Resonant Tunneling Diodes Research Articles

Fabrication of sub-micrometer 3D structures for terahertz oscillators by electron beam gray-tone lithography

More and more novel applications are appearing in the almost-unexplored-up-to-recent-times frequency range of around 0.3–3 terahertz (THz), where sub-millimeter radio waves meet far-infrared optical waves. Resonant tunneling diodes (RTDs) are considered one of the promising compact and coherent room temperature signal sources for terahertz applications. In this work, the fabrication process and fabrication challenges for an RTD THz oscillator with a cylindrical resonant cavity are discussed. Successful fabrication of 3D metallic structures with a height of 2–5 μm and a feature size down to 0.5 μm was achieved by combining the traditional trilayer resist process with the dose-modulated (gray-tone) electron-beam (EB) lithography process. It was shown that two-step EB exposure could be used in thick (>2.4 μm) PMMA resist to achieve predictable and controllable fabrication of V-shaped metallic structures with lateral sizes down to 0.5 μm. Applicability of the described fabrication approach was proven by the measurement of oscillation characteristics for the fabricated RTD THz oscillators. Successful operation of the RTD oscillator devices confirms good electrical contact between the top contact of the RTD mesa structure and the RTD pillar structure as well as between the resonant cavity and antenna parts. The fabrication approach described in this work allowed us to eliminate parasitic capacitance formed around RTD mesa in the first fabrication trial and achieve a frequency increase of up to 200 GHz for RTD THz oscillators operating at frequencies 1.5–1.7 THz. The described fabrication approach may also be applicable for the fabrication of 3D metallic structures with a feature size less than 0.5 μm and a height more than 2 μm with EB energies above 50 keV.

Transformation of polarons into magnetopolarons in GaAs quantum well

An experimental study is made of electron tunneling in a resonant-tunneling diode in magnetic fields directed parallel and perpendicular to the planes of the GaAs/AlXGa1−XAs heterostructure layers. In particular, phonon replicas on the current–voltage characteristics of the diode are investigated. In the second current derivatives a fine structure of replicas is found. The transformation of the structure of replicas in a perpendicular magnetic field can be qualitatively understood as a manifestation of the transition of polaron states to magnetopolaronic ones.

Modeling of the Degradation of Resonant-Tunneling Diodes Using Artificial Neural Networks

Modeling of the Degradation of Resonant-Tunneling Diodes Using Artificial Neural Networks

Tuning transport properties for 1D and 2D ψ-Graphene

In this work, we built two equivalent molecular devices via DFT/NEGF replicating the 2D and 1D ψ-Graphene unit cell composed by C12 and C24H6. Our results exhibit that the hydrogenation at the edge for 1D ψ-Graphene makes energetically stable, more structurally stable and reduces driving on device signature between 0 V and 0.25 V (Switching) and 0.25 V-0.5 V (resonant tunnel diode - RTD) while the 2D device has RTD signature. The density of states and transmission eingenchannels confirm metallic behavior and semiconductor–metal transition for two devices. Thenegative differential resistance (NDR) is identified in the two devices by transition voltage spectroscopy. Our discoveries make the1D ψ-Graphenehighly promising in nanoelectronics applications for tuning transport properties.

PAM4 48-Gbit/s wireless communication using a resonant tunneling diode in the 300-GHz band

In modern optical communications, pulse amplitude modulation 4 (PAM4) is employed to achieve higher data rates than that achieved by conventional non-return-to-zero format. Meanwhile, there is an increasing interest to convert wired connections to wireless in data centers using high-speed millimeter-wave and terahertz (THz) links. Here, we introduced the PAM4 modulation for THz wireless communications using a resonant tunneling diode (RTD) receiver. Compared with a Schottky-barrier diode receiver, the RTD receiver has higher sensitivity, and a stronger nonlinearity at low input power when it is operated with an amplified detection scheme. We achieved 24-Gbaud (48-Gbit/s) transmission in the 300-GHz band with a quasi-real-time digital signal processing (DSP), which is the fastest PAM4 wireless communication without an offline DSP to the best of our knowledge.

Modeling of Resonant Tunneling Diode Oscillators Based on the Time-Domain Boundary Element Method

We demonstrate how the coupling of a full-wave time-domain boundary element method (BEM) solver with a circuit solver can be used to model 1) the generation of high frequency oscillations in resonant tunneling diode (RTD) oscillators, and 2) the mutual coupling and synchronization of non-identical RTDs with significant differences in frequencies to achieve coherent power combination. Numerical simulations show a combined output power of up to 3.7 times a single oscillator in synchronized devices. The non-differential conductance of the RTD is modeled as a lumped component with a non-linear current-voltage relationship. The lumped element is coupled to the radiating structure using a finite-gap model in a consistent and discretisation independent manner. The resulting circuit equations are solved simultaneously and consistently with time-domain electric field integral equations that model the transient scattering of electromagnetic (EM) fields from conducting surfaces that make up the device. This paper introduces three novel elements: (i) the application of a mesh independent feed line to the modelling of feed lines of RTD devices, (ii) the coupling of the radiating system to a strongly non-linear component with negative differential resistance, and (iii) the verification of this model with circuit models where applicable and against the experimental observation of synchronisation when two RTDs are placed in close proximity. These three elements provide a methodology that create the capacity to model RTD sources and related technology.

Analytical Physics-Based Compact Current–Voltage Model for 2D-2D Resonant Tunneling Diodes

In this paper, we develop an analytical model for the resonant current-voltage ( <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I–V)</b> characteristics of 2D-2D Resonant Tunneling Diodes. Starting from the Tsu-Esaki formalism, we consider the overall electrical potential distribution in the device layer structure, including the quantized space charge region in the emitter layer. Additionally, to obtain a more realistic model, we also take into account the scattering experienced by electrons during tunneling process through the double barrier region. These additional features greatly improve the accuracy of the proposed model when compared with others approaches reported in the literature. The device model is fully physics-based, allowing the computation of the <bold xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I-V</b> curve accordingly to the geometry and device structure of the RTD. The model is fully analytical and explicit, thereby well suited for circuit simulator environment. The model is validated against experimental data from distinct RTDs structures, providing excellent agreement.

Ag/SeO2/C Avalanche Type Resonant Tunneling Schottky Barriers

Herein, the design and characterization of Ag/SeO2/C avalanche type resonant tunneling devices are reported. Thin pellets of SeO2 nano-powders pressed under hydraulic pressure of 1.0 MPa which is used as the active material are characterized. They showed tetragonal structure refereeing to space group of P42mbc and lattice parameters of a=b= 7.866 Å and c=5.336 Å. The current-voltage characteristic curves have shown that SeO2 can perform as active media to produce resonant tunneling diodes when forward biased and as avalanche type diode when reverse biased. The peak to valley current ratios of these diodes reached 18.3. In addition, the impedance spectroscopy measurements have shown that the device works in the low impedance mode when operated in the microwave range of frequency near 1.50 GHz. Negative conductance effect is observed in that frequency domain. The features of the Ag/SeO2/C nominate them for use as signal amplifiers and microwave oscillators.

Effect of the Conduction Band Offset on the Performance of GaAs/AlxGa1 – xAs Resonant Tunneling Diode

Effect of the Conduction Band Offset on the Performance of GaAs/AlxGa1 – xAs Resonant Tunneling Diode

Towards Practical Terahertz Imaging System With Compact Continuous Wave Transceiver

Terahertz (THz) imaging techniques have attracted significant attention and have developed rapidly in recent years. However, despite several advances, these techniques are still not mature, and their high cost and system complexity continue to limit their applications. In this article, the techniques for achieving a practical imaging system with a compact THz transceiver are addressed, while considering the limitations of the current technique. The aim is to provide a brief review of related topics, while also covering our recent progress, which can provide some general perspectives and contrasting approaches for realizing a practical THz imaging system. The continuous wave devices are mainly focused for their flexibility of balancing the imaging resolution and data acquisition time. The importance of transceiver integration is also discussed and illustrated by introducing a 600-GHz band micro-photonic interface for integrating a THz source and detector, with a single resonant tunneling diode as a transceiver. With regard to system issues, spatial sampling with mechanical beam-scanning is discussed as an intermediate approach for moving stage and array technology. The potential and limitations of this approach are evaluated, along with an elliptical reflector as an alternative to an f-theta lens owing to its low cost and simplicity. The combination of integrated devices, along with the mechanical beam-scanning, is also discussed for demonstrating our current concept of realizing a practical THz imaging system.

Analysis of output power characteristics for resonant-tunneling diode terahertz oscillator with cylindrical cavity resonator

An increasing number of novel applications has appeared in the previously unexplored frequency range of 0.3–3.0 THz, where sub-mm radio waves meet far-infrared optical waves. Resonant-tunneling diodes (RTDs) are considered as one of the promising compact and coherent room-temperature signal sources for terahertz (THz) applications. In this work, dependencies of output power on the resonator dimensions and output power limitation factors are analyzed for an RTD THz oscillator with a cylindrical cavity resonator, which can oscillate above 2 THz. Analysis of the output power dependencies on radius and height of the cylindrical resonant cavity shows that a decrease in the resonant cavity size could lead to an increase in the output power at a fixed frequency for this type of RTD oscillator. Moreover, in addition to the high-frequency oscillation limit, a rapid decrease in the output power in the lower frequency region was found for oscillator devices with larger RTD mesa areas. Rapid decrease in output power may occur even at frequencies around 1 THz, which could considerably limit the operational range for RTD oscillators with cavity-type resonators. To determine an approach for output power optimization and understand the nature of output power drop at lower frequencies, the output power behavior and connection with resonant cavity parameters were explained in detail. Results of the output power analysis and numerical calculation indicate that for the RTD structure and circular-resonator geometry considered in the present study, output powers up to 45 μW at 1.5 THz and up to 0.25 μW at 2.5 THz could be expected for single oscillator design.

Experimental Demonstration of a Hard-Type Oscillator Using a Resonant Tunneling Diode and Its Comparison with a Soft-Type Oscillator

Experimental Demonstration of a Hard-Type Oscillator Using a Resonant Tunneling Diode and Its Comparison with a Soft-Type Oscillator

Исследование робастности наноэлектронных структур на базе резонансно-туннельных элементов

Multi-input logic gates based on two-level logic cells MOBILE have short (picosecond) switching times and higher functionality due to the ability to implement logic functions with fewer gates. This creates good prospects for the development of ultra-high-speed FPGAs with a high degree of integration, which are required for organizing high-performance computing. However, the extremely high sensitivity of resonant tunneling elements to changes in the energies of quantum states requires an assessment of the stability of such structures to external influences in real operation. In this work, the problem of assessing the stability of nanoelectronic structures that include resonant tunneling elements is considered. The method for studying the robustness of logic cells MOBILE based on a resonant tunneling diode and an НВТ transistor was proposed, making it possible to find an external interval estimate of the output voltage of the device under study for given interval models of the initial components. The technique is based on the use of systems of topological and parametric equations written in finite increments. It was shown that the proposed decomposition principle for the initial interval model ensures the algorithmic solvability of the problem posed. A computational algorithm for calculating processes in a two-level logical cell MOBILE has been developed. The algorithm provides for step-by-step integration of interval differential equations and solution of interval nonlinear algebraic equations at each step of integration using Kaucher interval arithmetic. The obtained results of the study of processes in a two-level logic cell MOBILE create prerequisites for expanding the field of application of resonant tunneling devices in high-speed monolithic integrated circuits.

Performance enhancement of armchair graphene nanoribbon resonant tunneling diode using V-shaped potential well

We study the electron transport in armchair graphene nanoribbon (AGNR) resonant tunneling diode (RTD) using square and V-shaped potential well profiles. We use non-equilibrium Green’s function formalism to analyze the transmission and I–V characteristics. Results show that an enhancement in the peak current (I p ) can be obtained by reducing the well width (W w ) or barrier width (W b ). As W w decreases, I p shifts to a higher peak voltage (V p ), while there is almost no change in V p with decreasing W b . It is gratifying to note that there is an enhancement in I p by about 1.6 times for a V-shaped well over a square well. Furthermore, in the case of a V-shaped well, the negative differential resistance occurs in a shorter voltage range, which may beneficial for ultra-fast switching and high-frequency signal generation. Our work anticipates the suitability of graphene having better design flexibility, to develop ideally 2D RTDs for use in ultra-dense nano-electronic circuits and systems.

Influence of collector doping setback in the quantum transport characteristics of GaN/AlN resonant tunneling diodes

Harnessing resonant tunneling transport in III-nitride semiconductors to boost the operating frequencies of electronic and photonic devices, requires a thorough understanding of the mechanisms that limit coherent tunneling injection. Towards this goal, we present a concerted experimental and theoretical study that elucidates the impact of the collector doping setback on the quantum transport characteristics of GaN/AlN resonant tunneling diodes (RTDs). Employing our analytical model for polar RTDs, we quantify the width of the resonant-tunneling line shape, demonstrating that the setback helps preserve coherent injection. This design results in consistently higher peak-to-valley-current ratios (PVCRs), obtaining a maximum PVCR = 2.01 at cryogenic temperatures.

Characteristics of Series‐Connected Resonant Tunneling Diode‐Pair Configuration for High Output Power Operation

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.

Numerical constraints and non-spatial open boundary conditions for the Wigner equation

We discuss boundary value problems for the characteristic stationary von Neumann equation (stationary sigma equation) and the stationary Wigner equation in a single spatial dimension. The two equations are related by a Fourier transform in the non-spatial coordinate. In general, a solution to the characteristic equation does not produce a corresponding Wigner solution as the Fourier transform will not exist. Solution of the stationary Wigner equation on a shifted k-grid gives unphysical results. Results showing a negative differential resistance in IV-curves of resonant tunneling diodes using Frensley’s method are a numerical artefact from using upwinding on a coarse grid. We introduce the integro-differential sigma equation which avoids distributional parts at k=0 in the Wigner transform. The Wigner equation for k=0 represents an algebraic constraint needed to avoid poles in the solution at k=0. We impose the inverse Fourier transform of the integrability constraint in the integro-differential sigma equation. After a cutoff, we find that this gives fully homogeneous boundary conditions in the non-spatial coordinate which is overdetermined. Employing an absorbing potential layer double homogeneous boundary conditions are naturally fulfilled. Simulation results for resonant tunneling diodes from solving the constrained sigma equation in the least squares sense with an absorbing potential reproduce results from the quantum transmitting boundary with high accuracy. We discuss the zero bias case where also good agreement is found. In conclusion, we argue that properly formulated open boundary conditions have to be imposed on non-spatial boundaries in the sigma equation both in the stationary and the transient case. When solving the Wigner equation, an absorbing potential layer has to be employed.

Electroluminescence in Unipolar-Doped In0.53Ga0.47As/AlAs Resonant-Tunneling Diodes: A Competition between Interband Tunneling and Impact Ionization

We measure and analyze the light emission from a room-temperature n-type unipolar-doped ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}/\mathrm{Al}\mathrm{As}$ double-barrier resonant-tunneling diode (RTD) that occurs just above the ${\mathrm{In}}_{0.53}{\mathrm{Ga}}_{0.47}\mathrm{As}$ band edge and peaks around 1631 nm. The emission is attributed to electron-hole recombination emission made possible by holes generated in the high-field region on the collector side of the device by interband tunneling and impact ionization, which contribute comparable hole densities, according to our analysis. Although the external quantum efficiency (EQE) in our experimental configuration is rather low (\ensuremath{\approx}2 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}5}$ at 3.0-V bias), limited by suboptimal output coupling, the internal quantum efficiency (IQE) is much higher (\ensuremath{\approx}6% at 3.0-V bias), as derived from the experimental EQE and a radiometric analysis. To check this value and better understand the transport physics, we also carry out an independent estimate of the IQE using a combined interband-tunneling impact-ionization transport model, which yields an IQE of 10% at 3.0-V bias. The satisfactory agreement of theory with experimental data suggests that a RTD designed for better hole transport and superior optical coupling could become a useful light-emitting device, while retaining the intrinsic functionality of high-speed negative differential resistance, and all without the need for resistive p-type doping.

Room temperature asymmetric negative differential resistance characteristics of AlGaN/GaN resonant tunneling diodes grown by metal-organic chemical vapor deposition

A large area AlGaN/GaN resonant tunneling diode (RTD) grown by metal–organic chemical vapor deposition (MOCVD) is manufactured. The device adopts air-bridge structure suitable for the terahertz (THz) band, and a commercially available high-frequency GaN technology is applied in the process of device fabrication. Negative differential resistance (NDR) characteristics of the device are observed under the bipolar bias voltage at room temperature, which exhibits clear anti-symmetry in the peak-to-valley current ratio (PVCR), peak voltage and peak current density. The work systematically investigates how the built-in electric field introduced by polarization effects modulates the resonant tunneling transport utilizing TCAD tool Silvaco-ATLAS, and analyzes the enhancement of the resonant tunneling under negative bias voltage of the RTD with low-aluminum content AlGaN as barriers. The AlGaN/GaN RTD with conventional double-barrier single (DBS) quantum well (QW) structure shows reproducible NDR characteristics under the bipolar bias voltage at room temperature, which facilitates the design and extensive study of complicated device structures.

Simplified description of dynamics in neuromorphic resonant tunneling diodes.

In this article, the standard theoretical model accounting for a double barrier quantum well resonant tunneling diode (RTD) connected to a direct current source of voltage is simplified by representing its current-voltage characteristic with an analytically approachable, anti-symmetric N-shaped function. The time and variables involved are also transformed to reduce the number of parameters in the model. Responses observed in previous, more physically accurate studies are reproduced, including slow-fast dynamics, excitability, and bistability, relevant for spiking signal processing. A simple expression for the refractory time of the excitable response is derived and shown to be in good agreement with numerical simulations. In particular, the refractory time is found to be directly proportional to the circuit's intrinsic inductance. The presence or absence of bistability in the dependence of the parameters is also discussed thoroughly. The results of this work can serve as a guideline in prospective endeavors to design and fabricate RTD-based neuromorphic circuits for power and time-efficient execution of neural network algorithms.