Resonant Tunneling Diodes Research Articles

Quantum Transport Simulation of Nanowire Resonant Tunneling Diodes Based on a Wigner Function Model With Spatially Dependent Effective Masses

We present a self-consistent and three-dimensional quantum simulation for nanowire resonant tunneling diodes (RTDs) based on the Wigner transport equation with spatially dependent effective masses (SDEM), which is discretized by a third-order upwind difference scheme for high-accuracy calculation. Our calculation shows that the current density/voltage ( $J\text{--}V$ ) characteristics of nanowire RTDs with a radius ( $R$ ) $ 5 nm largely deviate from those of the one-dimensional RTDs. As $R$ decreases below 5 nm, the peak-to-valley ratio (PVR) of the low- $V$ negative differential resistance (NDR) decreases rapidly, and the high- $V$ NDR peak shifts sensitively to lower $V$ and becomes sharper. In this study, these results are shown to be closely related to the SDEM. According to our analysis, the SDEM along the transport direction contributes to the formation of an ‘effective double barrier’ in the energy subbands that is higher than the double barrier expected by the band offset, and the SDEM along the confinement direction reduces the height of the effective double barrier. The performance of nanowire RTDs with $R 5 nm was proven to be highly dependent on $R$ because of the SDEM along the confinement direction.

Degradation study of AlAs/GaAs resonant tunneling diode IV curves under influence of high temperatures

In the present work the thermal degradation of IV curves of AlAs/GaAs resonant tunneling diodes using artificial aging method was investigated. The dependency of AuGeNi specific ohmic contact resistance on time and temperature was determined.

Delay dynamics of neuromorphic optoelectronic nanoscale resonators: Perspectives and applications

With the recent exponential growth of applications using artificial intelligence (AI), the development of efficient and ultrafast brain-like (neuromorphic) systems is crucial for future information and communication technologies. While the implementation of AI systems using computer algorithms of neural networks is emerging rapidly, scientists are just taking the very first steps in the development of the hardware elements of an artificial brain, specifically neuromorphic microchips. In this review article, we present the current state of the art of neuromorphic photonic circuits based on solid-state optoelectronic oscillators formed by nanoscale double barrier quantum well resonant tunneling diodes. We address, both experimentally and theoretically, the key dynamic properties of recently developed artificial solid-state neuron microchips with delayed perturbations and describe their role in the study of neural activity and regenerative memory. This review covers our recent research work on excitable and delay dynamic characteristics of both single and autaptic (delayed) artificial neurons including all-or-none response, spike-based data encoding, storage, signal regeneration and signal healing. Furthermore, the neural responses of these neuromorphic microchips display all the signatures of extended spatio-temporal localized structures (LSs) of light, which are reviewed here in detail. By taking advantage of the dissipative nature of LSs, we demonstrate potential applications in optical data reconfiguration and clock and timing at high-speeds and with short transients. The results reviewed in this article are a key enabler for the development of high-performance optoelectronic devices in future high-speed brain-inspired optical memories and neuromorphic computing.

Full‐wave analysis of traveling pulses developed in a system of transmission lines with regularly spaced resonant‐tunneling diodes

SummaryIn 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.

New Tunneling Features in Polar III-Nitride Resonant Tunneling Diodes

Double barrier GaN/AlN resonant tunneling heterostructures have been grown by molecular beam epitaxy on the (0001) plane of commercially available bulk GaN substrates. Resonant tunneling diodes were fabricated; room temperature current-voltage measurements reveal the presence of a negative differential conductance region under forward bias with peak current densities of ~6.4 $kA/cm^2$ and a peak to valley current ratio of ~1.3. Reverse bias operation presents a characteristic turn-on threshold voltage intimately linked to the polarization fields present in the heterostructure. An analytic electrostatic model is developed to capture the unique features of polar-heterostructure-based resonant tunneling diodes; both the resonant and threshold voltages are derived as a function of the design parameters and polarization fields. Subsequent measurements confirm the repeatability of the negative conductance and demonstrate that III-nitride tunneling heterostructures are capable of robust resonant transport at room temperature.

GaSb/AlAsSb resonant tunneling diodes with GaAsSb emitter prewells

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=4.2 K, 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.

Temperature tuning from direct to inverted bistable electroluminescence in resonant tunneling diodes

We study the electroluminescence (EL) emission of purely n-doped resonant tunneling diodes in a wide temperature range. The paper demonstrates that the EL originates from impact ionization and radiative recombination in the extended collector region of the tunneling device. Bistable current-voltage response and EL are detected and their respective high and low states are tuned under varying temperature. The bistability of the EL intensity can be switched from direct to inverted with respect to the tunneling current and the optical on/off ratio can be enhanced with increasing temperature. One order of magnitude amplification of the optical on/off ratio can be attained compared to the electrical one. Our observation can be explained by an interplay of moderate peak-to-valley current ratios, large resonance voltages, and electron energy loss mechanisms, and thus, could be applied as an alternative route towards optoelectronic applications of tunneling devices.

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

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

Valley current characterization of high current density resonant tunnelling diodes for terahertz-wave applications

We report valley current characterisation of high current density InGaAs/AlAs/InP resonant tunnelling diodes (RTDs) grown by metal-organic vapour phase epitaxy (MOVPE) for THz emission, with a view to investigate the origin of the valley current and optimize device performance. By applying a dual-pass fabrication technique, we are able to measure the RTD I-V characteristic for different perimeter/area ratios, which uniquely allows us to investigate the contribution of leakage current to the valley current and its effect on the PVCR from a single device. Temperature dependent (20 – 300 K) characteristics for a device are critically analysed and the effect of temperature on the maximum extractable power (PMAX) and the negative differential conductance (NDC) of the device is investigated. By performing theoretical modelling, we are able to explore the effect of typical variations in structural composition during the growth process on the tunnelling properties of the device, and hence the device performance.

Strain-Induced Armchair Graphene Nanoribbon Resonant-Tunneling Diodes

The electronic properties of armchair graphene nanoribbons (AGNRs) can be changed and modified under the uniaxial strain. Taking this advantage, we propose a new platform of AGNR-based resonant-tunneling diode (RTD) using the effects of strain for the first time. In this RTD platform, barrier regions are composed of strained AGNR, whereas channel is made up by pristine AGNR. The calculated results show that the double barrier quantum well is performed for such device, and negative differential resistance property appears in ${I}$ – ${V}$ characteristic. In addition, performance of strain-induced 12-AGNR-RTD is explored under the variation of strain percentage ( $\varepsilon $ ). It is realized that the efficiency of such devices consist of peak to valley ratio can be engineered by setting strain percentage ( $\varepsilon $ ) to appropriate orders. Tight binding model coupled with nonequilibrium Green’s function formalism is derived for this paper.

Photocontrolled Terahertz Amplified Modulator via Plasma Wave Excitation in ORTD-Gated HEMTs

This paper theoretically explores photocontrolled terahertz amplified modulator in electron plasma wave optically switched resonant tunneling diode (ORTD) gate high-electron mobility transistor. We present a developed distributed circuit model based on the Khmyrova model. Photoexcitation causes ORTD operating state between negative differential conductivity and positive differential conductivity, which power gain of the device can be control by photoexcitation. Numerical and analytical results show that photoexcitation enabling amplified modulator can realize much larger modulation depth ( > 95%) than what has been reported in photocontrolled modulator. Our results show the potential of this device in several fields of terahertz technology, such as photocontrolled modulator, mixer, and other two port networks.

Characterization of a self‐aligned RTD using a SiNx sidewall process for high‐speed applications

AbstractIn 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.

Spectral Narrowing of a Varactor-Integrated Resonant-Tunneling-Diode Terahertz Oscillator by Phase-Locked Loop

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.

Design and fabrication of low power GaAs/AlAs resonant tunneling diodes

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 0.28V (0.53V) in forward (reverse) direction. The peak current density in forward bias is around 15.4kA/cm2 with variation of within 7%. As for reverse bias, the peak current density is around 22.8kA/cm2 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.

Graphene-based vertical-junction diodes and applications

In the last decade, graphene has received extreme attention as an intriguing building block for electronic and photonic device applications. This paper provides an overview of recent progress in the study of vertical-junction diodes based on graphene and its hybrid systems by combination of graphene and other materials. The review is especially focused on tunnelling and Schottky diodes produced by chemical doping of graphene or combination of graphene with various semiconducting/ insulating materials such as hexagonal boron nitrides, Si-quantum-dots-embedded SiO2 multilayers, Si wafers, compound semiconductors, Si nanowires, and porous Si. The uniqueness of graphene enables the application of these convergence structures in high-efficient devices including photodetectors, solar cells, resonant tunnelling diodes, and molecular/DNA sensors.

Efficient solution of the Wigner–Liouville equation using a spectral decomposition of the force field

The Wigner–Liouville equation is reformulated using a spectral decomposition of the classical force field instead of the potential energy. The latter is shown to simplify the Wigner–Liouville kernel both conceptually and numerically as the spectral force Wigner–Liouville equation avoids the numerical evaluation of the highly oscillatory Wigner kernel which is nonlocal in both position and momentum. The quantum mechanical evolution is instead governed by a term local in space and non-local in momentum, where the non-locality in momentum has only a limited range. An interpretation of the time evolution in terms of two processes is presented; a classical evolution under the influence of the averaged driving field, and a probability-preserving quantum-mechanical generation and annihilation term. Using the inherent stability and reduced complexity, a direct deterministic numerical implementation using Chebyshev and Fourier pseudo-spectral methods is detailed. For the purpose of illustration, we present results for the time-evolution of a one-dimensional resonant tunneling diode driven out of equilibrium.

Transport in quantum dots resonant tunneling diodes in non-interacting regime

Transport in quantum dots resonant tunneling diodes in non-interacting regime

Mapping of electromagnetic waves generated by free-running self-oscillating devices

Near-field mapping has proven to be a powerful technique for characterizing and diagnosing antennas in the microwave frequency range. However, conventional measurement methods based on a network analyzer cannot be applied to on-chip antenna devices extensively studied for future wireless communication in the millimeter wave (mm-wave) (30–300 GHz) and terahertz (THz) wave (0.1–10 THz) frequency regions. Here, we present a new asynchronous mapping technique to investigate the spatial distribution of not only the amplitude but also the phase of the electric field generated by free-running, self-oscillating generators including CMOS oscillators, Gunn oscillators, resonant tunneling diodes, and quantum cascaded lasers. Using a photonic-electronic hybrid measurement system, a wide frequency coverage, minimal invasiveness of the field to be measured, and phase distribution measurements with a theoretically-limited sensitivity are simultaneously achieved. As a proof-of-concept experiment, we demonstrate the mapping of a mm-wave (77 GHz) generated by a free-running Gunn oscillator and antenna characterization based on near-to-far field transformation.

Resonant Tunneling and Quantum Cascading for Optimum Room-Temperature Generation of THz Signals

We report on the results of a numerical study of quantum transport in ZnSe-based resonant-tunneling diodes (RTDs) and quantum cascade oscillators (QCOs) with fixed and unequal barrier heights. It is found that the negative differential resistance exists up to room temperature in the current–voltage characteristics of the RTD and QCO devices with unequal barrier heights. Further, we demonstrate that QCOs with unequal barrier heights have a better frequency and power performance characteristics compared with RTDs and are more beneficial for high-power terahertz generation at room temperature. For the best QCO device with 100 periods of quantum cascading, a maximum output power of ~7–9 $\mu \text{W}$ for the operating frequency range from 0.1 to ~6 THz at room temperature was achieved.

Understanding the behavior of RTD-loaded NMOS inverter through compact-form analysis

Abstract Resonant-tunneling diodes (RTDs) find a wide variety of applications in oscillators, digital circuits, and latches due to their reduced circuit complexity, low power, and high speed. Since the inverter is the main building block in digital design, in this paper, the analysis of the RTD-loaded NMOS inverter is addressed quantitatively with its static and dynamic characteristics investigated and compared with that of the conventional static CMOS inverter. Specifically, compact-form expressions are derived for the critical points of the voltage-transfer characteristics (VTC), the low-to-high, and the high-to-low propagation delays along with the fan-out. Also, the static- and dynamic–power consumption are investigated and equations are derived for them. The results obtained from this analysis can be generalized to RTD-MOS logic-circuit family in which the pull-down network contains properly connected and appropriately sized NMOS transistors and the pull-up network is the RTD device. The results of the quantitative analysis are discussed and verified by comparison with the simulation results using the Berkeley predictive-technology model (BPTM) of the 45 nm CMOS technology with a power-supply voltage of 1 V.