S-shaped Negative Differential Resistance Research Articles

GaAs/InGaAs Optoelectronic Switch for Triple-Logic Applications

A new GaAs/InGaAs triangular barrier optoelectronic switch combined with tri-state characteristic is fabricated and demonstrated. Two GaAs/InGaAs barriers are employed to provide potential barriers for electron thermionic emission and hole confinement, respectively. Applying a sufficient DC voltage to this device, a double S-shaped negative differential resistance (NDR) phenomenon with nearly equal switching voltage difference is appeared at room temperature. This unique NDR property can be introduced to triple stable regions into the device circuit design. Based on a proper circuit design with suitable load line, the studied device has potential for triple-logic applications.

GaAs/InGaAs Optoelectronic Switch for Triple-Logic Applications

A new GaAs/InGaAs triangular barrier optoelectronic switch combined with tri-state characteristic is fabricated and demonstrated. Two GaAs/InGaAs barriers are employed to provide potential barriers for electron thermionic emission and hole confinement, respectively. Applying a sufficient DC voltage to this device, a double S-shaped negative differential resistance (NDR) phenomenon with nearly equal switching voltage difference is appeared at room temperature. This unique NDR property can be introduced to triple stable regions into the device circuit design. Based on a proper circuit design with suitable load line, the studied device has potential for triple-logic applications.

The S-Shaped Negative Differential Resistance during the Electrooxidation of H2/CO in Polymer Electrolyte Membrane Fuel Cells: Modeling and Experimental Proof

In the present contribution, the operation of a low temperature polymer electrolyte membrane fuel cell (PEMFC) with H2/CO and N2/CO mixtures is studied. The aim of the work is to clarify a discrepancy between earlier experiments with conventional electrochemical arrangements and experiments carried out with PEMFCs: Investigations of CO oxidation with rotating disk electrode (RDE) setups in acidic electrolytes revealed a hysteresis in the polarization curve around the onset overpotential of CO oxidation, which was not found in PEMFCs so far. The proof of a hysteresis in the PEMFC polarization curve could be interesting in terms of fuel cell performance, because it was argued that its presence also influences the oxidation of H2/CO mixtures. Indeed, in the present work, this hysteresis during operation with a N2/CO mixture is found, but it is seen that it disappears once H2 is added. To understand the background of these studies, a spatially lumped model is derived.

Pnpn and npn Heterostructural Optoelectronic Devices

An ohmic-contact pnpn triangular-barrier optoelectronic switch (TBOS) has been fabricated. The triangular barrier is formed by inserting an InGaAs p-type delta-doped (δ(p+)) quantum well between two n--GaAs layers. Due to the hole accumulation in the δ(p+) well and the avalanche multiplication in the reverse-biased region, an S-shaped negative differential resistance (NDR) is obtained in the device characteristics. With a replacement of the ohmic-contact p-type cap layer with a Schottky-contact n-type layer in the pnpn TBOS, a Schottky-contact npn TBOS has also been proposed. A double S-shaped NDR phenomenon is observed in the npn TBOS characteristics due to the hole accumulation in the δ(p+) well and the sequential avalanche multiplications in the reverse-biased region and Schottky-contact junction. Both devices show a flexible optical function as a result of the triangular barrier heights associated with incident light.

Electrodeposition of Ni in Submicrometer Trenches

A survey of the effect of cationic, anionic, and nonionic surfactants on the rate and morphological evolution of nickel electrodeposition is presented. Attention is given to the prospect for void-free filling of submicrometer trenches. Cationic species such as polyethyleneimine (PEI) and cetyl-trimethyl-ammonium yield significant inhibition of nickel deposition. For a range of concentrations the single cationic surfactant systems exhibit hysteretic voltammetric curves that, when corrected for ohmic electrolyte losses, reveal an S-shaped negative differential resistance. Void-free bottom-up superconformal feature filling is observed when operating at potentials within the hysteretic regime whereby metal deposition begins preferentially in the most densely patterned regions of the wafer followed by propagation of the growth front laterally across the wafer surface. In contrast, at low overpotentials and concentrations, sulfur-bearing additives such as thiourea (TU) exert a depolarizing effect on nickel deposition and negligible hysteresis. When PEI and TU are both present, the suppression provided by PEI is diminished and feature filling leads to uniform deposition on the wafer scale. Suitable combinations of PEI and TU enable near void-free filling of wide trenches with sloping sidewalls. Initial conformal growth is followed by geometric leveling once the deposits on the sloping sidewalls meet.

Dynamic Instabilities and Mechanism of the Electrochemical Oxidation of Thiosulfate

The electrochemical oxidation of thiosulfate is revealed to have two distinct oscillatory regimes in both linear potential and galvanic voltammograms, where various nonlinear behaviors such as period-2, mixed-mode and quasi-periodic oscillations, and chaos are observed under potentiostatic or galvanostatic conditions. Electrochemical impedance spectroscopy and iR compensation characterization indicate that, depending on the operating conditions, the system could be either a strictly potentiostatic oscillator or an S-shaped negative differential resistance oscillator. Chronoamperometry measurements reveal that the first oscillatory process involves a single-electron transfer, whereas within the second oscillatory regime the average number of electrons transferred is around 3.8. Measurements with capillary electrophoresis and chemical methods illustrate that the oxidation products include S2O6(2-), S4O6(2-), S5O6(2-), S3O6(2-), and SO4(2-).

Negative differential resistance related to self-organization phenomena in a dc gas discharge

Oscillations stimulated by an S-shaped negative differential resistance originating from the self-assembly and decay of a self-organized space charge configuration in front of the anode of a dc gas discharge were observed. The oscillations appear in a resonant circuit, appropriate for the discharge, which contains a capacitor having a plate charged by secondary electron emission. The plate can be a metallic or a dielectric one because the negative differential resistance operates at zero dc current to the plate.

Negative differential resistance in single-wall carbon nanotube network

The current-voltage (I-V) characteristic of single-wall carbon nanotube (SWNT) network was investigated at low temperatures. At T∼ 1.6K, S-shaped negative differential resistance (NDR) was observed, which was systematically examined under the variation of temperature, magnetic field in transverse direction and distances between probes. S-shaped NDR was explained by the nondestructive impact ionization of carriers and the resulting rmation of inhomogeneous spatial structures such as high-current filaments. Critical electric field for impact ionization was estimated to be about IV/ cm.

S-shaped negative differential resistance in 650 nm quantum well laser diodes

The appearance of an S-shaped negative differential resistance (NDR) in GaInP/AlGaInP quantum well (QW) lasers has been observed in both single and multi-QW devices at temperatures between 100 and 200 K. Light–current ( L– I) and current–voltage ( I– V) relationships have been measured in detail at temperatures from 100 to 250 K, using three QW samples with different values of cladding layer thickness (1.0, 0.5 and 0.3 μm). The dependence of the S-shaped NDR characteristics on cladding thickness and laser output intensity shows that the high resistivity p-cladding layer is the cause of NDR. It is because of the impact ionisation of impurities in the p-cladding layer that an S-shaped NDR is produced. An impurity ionisation energy of ∼62 meV in p-type (Al 0.7Ga 0.3) 0.52In 0.48P is obtained from the I–V curve, which is consistent with that obtained from a hydrogenic model and accepted values.

GaAs Triac-like Triangular Barrier Switch Prepared by Molecular Beam Epitaxy

A new S-shaped negative differential resistance (NDR) switching device, prepared by molecular beam epitaxy (MBE), has been successfully developed in a GaAs double triangular barrier structure. Symmetrical bidirectional S-shaped NDR characteristics are observed experimentally. The bidirectional current-voltage (I–V) characteristics exhibit a new type of NDR caused by an avalanche multiplication process in reverse biased base-collector region and barrier redistribution. Under a base current injection with respect to the cathode, the device exhibits a conventional transistor with a current gain of 1.2 at room temperature. The experimentally electrical results can be easily understood by an equivalent circuit. In addition, a new optoelectronic switching device is also proposed which may have the potential for bidirectional wave length emission.

A novel resonant tunneling base transistor with bi-directional negative-differential-resistance phenomena

In this article, a new functional resonant tunneling base transistor (RTBT) with an AlGaAs/GaAs/InGaAs heterostructure emitter and an ultra-thin base layer was fabricated and demonstrated. The bi-directional negative-differential-resistance (NDR), i.e., N- and S-shaped NDR, are observed under the normal and inverted operation mode, respectively. In this device, typical transistor performances with common-emitter current gain of 140 and a low offset voltage of 100 mV, incorporating an interesting N-shaped NDR phenomenon with peak-to-valley current ratio of 5.3, are attained at room temperature. The behavior is mainly attributed from electrons resonant tunneling from depleted GaAs emitter to collector barriers, through InGaAs quantum well (QW) and ultra-thin base layer. On the contrary, the multiple S-shaped NDR behaviors resulting from an avalanche multiplication and successive two-stage barrier lowing process under inverted operation mode are observed.

Functional AlGaAs/GaAs heterostructure-emitter bipolar transistor with a pseudomorphic InGaAs/GaAs quantum-well base

Abstract A new functional AlGaAs/GaAs heterostructure-emitter bipolar transistor (HEBT) with a pseudomorphic InGaAs/GaAs quantum-well (QW) base structure is presented. Due to the insertion of an InGaAs QW between the emitter–base (E–B) junction, the valence band discontinuity can be enhanced. The excellent transistor characteristics including a high current gain of 280 and a low offset voltage of 100 mV are obtained. In addition, an interesting multiple S-shaped negative differential resistance (NDR) phenomenon is observed under the inverted operation mode. This may be attributed to an avalanche multiplication and sequential two-stage barrier lowering effect.

Multiple wavelength electroluminescence and laser generation in p-i-n resonant tunneling heterostructures

We report a design and electroluminescence (EL) investigation of a p-i-n resonant tunneling device based on an Al0.4Ga0.6As/GaAs graded-index waveguide heterostructure. The intrinsic region of the structure consists of a quantum well (QW) surrounded by multiple barrier energy filters providing simultaneous resonant occupation of electron and heavy-hole second excited subbands in the QW. Several peaks are observed in the EL spectra, confirming occupation of the excited subbands. The EL efficiency displays a resonant behavior accompanied by an S-shaped negative differential resistance region in the voltage–current characteristic. Current bistability is demonstrated, leading to bistability in the EL and laser generation spectra.

Functional heterostructure-emitter bipolar transistor (HEBT) with graded-confinement and pseudomorphic-base structure

In this paper, a new functional heterostructure-emitter bipolar transistor (HEBT) with a graded AlxGa1−xAs confinement layer and a pseudomorphic InGaAs/GaAs base structure is fabricated and demonstrated. Due to the insertion of an InGaAs quantum well (QW) between the emitter–base (E–B) junction, the valence band discontinuity can be enhanced and a high emitter injection efficiency may be achieved. Furthermore, the potential spike of the graded AlxGa1−xAs/GaAs heterojunction is expected to be smoothed out which results in a lower offset voltage. The excellent transistor characteristics include a high current gain of 120 and a low offset voltage of 100 mV. In addition, an interesting S-shaped multiple negative differential resistance (MNDR) phenomenon is observed in the inverted operation mode. This may be attributed to an avalanche multiplication and a sequential two-stage barrier lowering effect resulting from the accumulation of holes at the base and electrons at the InGaAs QW, respectively. Consequently, owing to the remarkable transistor performance and MNDR characteristics, the studied HEBT shows good promise for applications in amplifiers and multiple-valued logic circuits.

Switches with double carrier confinement structures for multiple-valued logic applications

Two switching devices with different double carrier confinement structures were fabricated. One is a GaAs switch with double InGaP barriers, another with double GaAs-InGaAs delta-doped (-doped) quantum wells. In the characteristics of the former, because of the two-step barrier lowering induced by the processes of the carrier charging to the injection junction and carrier confinement in wells, a double S-shaped negative differential resistance (NDR) phenomenon is observed after the onsets of successive avalanche multiplications. However, only a single NDR performance is obtained in the characteristics of the latter due to one-step barrier lowering, even though successive avalanche multiplications also occur in the device operation.

Heterostructure optoelectronic switch with lightcontrollable S-shaped negative differential resistance

A heterostructure optoelectronic switch, grown by molecular beam epitaxy, has been fabricated. Owing to the carrier confinement and avalanche multiplication in the transport mechanism, S-shaped negative-differential-resistance performances are observed in the current–voltage (I–V) characteristics. The device shows a flexible optical function related to the potential barrier height and breakdown voltage controllable by incident light. The dependence of the I–V characteristics on illumination is attributed to the carrier confinement effect in the device operation.

Regenerative switching phenomenon of a graded Al xGa 1− xAs/InGaAs/GaAs heterostructure

In this paper, a new regenerative switching device with a graded Al x Ga 1− x As/InGaAs/GaAs heterostructure has been fabricated and demonstrated. A thin p +-InGaAs quantum well (QW) is employed between n-GaAs and p +-GaAs layers to provide a significant confinement effect for electrons. An interesting S-shaped multiple negative differential resistance (NDR) phenomenon and three operation points are observed, due to an avalanche multiplication and sequential two-stage barrier lowering effect resulting from the successive accumulation of holes and electrons at the p +-GaAs base regime and InGaAs quantum well respectively. Consequently, with appropriate adjustments of device parameters, the proposed heterostructure provides a good promise for multiple-valued logic circuit applications.

Symmetric triangular-barrier optoelectronic switch (S-TOPS) by gas source MBE

We report a novel optical functional device, a symmetric triangular-barrier optoelectronic switch (S-TOPS), which consists of a symmetric n + -i-δp + -i-n + structure of In 0.53 Ga 0.47 As, grown by a gas source molecular beam epitaxy (GSMBE). We fabricated the S-TOPS by changing the sheet-carrier concentration of the δp + gate layer to introduce an avalanche multiplication for positive feedback in the operation. In the current vs. voltage characteristics of the S-TOPS under illumination of 1.55 μm wavelength light, we successfully confirmed a bipolar S-shaped negative differential resistance (NDR) characteristics on a certain range of sheet-carrier concentration of the δp + gate layer. Clear latch characteristics in the input-light power vs. current characteristics were also obtained in both the positive and negative biased conditions. We found that it is very important to optimize the sheet-carrier concentration to fabricate the S-TOPS.

Multiple Switching Phenomena of AlGaAs/InGaAs/GaAs Heterostructure Transistors

An interesting multiple negative-differential-resistance (NDR) phenomenon is observed in AlGaAs/InGaAs/ GaAs heterostructure bipolar transistors with abrupt or graded AlGaAs confinement layers under the inverted operation mode. The switching behaviors are mainly due to avalanche multiplication and a two-stage barrier lowering effect. The switching properties of the first S-shaped NDR observed in the device with the abrupt confinement layer are better than those observed in the device with the graded confinement layer, due to the former device's superior confinement effect on holes and electrons. The control voltage efficiency of the second S-shaped NDR is nearly equal in the studied devices because the InGaAs quantum well dominates the properties of this NDR.

GaAs three-terminal double delta-doped triangular barrier switch prepared by molecular beam epitaxy

A novel S-shaped negative differential resistance (NDR) switching device, prepared by molecular beam epitaxy (MBE), has been successfully developed in a GaAs double delta-doped triangular barrier (TB) structure. All symmetrical S-shaped NDR characteristics, either unidirectional or bidirectional bistability phenomena, are observed experimentally. The bidirectional current-voltage (I–V) characteristics exhibit a new type of NDR caused by an avalanche multiplication process in the reverse-biased base-collector region and by barrier redistribution. Under a base current injection with respect to the cathode, the device behaves as a conventional transistor with a current gain of 1.2 at room temperature. Experimentally, electrical results can be easily understood by an equivalent circuit and a proposed model. In addition, a new optoelectronic switching device is also proposed which may have the potential for bidirectional wavelength emission. On the basis of the first utilization of a double delta-doped TB structure, a conceptual understanding of such a set of results enhances our understanding of the physics of double delta-doped TB devices in general.