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We proposed and fabricated large-scale arrays of resonant tunneling diode (RTD) oscillators for high-output-power terahertz (THz) sources. The array element is composed of an RTD, a slot resonator, and a dipole array antenna on a dielectric layer stacked on the RTD. In this structure, the output power is radiated in the upward direction of the substrate without a hemispherical silicon lens. The dipole array antenna was designed so that the average output power determined by the variation in the size of the RTD in the array was maximized. The experimental output power was proportional to the element number, and its value was 0.73 mW for an 89-element array at ∼1 THz in a pulsed mode with a repetition rate of 300 Hz and a duty ratio of 10%. Multiple peaks were observed in the oscillation spectra, because the elements were not intentionally coupled with each other. The average output power per element was 9 μW in the array, which was lower than that of the separated single oscillators (21 μW). Possible causes of this difference are discussed. [ABSTRACT FROM AUTHOR]

The influence of a GaN layer as a sub-quantum well for an AlGaN/GaN/AlGaN double barrier resonant tunneling diode (RTD) on device performance has been investigated by means of numerical simulation. The introduction of the GaN layer as the sub-quantum well turns the dominant transport mechanism of RTD from the 3D-2D model to the 2D-2D model and increases the energy difference between tunneling energy levels. It can also lower the effective height of the emitter barrier. Consequently, the peak current and peak-to-valley current difference of RTD have been increased. The optimal GaN sub-quantum well parameters are found through analyzing the electrical performance, energy band, and transmission coefficient of RTD with different widths and depths of the GaN sub-quantum well. The most pronounced electrical parameters, a peak current density of 5800 KA/cm2, a peak-to-valley current difference of 1.466 A, and a peak-to-valley current ratio of 6.35, could be achieved by designing RTD with the active region structure of GaN/Al0.2Ga0.8N/GaN/Al0.2Ga0.8N (3 nm/1.5 nm/1.5 nm/1.5 nm). [ABSTRACT FROM AUTHOR]

Herein, a series‐connected resonant tunneling diode (RTD)‐pair configuration is characterized using the dynamic capacitance characteristics of the RTD. To achieve enhanced output power characteristics, the RTD‐pair cell is utilized in the RTD oscillator design to increase the voltage span and current span of the negative differential conductance (NDC) region. The results indicate that not only the output power can be significantly improved compared with the single‐RTD oscillator, but also it operates at a small capacitance region, making the RTD‐pair oscillator beneficial for high frequency and high output power operation. [ABSTRACT FROM AUTHOR]

AlN/GaN resonant tunneling diodes (RTDs) were grown separately on freestanding GaN (FS-GaN) substrates and sapphire substrates by plasma-assisted molecular-beam epitaxy (PA-MBE). Room temperature negative differential resistance (NDR) was obtained under forward bias for the RTDs grown on FS-GaN substrates, with the peak current densities (J p) of 175â€"700 kA/cm2 and peak-to-valley current ratios (PVCRs) of 1.01â€"1.21. Two resonant peaks were also observed for some RTDs at room temperature. The effects of two types of substrates on epitaxy quality and device performance of GaN-based RTDs were firstly investigated systematically, showing that lower dislocation densities, flatter surface morphology, and steeper heterogeneous interfaces were the key factors to achieving NDR for RTDs. [ABSTRACT FROM AUTHOR]

We investigate electron transport in epitaxially grown nitride-based resonant tunneling diodes (RTDs) and superlattice sequential tunneling devices. A density-matrix model is developed, and shown to reproduce the experimentally measured features of the current–voltage curves, with its dephasing terms calculated from semi-classical scattering rates. Lifetime broadening effects are shown to have a significant influence in the experimental data. Additionally, it is shown that the interface roughness geometry has a large effect on current magnitude, peak-to-valley ratios and misalignment features; in some cases eliminating negative differential resistance entirely in RTDs. Sequential tunneling device characteristics are dominated by a parasitic current that is most likely to be caused by dislocations; however, excellent agreement between the simulated and experimentally measured tunneling current magnitude and alignment bias is demonstrated. This analysis of the effects of scattering lifetimes, contact doping and growth quality on electron transport highlights critical optimization parameters for the development of III–nitride unipolar electronic and optoelectronic devices. [ABSTRACT FROM AUTHOR]

Among the four features unique to hot-carrier solar cells (HC-SCs): (i) carrier thermalization time and (ii) carrier equilibration time in the absorber, (iii) energy-selection width and (iv) conductance of the energy-selective contacts (ESCs), requisites of (i)-(iii) for high conversion efficiency have been clarified. We have tackled the remaining issues related to (iv) in the present study. The detailed balance model of HC-SC operation has been improved to involve a finite value of the ESC conductance to find the required values, which in turn has been revealed to be feasible using resonant tunneling diodes (RTDs) consisting of semiconductor quantum dots (QDs) and quantum wells (QWs) by means of a formulation to calculate the conductance of the QD- and QW-RTDs derived using the rigorous solutions of the effective-mass Hamiltonians. Thus, all of the four requisites unique to HC-SCs to achieve high conversion efficiency have been elucidated, and the two requisites related to the ESCs can be fulfilled using the QD- and QW-RTDs. [ABSTRACT FROM AUTHOR]

In this paper, we have investigated the effect of Be acceptors on the electroluminescence and the spin polarization in GaAs/AlAs p-i-n resonant tunneling diodes. The quantum well emission comprise two main lines separated by ~20meV attributed to excitonic and Be-related transitions, which intensities show remarkably abrupt variations at critical voltages, particularly at the electron resonant peak where it shows a high-frequency bistability. The circular-polarization degree of the quantum-well electroluminescence also shows strong and abrupt variations at the critical bias voltages and it attains relatively large values (of ~–75% at 15 T). These effects may be explored to design novel devices for spintronic applications such as a high-frequency spin-oscillators. [ABSTRACT FROM AUTHOR]

The objective of the present study is to evaluate the fluid dynamics of a flotation column (2" in diameter and 6 m in height) based on gas hold-up measurements and, in particular, by applying the liquid-phase RTD (residence time distribution) technique. Additionally, a comparative study is performed between different methodologies used to determine RTD hydrodynamic parameters such as fitting of tanks-in-series and axial dispersion models to the experimental data. The evidence indicates that the axial dispersion and tanks-in-series models agreed well with the experimental data, with a slight advantage of the first model, in terms of obtaining the fluid dynamic parameters. It was proven that the axial dispersion model parameters (Pe or Nd) can be obtained with good precision (R2 > 0.99) within the evaluated ranges from correlations that use the number of tanks in series (N) values estimated by the tanks-in-series model. [ABSTRACT FROM AUTHOR]

We propose and fabricate a novel structure-simplified resonant tunneling diode (RTD) terahertz (THz) oscillator without metal-insulator-metal (MIM) capacitors. In conventional RTD THz oscillators, the DC and RF circuits are separated by MIM capacitors. In the proposed oscillator, the DC-to-RF separation is based on the difference between the impedances of the antenna and biasing wire inductances in the low-frequency range and those in the THz range. Thus, by removing MIM capacitors, we simplify the device structure. We also introduce an InP etch stopper layer in the RTD epitaxial structure for easy formation of an airbridge. Owing to the simple structure and the introduction of the etch stopper layer, the fabrication process is significantly simplified. We successfully fabricated novel RTD oscillators using a simple fabrication process. Oscillation frequency of up to 742 GHz and output power of ~ 10 μW up to 600 GHz were obtained, respectively. The novel RTD oscillator is expected to be suitable for massive array configurations that could enable a wide range of practical THz applications. [ABSTRACT FROM AUTHOR]

In this letter, we propose an area-efficient series-connected resonant tunneling diode pair device (SCRTD) to utilize as a binary neuron device in cellular neural networks (CNNs). The proposed SCRTD (P-SCRTD) consists of two RTDs interconnected by a floating metal layer with a high-doped epitaxial layer. The P-SCRTD shows a 48 % reduction in area by eliminating the isolation area in the conventional SCRTD (C-SCRTD) owing to the symmetrical I-V characteristic of the RTD, without any performance penalty on the DC and RF characteristics by means of adopting the floating metal layer, compared to the C-SCRTD. CNN application of the P-SCRTD is investigated, showing inherent threshold operation and high-speed (12.5 Gbps) low-power (21 mW) performance, despite using the $2~\mu \text{m}$ technology node, for a unit cell for edge extraction. [ABSTRACT FROM AUTHOR]

Resonant tunneling diodes with negative differential resistance (NDR) have attracted significant attention due to their unique quantum resonant tunneling phenomena and potential applications in terahertz emission/detection and high‐density logic/memory. In this paper, resonant tunneling devices, where the carriers tunnel through a hexagonal boron nitride (hBN) barrier sandwiched by two black phosphorus (BP) layers, are explored. The resonance occurs when the energy bands of the two black phosphorus layers are aligned. The conductive atomic force microscopy (CAFM) measurements reveal prominent NDR peaks with large peak‐to‐valley ratios at room temperature. It is found that the positions of the NDR peaks are very sensitive to the amplitude and the shape of the voltage waveform used in CAFM, which can be explained by the charge trapping effect. Furthermore, resonant tunneling transistors are demonstrated based on BP/hBN/BP stacks in which the locations of the NDR peaks are tunable by the electrostatic gating. As compared to the traditional tunneling diodes based on bulk materials, the tunneling devices based on thin boron nitride tunneling barrier and high mobility black phosphorus offer ultra‐high‐speed response. This feature, together with the NDR characteristics, provides the potential for applications in THz oscillators and multi‐value logic devices. [ABSTRACT FROM AUTHOR]

Optical modulation characteristics of resonant tunneling diode photodetectors (RTD‐PD) are investigated. Intensity modulated light excites the RTD‐PDs to conduct data experiments. Simple and complex data patterns are used with results showing data rates up to 80 and 200 Mbit/s, respectively. This is the first demonstration of complex modulation using resonant tunneling diodes. [ABSTRACT FROM AUTHOR]

We report repeatable AlGaN/GaN resonant tunneling diodes (RTDs) grown on a sapphire substrate by metal organic chemical vapor deposition. The RTDs exhibit clear negative differential resistance at low temperature in both bias directions. Peak-to-valley current ratios of 1.4 and 1.08 and peak current densities of 6 kA/cm2 and 0.65 kA/cm2 are extracted at 6.5 K for forward and reverse bias, respectively. The polarization fields in III-nitrides are found to affect the diode electrical behaviors by modulating the symmetry of the two barriers and altering the thickness of the depletion region, which eventually triggers asymmetric resonant tunneling transport. [ABSTRACT FROM AUTHOR]

The band gap of armchair graphene nanoribbons (AGNRs) can be modulated by replacing the carbon atoms with boron/nitride (BN) atoms to produce the compound nanoribbons, while the width of ribbons remains constant. By introducing BN doping atoms in the proper positions along the ribbon length, a double-barrier quantum-well structure is constructed. Consequently, negative differential resistance properties can be obtained by a combination of armchair BN nanoribbons (ABNNRs) and AGNRs as the compound ABNxGyNRs, in which x and y denote the number of BN and C atoms in the ribbon width, respectively. The proposed resonant tunneling diode (RTD), called an armchair BN graphene nanoribbon resonant tunneling diode (ABNGNR-RTD), is investigated in three different platforms including W, S, and H shapes. The numerical tight-binding model along with non-equilibrium Green's function formalism is taken into account to study the electronic properties of the proposed RTD. The performance of the ABNGNR-RTD is examined in terms of device characteristics such as peak-to-valley ratio (PVR) and power dissipation. Based on the presented results, the performance of H-shaped devices is better than those of the other two cases in terms of PVR and power dissipation. In addition, the electronic properties of ABNGNR-RTDs can be modified by varying the relative width of ABNNRs with respect to AGNRs. [ABSTRACT FROM AUTHOR]

This paper reports on a 15-Gb/s wireless link that employs a high-power resonant tunneling diode (RTD) oscillator as a transmitter (Tx). The fundamental carrier frequency is 84 GHz and the maximum output power is 2 mW without any power amplifier. The reported performance is over a 50-cm link, with simple amplitude shift keying modulation utilized. The 15-Gb/s data link shows correctable bit error rate (BER) of $4.1 \times 10^{-3}$ , while the lower data rates of 10 and 5 Gb/s show a BER of $3.6 \times 10^{-4}$ and $1.0 \times 10^{-6}$ , respectively. These results demonstrate that the RTD Tx is a promising candidate for the next-generation low-cost, compact, ultrahigh data rates wireless communication systems. [ABSTRACT FROM AUTHOR]

We propose a resonant tunneling diode (RTD)-based relaxation oscillator and an oscillator-based terahertz (THz) wireless link that compensate for shortfalls in RF-oscillator emission power by adding several relaxation carrier wave harmonic modes. We believe that the proposed link can perform as well as or superior to large component-based wireless links that suppress power dissipation using collimating lenses and/or discrete antennas to add the power output of a relaxation oscillator. This hypothesis is investigated analytically using a physics-based equivalent circuit model of the proposed oscillator and a link budget analysis of the relaxation carrier wave. The model, which incorporates effects such as the non-linearity of the tunneling diode and the electromagnetic properties of the integrated bow-tie antenna on the oscillator, is used to quantitatively investigate the link characteristics in terms of the signal-to-noise ratio (SNR) and other parameters and to demonstrate that, by applying an appropriate device size and number of harmonic modes in the occupied bandwidth, link performance comparable to that of previously reported wireless links can be achieved. Based on these results, we discuss the potential for practical implementation of the proposed link configuration in mobile link applications for use in environments such as the Internet of Things (IoT). [ABSTRACT FROM AUTHOR]

This paper brings deep study focused on dynamical behavior associated with a multi-valued static memory (MVSM) cell composed by a pair of the resonant tunneling diodes. Ampere-voltage characteristic of these diodes is approximated by the piecewise-linear scalar function with two positive breakpoints. Initial linear analysis supposes separation of a state space into nine affine segments, while local vector field geometry and dynamical flow in each region are graphically visualized. It is shown that discovered chaotic solution is not a numerical artifact, is robust and can generate various dense structurally stable strange attractors. For numerical investigations concepts of the largest Lyapunov exponent, the basins of attraction and a conventional signal analysis are adopted. MVSM dynamics is then implemented as the lumped electronic circuits using different but universal analog design approaches. Proposed oscillators are optimized with respect to the simple one-to-one relations between the mathematical model parameters and adjustable-by-hand circuit components. The constructed chaotic oscillators strictly utilize only commercially available active devices. This makes true experimental verification of idea that chaos exists inside narrow regions of a hyperspace of the internal MVSM system parameters possible and easy task. [ABSTRACT FROM AUTHOR]

AlAs/InGaAs/AlAs resonant tunneling materials was optimized, and terahertz resonant tunneling diodes grown by MOCVD is first fabricated. The epitaxial layers of the RTD were grown on semi-insulating InP substrate. InP-based RTD were fabricated by using contact lithography and air bridge technology.The electrical characteristics of the device were tested at room temperature. The peak current density of our RTD exceeded 400 kA/cm2, and the peak to valley current ratio of our RTD was 2.4.

Flotation performance in mechanical cells is strongly influenced by hydrodynamics. Hydrodynamics is driven by the action of the impeller which is responsible for mixing in the cell. This paper evaluates key mixing parameters in three industrial Wemco mechanical flotation cells (#144, #164, #190; 14.2, 28.3, 42.5 m 3 ). Residence time distributions, circulation times, impeller pumping capacities, impeller Flow numbers and power intensities are presented. RTD studies are based on LiCl tracer measurements. RTD studies showed that the #190, #164 & #144 behave as 2.1–2.4, 1.2 and 1.0 tanks in series respectively. Circulation times for the #164 and #144 were much shorter than the #190. The #190 and #164 had the highest pumping capacities. Dimensionless Flow numbers varied from 0.12 to 0.22. Iron losses by entrainment of fine particles (<45 µm) were higher in the #164, due to the higher intensity of mixing. [ABSTRACT FROM AUTHOR]

We demonstrate theoretically a viable design of a superior thermoelectric generator based on “antireflection” enabled lineshape engineering. Using a minimal double-barrier structure, we show that enabling an antireflection design over it facilitates a bandpass transmission spectrum, which significantly enhances the thermoelectric performance. It is demonstrated that the best operating regime of this device offers a maximum power in the range of 0.7–0.9 MW/m2 at efficiencies ranging from 46% to 54% of Carnot efficiency. The physics of charge and heat transport in the ballistic regime of operation helps us to gain additional insights on how a large number of transverse current carrying modes boost the output power, and simultaneously, how the diminishing effects of high-energy parasitic currents aid the efficiency. Our device design also manifests a high immunity to the destructive effect of self-consistent charging, which aids the enhancement of the output power at high efficiencies. Finally, a comparative study with a conventional double barrier thermoelectric is presented in terms of standard performance parameters, which clarifies the potential benefits of enabling the antireflective region. [ABSTRACT FROM AUTHOR]

We present resonant tunneling diode-photodetectors (RTD-PDs) with GaAs0.15Sb0.85/AlAs0.1Sb0.9 double barrier structures combined with an additional quaternary Ga0.64In0.36As0.33Sb0.67 absorption layer covering the fingerprint absorption lines of various gases in the mid-infrared wavelength spectral region. The absorption layer cut-off wavelength is determined to be 3.5 μm, and the RTD-PDs show peak-to-valley current ratios up to 4.3 with a peak current density of 12 A/cm−2. The incorporation of the quaternary absorption layer enables the RTD-PDs to be sensitive to illumination with light up to the absorption lines of HCl at 3395 nm. At this wavelength, the detector shows a responsivity of 6.3 mA/W. At the absorption lines of CO2 and CO at 2004 nm and 2330 nm, respectively, the RTD-PDs reach responsivities up to 0.97 A/W. Thus, RTD-PDs pave the way towards high sensitive mid-infrared detectors that can be utilized in tunable laser absorption spectroscopy. [ABSTRACT FROM AUTHOR]

Abstract: In this letter, the temperature dependence of a microwave push–push oscillator based on a resonant tunneling diode (RTD) has been investigated from 30°C to 80°C. An output power characteristic curve of the second‐harmonic RTD oscillator has been shifted to the higher bias voltages as temperature increases. An oscillation frequency shift of ΔfOSC has been obtained to be –9.8 MHz/K which is equivalent to the ΔfOSC of –4.9 MHz/K at the fundamental oscillation frequency of 14.45 GHz. To authors’ knowledge, this is the first report on the temperature dependent characteristics of the second‐harmonic push–push RTD oscillator. [ABSTRACT FROM AUTHOR]

We discuss the modelling of high current density InGaAs/AlAs/InP resonant tunneling diodes to maximize their efficiency as THz emitters. A figure of merit which contributes to the wall plug efficiency, the intrinsic resonator efficiency, is used for the development of epitaxial designs. With the contribution of key parameters identified, we analyze the limitations of accumulated stress to assess the manufacturability of such designs. Optimal epitaxial designs are revealed, utilizing thin barriers, with a wide and shallow quantum well that satisfies the strained layer epitaxy constraint. We then assess the advantages to epitaxial perfection and electrical characteristics provided by devices with a narrow InAs sub-well inside a lattice-matched InGaAs alloy. These new structures will assist in the realization of the next-generation submillimeter emitters. [ABSTRACT FROM AUTHOR]

Summary: In a system of transmission lines with regularly spaced resonant‐tunneling diodes (RTDs), where several straight RTD lines are connected halfway to a closed RTD line, a pulse‐shaped rotary traveling wave develops on the closed line by mutual synchronization of the oscillatory edge developed in each straight RTD line. The oscillating edge on each straight line is synchronized with the traveling pulse, such that the system has the potential to generate multiphase oscillatory signals in millimeter‐wave frequencies. To examine the dynamics of traveling pulses at such high frequencies, the system is modeled in the framework of the finite‐difference time‐domain method. It is found that a traveling pulse develops in the closed RTD line synchronized with the oscillatory edges moving in the straight lines, assuming a microstrip structure for each RTD line. We then compare the results of the finite‐difference time‐domain calculation with those predicted by the transmission line theory with parameter values obtained by the quasi‐transverse electromagnetic estimation. In addition, the RTD line that compactly confines the electromagnetic fields is shown to have the potential to generate multiphase oscillatory signals at submillimeter‐wave frequencies. [ABSTRACT FROM AUTHOR]

We have investigated the spin properties of a two-dimensional hole gas (2DHG) formed at the contact layer of a p-type GaAs/AlAs resonant tunneling diode (RTD). We have measured the polarized-resolved photoluminescence of the RTD as a function of bias voltage, laser intensity and external magnetic field up to 15T. By tuning the voltage and the laser intensity, we are able to change the spin-splitting from the 2DHG from almost 0 meV to 5 meV and its polarization degree from − 40% to + 50% at 15T. These results are attributed to changes of the local electric field applied to the two-dimensional gas which affects the valence band and the hole Rashba spin-orbit effect. [ABSTRACT FROM AUTHOR]

In this article, an InP-based self-aligned resonant-tunneling-diode (RTD) using a SiNX sidewall process has been fabricated and characterized for high-speed microwave applications. A plasma-enhanced dry-etch technique has been used to form a vertical emitter mesa. The measured peak currents are almost constant at a peak voltage due to negligible thermionic emission in a wide temperature range up to 125°C. The extracted series resistance of the fabricated self-aligned RTD is 14.5 Ω, which is reduced by 27% compared with that of the nonself-aligned RTD by reducing the spacing between emitter mesa and collector metal. To the best of authors' knowledge, this is the first attempt to characterize a self-aligned RTD using the dry-etched mesa and the SiNX sidewall structure. [ABSTRACT FROM AUTHOR]

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 In 0.8 Ga 0.2 As/AlAs RTD with 20 nm emitter spacer thickness and 5 μm 2 mesa area theoretically possesses the capability to reach 3.1 mW at 300 GHz and 1.8 mW at 600 GHz. [ABSTRACT FROM AUTHOR]

Spectral narrowing of a resonant-tunneling-diode (RTD) terahertz oscillator, which is useful for various applications of terahertz frequency range, such as an accurate gas spectroscopy, a frequency reference in various communication systems, etc., was achieved with a phase-locked loop system. The oscillator is composed of an RTD, a slot antenna, and a varactor diode for electrical frequency tuning. The output of the RTD oscillating at 610 GHz was down-converted to 400 MHz by a heterodyne detection. The phase noise was transformed to amplitude noise by a balanced mixer and fed back into the varactor diode. The loop filter for a stable operation is discussed. The spectral linewidth of 18.6 MHz in free-running operation was reduced to less than 1 Hz by the feedback. [ABSTRACT FROM AUTHOR]

In this paper, novel features offered by Resonant Tunneling Diode (RTD) are reviewed by simulating it under different conditions. GaAs/AlGaAs based RTD is used as the reference one to obtain the characteristics due to parametric variations. To fulfil this purpose a simple model of resonant electronic transport through a double-barrier structure is developed. I-V characteristics are studied by varying barrier parameters and well width. Different peak and valley currents are measured under these conditions. For the same set of parameters both symmetric and asymmetric cases are considered. Different materials of lower effective mass are also taken into consideration to improve Peak to Valley Ratio (PVR). The Indium (In) based materials are considered to compare the characteristics obtained from the conventional GaAs based RTD structure. All these proposed structures are simulated using Silvaco Atlas software. [ABSTRACT FROM AUTHOR]

A very low peak voltage GaAs/AlAs resonant tunneling diode (RTD) grown by molecular beam epitaxy (MBE) has been studied in detail. Excellent growth control with atomic-layer precision resulted in a peak voltage of merely 0.28 V (0.53 V) in forward (reverse) direction. The peak current density in forward bias is around 15.4 kA/cm 2 with variation of within 7%. As for reverse bias, the peak current density is around 22.8 kA/cm 2 with 4% variation which implies excellent scalability. In this work, we have successfully demonstrated the fabrication of a GaAs/AlAs RTD by using a conventional optical lithography and chemical wet-etching with very low peak voltage suitable for application in low dc input power RTD-based sub-millimetre wave oscillators. [ABSTRACT FROM AUTHOR]

We investigate the electronic transport properties of GaSb/AlAsSb double barrier resonant tunneling diodes with pseudomorphically grown ternary GaAsxSb1-x emitter prewells over a broad temperature range. At room temperature, resonant tunneling is observed and the peak-to-valley current ratio (PVCR) is enhanced with the increasing As mole fraction from 1:88 (GaAs0.07Sb0.93 prewell), to 2:08 (GaAs0.09Sb0.91 prewell) up to 2:36 (GaAs0.11Sb0.89 prewell). The rise in PVCR is attributed to an enhanced carrier density at the Γ-valley within the emitter prewell. On the contrary at cryogenic temperatures, increasing the As mole fractions reduces the PVCR. At a temperature of T = 4:2K, reference samples without incorporation of an emitter prewell containing As show PVCRs up to 20:4. We attribute the reduced PVCR to a degraded crystal quality of the resonant tunneling structure caused by As incorporation and subsequently an enhanced defect scattering at the interfaces. [ABSTRACT FROM AUTHOR]

Multi-valued logic (MVL) is one of the promising alternatives of binary logic since it has a high-logic density, which brings a vision of simpler circuit structure. Based on a novel concept as double negative differential resistance field effect transistor and its analytic description, a design methodology utilising the mechanism of monostable-to-multistable transition logic element (MMLE) for MVL is proposed in this study. The basic ternary logic gates are designed as a complete logic set, and a compact ternary four-input full adder is constructed as an example of function circuits. Comparing with binary circuit and ternary circuit based on conventional design, the circuit shows advantage in device cost and other properties. The results verify that this MMLE design methodology has a good information load ability, which will have a brilliant prospect in MVL circuits. [ABSTRACT FROM AUTHOR]

In this paper, we report on a terahertz (THz) sensing system. Compared to previously reported systems, it has increased system sensitivity and reduced size. Both are achieved by using a photonic crystal (PC) cavity as a resonator and compact resonant tunneling diodes (RTDs) as signal source and as detector. The measured quality factor of the PC cavity is higher than 10,000, and its resonant frequency is 318 GHz. To demonstrate the operation of the refractive index sensing system, dielectric tapes of various thicknesses are attached to the PC cavity and the change in the resonator's refractive index is measured. The figure of merit of refractive index sensing using the developed system is one order higher than that of previous studies, which used metallic metamaterial resonators. The frequency of the RTD-based source can be swept from 316 to 321 GHz by varying the RTD direct current voltage. This effect is used to realize a compact frequency tunable signal source. Measurements using a commercial signal source and detector are carried out to verify the accuracy of the data obtained using RTDs as a signal source and as a detector. [ABSTRACT FROM AUTHOR]

Negative differential resistance (NDR) has large potential for versatile device applications, including high-frequency oscillators, memories, fast switches, and multilevel logic circuits. NDRs are observed at heteromaterial interfaces in resonant tunneling diodes or Esaki diodes consisting of compound semiconductors or two-dimensional (2D) atomic thin films. However, these devices suffer from poor peak-to-valley ratios (PVR) at room temperature; a cryogenic temperature is needed to improve the PVR. These negative factors are obstacles to practical applications. Here, a new NDR transistor is proposed, in which a p-n heterojunction of organic semiconductors plays a key role. Well-balanced carrier transport is manipulated at the organic p-n junction to realize outstanding NDR. Experimental and simulation analyses reveal that the observed NDR can be explained by analogy with the shoot-through current mechanism in complementary metal-oxide- semiconductor (CMOS) devices. As a result, the NDR transistor shows large PVRs of up to about 104 even at room temperature. [ABSTRACT FROM AUTHOR]

Within the framework of the coherent quantum-mechanical model, which is based on the solution of the time-dependent Schrödinger equation with exact open boundary conditions, we have numerically analyzed the behavior of the InGaAs/AlAs resonant-tunneling diode (RTD) in an alternating-current electric field in the presence of spacer layers. It is shown that the interaction of the energy levels in a quantum well of the RTD, and the same in a quantum well of the emitter spacer, leads to splitting of these levels, which sharply increases the active current of RTD. In this case, the value of the active current at the finite frequencies may exceed the active current within the low-frequency limits. For RTD, heterostructure considered active current at frequency 1.77 THz takes its maximum value at the thickness of the emitter spacer equals to 6.5 nm exceeding the corresponding value in the absence of the spacer by the factor of 32. The results obtained in this paper may be useful to improve the frequency characteristics of resonant-tunneling nanostructures. [ABSTRACT FROM PUBLISHER]

Resonant tunneling through a 4 nm nanocrystal Ge (nc-Ge) layer and a 2.4 nm monolayer of Si colloidal quantum dots (QD) is achieved with 0.7 nm amorphous Al2O3 (a-Al2O3) barriers. The nc-Ge resonant tunneling diode (RTD) demonstrates a peak-to-valley current ratio (PVCR) of 8 and a full width at half maximum (FWHM) of 30 mV at 300 K, the best performance among RTDs based on annealed nanocrystals. The Si QD RTD is first achieved with PVCRs up to 47 and FWHMs as small as 10 mV at room temperature, confirming theoretically expected excellences of 3D carrier confinements. The high performances are partially due to the smooth profile of nc-Ge layer and the uniform distribution of Si QDs, which reduce the adverse influences of many-body effects. More importantly, carrier decoherence is avoided in the 0.7 nm a-Al2O3 barriers thinner than the phase coherence length (≈1.5 nm). Ultrathin a-Al2O3 also passivates well materials and suppresses leakage currents. Additionally, the interfacial bandgap of ultrathin a-Al2O3 is found to be similar to the bulk, forming deep potential wells to sharpen transmission curves. This work can be easily extended to other materials, which may enable resonant tunneling in various nanosystems for diverse purposes. [ABSTRACT FROM AUTHOR]

A scheme is proposed for generating multiphase oscillatory signals in millimeter-wave frequencies based on the dynamics of a traveling pulse developed in a closed transmission line periodically loaded with resonant-tunneling diodes (RTDs) that is coupled with several straight RTD lines. When supplied with an appropriate voltage at the end of an RTD line, a pulse edge is shown to exhibit a spatially extended limit-cycle oscillation on the line. We consider the case where several RTD lines are connected halfway to a closed one at even intervals. In this case, the oscillatory edge developed in each straight RTD line is mutually synchronized such that a pulse-shaped rotary traveling wave develops on the closed RTD line. The oscillating edge on each straight line is also synchronized with the traveling pulse on the closed line, such that the leading edge of the traveling pulse on the closed line and the forward edge on the straight line pass the cross point simultaneously. As a result, when N straight lines are connected to the closed line, the phase difference between two adjacent oscillatory edges becomes 2 π/ N . On the other hand, the trailing edge of the traveling pulse at the cross point breaks the voltage wave on the straight line into two pieces, one of which travels forward to form a solitary wave and the other of which travels backward to reach the input end, where it is reflected and starts to travel forward and this forward moving edge is supposed to be synchronized with the leading edge of the traveling pulse. It means that a back-and-forth edge and a forward-moving solitary wave develop periodically on each straight line. Because the time required for the traveling pulse to go around the closed line must be coincident with the period of the edge oscillation on each straight line, a unique traveling pulse cannot synchronize with each oscillating edge when the cell size of the closed line becomes large, resulting in the development of multiple traveling pulses on the closed line. In this paper, the design criteria are discussed concerning the connecting point between the straight and closed lines, the number of straight lines, and the size of the closed line. In addition, we describe several measurement results that validate the essential properties of the traveling pulse and then show several results of full-wave analysis of a monolithically integrated RTD line. [ABSTRACT FROM AUTHOR]

A strain sensor was fabricated based on a resonant tunneling diode (RTD) oscillator. This sensor uses strain-dependent frequency change of the RTD oscillator, and the frequency delta-sigma modulation technique, which can convert a frequency to a digital signal with wide dynamic range and wide bandwidth. The sensor consists of an oscillator using an LC resonator and an RTD loaded on a beam of the cantilever. The cantilever was fabricated using selective wet etching of the InP substrate. In this sensor, the strain applied to the cantilever changes the current-voltage characteristics of the RTD due to the piezoelectric effect, and hence it changes the oscillation frequency. In this paper, it was demonstrated that the acoustic strain signal can be successfully detected by this sensor with noise shaping behavior, which is a unique nature of the delta-sigma modulation. [ABSTRACT FROM AUTHOR]

The Wigner transport equation based on the Wigner function which is defined on the phase space describes two actions in orthogonal directions of the phase space: movement (diffusion) in position space and transition in momentum space. Here, we show that for the proper analysis of a resonant tunneling diode using the finite-difference-based solution of the Wigner transport equation, the degree of numerical accuracy in the calculation of the movement in position space should be balanced with that in the evaluation of the transition in momentum space. [ABSTRACT FROM AUTHOR]

The authors report the growth by rf-plasma assisted molecular beam epitaxy of AlN/GaN/AlN resonant tunneling diodes which exhibit stable, repeatable, and hysteresis-free negative differential resistance (NDR) at room temperature for more than 1000 bias sweeps between −2.5 and +5.5 V. The device layers were grown on freestanding, Ga-polar GaN substrates grown by hydride vapor phase epitaxy and having a density of threading dislocations between 106 and 107 cm−2. The authors speculate that the repeatable NDR is facilitated by the low-dislocation density substrates. [ABSTRACT FROM AUTHOR]

Radiation from antennas integrated with indium phosphide (InP)-based resonant tunnelling diode (RTD) oscillators is mainly through the substrate because of the effects of the large dielectric constant. Therefore, hemispherical lenses are used to extract the signal from the backside of the substrate. In this study the authors present a broadband bow-tie slot antenna with a tuning stub which is diced and mounted on a ground plane to alleviate the substrate effects. Here, the large dielectric constant substrate around the antenna conductor is removed. In addition, the ground plane underneath the diced substrate acts as a reflector and, ultimately, the antenna radiates to the air-side direction. Antenna integration with RTD oscillators is described in this study as well. Two-port bow-tie slot antennas were designed and characterised and showed the suitability of integration with power combining RTD oscillator circuits which are based on mutual coupling. The antennas were fabricated using electron beam lithography on a semi-insulating InP substrate. Simulated and measured bandwidth almost extends the entire frequency range 230-325 GHz. Simulations shows air-side radiation pattern and antenna gain of around 11 dB at 280 GHz. Simulations also show that the antenna may be fed with a 50-O or 30-O feed line, i.e. suitable feed lines, without compromising its performance which may prove beneficial for optimum loading of RTD oscillators. [ABSTRACT FROM AUTHOR]

In this article, we present a model of Triple barrier Resonant tunneling diodes (TBRTDs) and characterize its electrical properties and their dependence of operating temperature and barrier length associated with varying structural parameters of the device. This paper also demonstrates the potential impact of doping concentration on current density of the device. Furthermore, Local density of states (LDOS) and transmission coefficient at various operating temperatures are analysed. Quantum tunneling mechanism results, based on Landauer-Buttiker formalization of non-equilibrium Green's function within ballistic limits, shows high peak current with 2 nm triple barrier structure and achieve high peak to valley ratio as compared to other barrier structure. Significantly, comparison helps in analyzing the better device between the proposed device structures. Simulation of the device has been performed with the use of Nextnano3 tool which confirms the various results presented in this research. [ABSTRACT FROM AUTHOR]

We report on a simulation for an aluminum gallium nitride (AlGaN)/gallium nitride (GaN) resonant tunneling diode (RTD) with a step heterojunction emitter spacer (SHES) at room temperature. An SHES and low Al component barriers were introduced in to AlGaN/GaN RTDs to improve the electronic injection efficiency in to the emitter, reduce the transit time in the collector depletion region, and achieve lattice matching. The substitution of the emitter spacer for the SHES alters the dominant transport mechanism, increases the tunneling current, and restrains the thermionic current. As a result, the peak current was 1.683 A at 0.39 V and the peak-to-valley current difference was 0.93 A. [ABSTRACT FROM PUBLISHER]

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. [ABSTRACT FROM PUBLISHER]