S-type Negative Differential Resistance Research Articles

Speed enhancement of magnetic logic-memory device by insulator-to-metal transition

Complementary metal-oxide-semiconductor logic circuits used in conventional computers require frequent communication with external nonvolatile memory, causing the memory wall problem. Recently reported magnetic logic with reconfigurable logic operation and built-in nonvolatile memory can potentially bridge this gap. However, its high-frequency performance is not well studied. Here, we first perform experimental and theoretical investigation on the switching time of magnetic logic-memory devices combining magnetic units and negative differential resistance (NDR) of semiconductors. It is found that the switching time of S-type NDR (transistor circuits) in logic operations is ∼300 ns and determined by the transistor's internal turn-on properties. We then propose a magnetic logic-memory device by coupling the anomalous Hall effect in magnetic materials and the insulator-to-metal transition in VO2. Our device realizes reliable output (output ratio > 1000%), a low work magnetic field (<20 mT), and excellent high-frequency performance (switching time = 1–10 ns).

Schottky-Barrier-Induced Asymmetry in the Negative-Differential-Resistance Response of Nb/NbOx / Pt Cross-Point Devices

The negative differential resistance (NDR) response of Nb/NbOx/Pt cross-point devices is shown to have a polarity dependence due to the effect of the metal/oxide Schottky barriers on the contact resistance. Three distinct responses are observed under opposite polarity testing: bipolar S-type NDR, bipolar snap-back NDR, and combined S-type and snap-back NDR, depending on the stoichiometry of the oxide film and device area. In-situ thermoreflectance imaging is used to show that these NDR responses are associated with strong current localisation, thereby justifying the use of a previously developed two-zone, core shell thermal model of the device. The observed polarity dependent NDR responses, and their dependence on stoichiometry and area are then explained by extending this model to include the effect of the polarity dependent contact resistance. This study provides an improved understanding of the NDR response of metal/oxide/metal structures and informs the engineering of devices for neuromorphic computing and non-volatile memory applications.

Electrical switching in metal-oxide-metal structures based on anodic niobium oxide

Nonlinear current-voltage (I–V) characteristics with S-type negative differential resistance, as a characteristic feature of the switching effect, are inherent in a variety of transition metal oxides, niobium oxide included. Although this phenomenon has been known for a long time, recent proposals to use oxide-based switching devices as elements of oscillatory neural networks have resumed the interest in this area. In this work, electrical switching in sandwich structures based on anodic films of niobium oxide is studied. After being electroformed, these structures exhibit S-shaped I–V characteristics. As the temperature increases, the threshold voltage decreases, presumably tending to zero at a critical temperature T0, which correlates with the temperature of metal–insulator phase transition of niobium dioxide. Channels consisting of NbO2 are formed in the initial anodic films during the process of electroforming.

Spontaneous current constriction in threshold switching devices

Threshold switching devices are of increasing importance for a number of applications including solid-state memories and neuromorphic circuits. Their non-linear characteristics are thought to be associated with a spontaneous (occurring without an apparent external stimulus) current flow constriction but the extent and the underlying mechanism are a subject of debate. Here we use Scanning Joule Expansion Microscopy to demonstrate that, in functional layers with thermally activated electrical conductivity, the current spontaneously and gradually constricts when a device is biased into the negative differential resistance region. We also show that the S-type negative differential resistance I–V characteristics are only a subset of possible solutions and it is possible to have multiple current density distributions corresponding to the same value of the device voltage. In materials with steep dependence of current on temperature the current constriction can occur in nanoscale devices, making this effect relevant for computing applications.

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.

An accurate locally active memristor model for S-type negative differential resistance in NbOx

A number of important commercial applications would benefit from the introduction of easily manufactured devices that exhibit current-controlled, or “S-type,” negative differential resistance (NDR). A leading example is emerging non-volatile memory based on crossbar array architectures. Due to the inherently linear current vs. voltage characteristics of candidate non-volatile memristor memory elements, individual memory cells in these crossbar arrays can be addressed only if a highly non-linear circuit element, termed a “selector,” is incorporated in the cell. Selectors based on a layer of niobium oxide sandwiched between two electrodes have been investigated by a number of groups because the NDR they exhibit provides a promisingly large non-linearity. We have developed a highly accurate compact dynamical model for their electrical conduction that shows that the NDR in these devices results from a thermal feedback mechanism. A series of electrothermal measurements and numerical simulations corroborate this model. These results reveal that the leakage currents can be minimized by thermally isolating the selector or by incorporating materials with larger activation energies for electron motion.

A comparative study of electrical and optical properties of InN and In0.48Ga0.52N

We present results of our studies concerning electrical and optical properties of In0.48Ga0.52N and InN. Hall measurement were carried out at temperatures between T=77 and 300K. Photoluminescence (PL) spectrum in InN and In0.48Ga0.52N. InN has a single peak at 0.77eV at 300K. However, the PL in In0.48Ga0.52N has two peaks; a prominent peak at 1.16eV and a smaller peak at 1.55eV. These two peaks are attributed to Indium segregation corresponding to a high Indium concentration of 48% and a low concentration of 36%. High electric field measurements indicate that drift velocity that tends to saturate at around Vd=1.0×107cm/s at 77K in InN at an electric field of F=12kV/cm. However, in In0.48Ga0.52N the I–V curve is almost linear up to an electric field of F=45kV/cm, where the drift velocity is Vd=1.39×106cm/s. At applied electric fields above this value a S-type negative differential resistance (NDR) is observed leading to an instability in the current and to the irreversible destruction of the sample.

Negative and positive electroluminescence from a compensated p-type germanium in terahertz frequencies

Negative luminescence (NL) and positive luminescence in terahertz frequencies were observed from a compensated p-type Ge under various levels of pulse excitations at cryogenic temperatures. The NL spectra accompanying an S-type negative differential resistance phenomenon were attributed to the reduced emission transitions from the excited states to the ground states of the acceptors during the excitation. A possible excitation mechanism is also presented according to the measured data.

Study of the behaviour of a DI diode under the influence of a surface magnetic field

Double injection diodes consisting of an electron injecting cathode (n+) and a hole injecting anode (p+) exhibit s-type negative differential resistance under proper conditions similar to a silicon controlled rectifier. These devices usually have high threshold voltages and high holding voltages. This work explores the modulation of threshold voltage, in particular, with a surface magnetic field. For these devices, the threshold voltage variation under the influence of magnetic field is investigated for different anode to cathode separations.

Pattern Formation in Globally Coupled Electrochemical Systems with an S-Shaped Current-Potential Curve

The impact of global coupling on pattern formation in electrochemical systems with an S-shaped current potential curve is investigated theoretically and compared with the corresponding behavior in systems with N-shaped current potential characteristics. The global coupling, present under many experimental conditions, arises either owing to the galvanostatic operation mode or owing to the use of a Haber−Luggin capillary in a potentiostatic experiment. In the galvanostatic operation mode, any homogeneous current distribution of an S-NDR (S-type negative differential resistance) system is unstable in nearly the whole range of current values that lie on the NDR branch of the current potential curve. The system evolves either to a state composed of two stationary domains of low and high current density or to a more complicated stationary pattern with a larger wave number. The first attractor only exists in the presence of the global coupling, whereas the latter one is associated with a Turing-type instability ...

Imaging of domains and current filaments in GaAs/AlxGa1-xAs multi-quantum wells

The field contrast mode of a scanning electron microscope has been used to image high-field domains and current filaments in GaAs/AlxGa1-x As multi-quantum wells. In samples where negative differential resistance and current instabilities occur, high-field domains form. However, these domains are shown to be either stationary or propagating slowly along the samples, in stark contrast to the case in bulk material. In samples where n-type negative differential resistance, caused by either real-space transfer or intervalley transfer, is inhibited, high-field domains do not form. In the case of moving domains they are shown to annihilate before reaching the anode, probably by a lateral dissipative mechanism. In samples where s-type negative differential resistance occurs current filaments are shown to form, which leads to the irreversible destruction of the material.

Semiconducting structures and devices based on gallium arsenide with deep centers

We have examined the characteristics of nontraditional gallium arsenide π-v-n type device structures, in which p-n junctions are formed by doping the semiconductor with deep centers. The properties of π-v-n structures are established by processes of charge trapping at deep centers under the effect of an external bias or exposure to a wide spectral range of electromagnetic radiation. The nature of formation of S-type negative differential resistance for a reverse biased π-v-n structure and the mechanism of high speed switching (with a switching time of (0.4–5)×10−10 sec) were analyzed. The results of an investigation of the effects of optical radiation, x-rays, γ-radiation and high energy charged particles on the structures are discussed and the characteristics of new devices constructed using π-v-n structures are presented. These devices include high speed avalanche S-diodes and triodes, wide spectral band photodiodes (λ=0.2–10 µm) and detectors for x-rays, γ-radiation, and relativistic electrons.

Relationship between the chaotic oscillation and side-gating effect S-type negative differential resistance in GaAs MESFETs

It is shown that GaAs MESFET drain current low-frequency oscillations (LFOs) correlate with side-gating effect S-type negative differential resistances (SNDRs). The LFOs are observed only around the SNDR region, which appears near an extrapolated cross-point for a low-voltage Ohm's law curve and a high-voltage super linear injection current curve. The LFO waveforms are chaotic and they vary with temperature and applied side-gate voltage in a complicated way. Fractal dimensions for the LFO waveform are estimated to be 1.7-3.6, which may support the waveform complexity.

A novel quantum-well optoelectronic switching device with stimulated emission

A novel GaAs/GaAlAs quantum well optoelectronic switching device which exhibits an s-type negative differential resistance at room temperature and emits a high performance stimulated emission was demonstrated. The device structure is similar to the conventional separate confinement heterojunction quantum-well laser and thus can be easily integrated with the optoelectronic integrated circuits. The devices can be switched optically and/or electrically. Threshold current densities of 1.2 kA/cm2, differential quantum efficiencies as high as 67% (0.5 W/A slope efficiency per facet) and output power in excess of 50 mW per facet were obtained. To date, this is one of the best overall performances in terms of output power, differential quantum efficiency and threshold current density for the optoelectronic switching devices exhibiting stimulated emission capability.

Negative differential resistance of a delta-doping-induced double-barrier quantum-well diode at room temperature

A resonant-tunneling homojunction diode, which is a delta-doping-induced double-barrier quantum-well (D/sup 3/BQW) diode, is presented. The barrier uses the delta n/sup +/-i- delta p/sup +/-i- delta n/sup +/ structure. The current-voltage characteristics exhibit three sections of negative differential resistance (NDR) phenomena. At low bias, N-type NDR is demonstrated due to the resonant-tunneling effect. At higher bias, another N-type NDR appears due to the heating effect in the high electric field. As the external bias increases further, an S-type NDR is observed. This result is attributed to the impact ionization effect of thermionic electrons, and then trapping of holes in the maxima of the valence bands, resulting in barrier lowering and redistribution of voltage. >

Gallium arsenide avalanche-type S-diode based on a n+-?-?-n structure

The results are presented of an investigation of the electrical characteristics of avalanche S-diodes based on a new type of structure (+-π-ν-n), which is obtained by diffusing iron in GaAs with n = 1.1017 cm−3. The diodes have regions of S-type negative differential resistance (NDR), for both bias polarities and have a number of other features by comparison with diodes based on the π-ν-n structures with fused contacts. We discuss the mechanism for the formation of the NDR region of the voltage-current characteristics of the diode.