbar applications

Although TaOx memristors have demonstrated encouraging write/erase endurance and nanosecond switching speeds, the linear current-voltage (I-V) characteristic in the low resistance state limits their applications in large passive crossbar arrays. We demonstrate here that a TiO2-x/TaOx oxide heterostructure incorporated into a 50 nm× 50 nm memristor displays a very large nonlinearity such that I(V/2) ≈ I(V)/100 for V ≈ 1 volt, which is caused by current-controlled negative differential resistance in the device.

Current-controlled negative differential resistance due to Joule heating in TiO2

We show that Joule heating causes current-controlled negative differential resistance (CC-NDR) in TiO2 by constructing an analytical model of the voltage-current V(I) characteristic based on polaronic transport for Ohm’s Law and Newton’s Law of Cooling and fitting this model to experimental data. This threshold switching is the “soft breakdown” observed during electroforming of TiO2 and other transition-metal-oxide based memristors, as well as a precursor to “ON” or “SET” switching of unipolar memristors from their high to their low resistance states. The shape of the V(I) curve is a sensitive indicator of the nature of the polaronic conduction.

Coexistence of Memristance and Negative Differential Resistance in a Nanoscale Metal‐Oxide‐Metal System

We experimentally demonstrate and present an analytical model for a nanoscale metal/oxide/metal device that simultaneously exhibits memristance, based on oxygen vacancy drift, and current-controlled negative differential resistance, based on a metal-insulator transition instability. We show that this oxide nanodevice can be used to fabricate a continuously tunable voltage-controlled oscillator.

Current-controlled negative differential resistance effect induced by Gunn-type instability in n-type GaN epilayers

High-field carrier transport properties probed by pulsed current-voltage characteristics in n-type GaN epilayers have been investigated at room temperature. The threshold electric field of the Gunn effect is experimentally determined to be about 400 kV/cm with an electron peak velocity of about 1.9×107 cm/s. The current-controlled negative differential resistance effect induced by the Gunn-type instability in n-type GaN is observed, which takes the early electric breakdown of the GaN epilayers in charge.

Investigation on Resistive Memory Switching Mechanism of NiO

ABSTRACT Experimental investigations on the resistive memory switching in sub-micron sized NiO memory cell are presented to elucidate the resistive memory switching mechanism. The voltage or current-biased I-V measurements show that the resistive switching transitions can be regarded as the combination of a voltage-controlled negative differential resistance phenomenon and a current-controlled negative differential resistance phenomenon. Along with experimental observations of multiple resistance states, these indicate that the memory switching in NiO would come from the percolative formation and rupture of filamentary conducting paths. Pulse experiments further suggest that the memory switching would come from local domains inside filaments.

Influence of temperature and interelectrode distance on the negative differential resistance in metal-chalcogenide glassy semiconductors

An investigation has been carried out, to elucidate some aspects of the current-controlled negative differential resistance (CCNDR) effect in bulk metal-chalcogenide glassy semiconductors. Because this phenomenon has been shown to be mainly of a thermal nature, a model, from the thermodynamic point of view, was developed, including some aspects related to the thermistors theory. The main conclusion from this model is the appearance of a current filament, which showed up when the material switched from the high electrical resistance to the low electrical resistance state, forming a crystalline filament between both electrodes. The variation of the CCNDR parameters with temperature and interelectrodic distance was studied, using both coplanar point electrodes and coplanar disc electrodes. The experimental results show a good agreement with the expected behaviour from the proposed thermal model (especially when natural convection was considered as the heat-exchanging process between the material and the ambient surrounding). In addition an algorithm was found to simulate the phenomenon computationally, using the experimentally determined physical parameters for the samples under study.