Resonant Tunneling Diodes Research Articles

Implementation of Flip-Chip Microbump Bonding between InP and SiC Substrates for Millimeter-Wave Applications.

Flip-chip microbump (μ-bump) bonding technology between indium phosphide (InP) and silicon carbide (SiC) substrates for a millimeter-wave (mmW) wireless communication application is demonstrated. The proposed process of flip-chip μ-bump bonding to achieve high-yield performance utilizes a SiO2-based dielectric passivation process, a sputtering-based pad metallization process, an electroplating (EP) bump process enabling a flat-top μ-bump shape, a dicing process without the peeling of the dielectric layer, and a SnAg-to-Au solder bonding process. By using the bonding process, 10 mm long InP-to-SiC coplanar waveguide (CPW) lines with 10 daisy chains interconnected with a hundred μ-bumps are fabricated. All twelve InP-to-SiC CPW lines placed on two samples, one of which has an area of approximately 11 × 10 mm2, show uniform performance with insertion loss deviation within ±10% along with an average insertion loss of 0.25 dB/mm, while achieving return losses of more than 15 dB at a frequency of 30 GHz, which are comparable to insertion loss values of previously reported conventional CPW lines. In addition, an InP-to-SiC resonant tunneling diode device is fabricated for the first time and its DC and RF characteristics are investigated.

Deep-Level Defects Induced Degradation of Negative Differential Resistance in GaN-Based Resonant Tunneling Diodes

The degradation of the negative differential resistance (NDR) of GaN-based resonant tunneling diodes (RTDs) is a primary factor that limits their development into terahertz light source devices. In this work, on the basis of experiments and first-principles calculations, we proposed that deep-level defects in epitaxial materials can significantly affect the NDR of GaN-based RTDs. According to a low-frequency noise test, we confirmed the existence of 0.21 and 0.54 eV deep-level defects at the active region tunneling interface. The first-principles study of defect calculation based on the mixed functional theory reveals that the two deep-level defects originate from the interstitial defect Ni and the defect pair VN-NGa of the GaN side in the tunneling interface, respectively. In addition, AFM and TEM results indicate that threading dislocations are not the main factor fueling NDR degradation in the prepared RTDs. Our work suggests that the elimination of deep-level defects is the key to obtaining steady and reliable NDR in GaN-based RTDs.

Passive mode-locking and terahertz frequency comb generation in resonant-tunneling-diode oscillator

Optical frequency combs in the terahertz frequency range are long-awaited frequency standards for spectroscopy of molecules and high-speed wireless communications. However, a terahertz frequency comb based on a low-cost, energy-efficient, and room-temperature-operating device remains unavailable especially in the frequency range of 0.1 to 3 THz. In this paper, we show that the resonant-tunneling-diode (RTD) oscillator can be passively mode-locked by optical feedback and generate a terahertz frequency comb. The standard deviation of the spacing between the comb lines, i.e., the repetition frequency, is reduced to less than 420 mHz by applying external bias modulation. A simulation model successfully reproduces the mode-locking behavior by including the nonlinear capacitance of RTD and multiple optical feedback. Since the mode-locked RTD oscillator is a simple semiconductor device that operates at room temperature and covers the frequency range of 0.1 to 2 THz (potentially up to 3 THz), it can be used as a frequency standard for future terahertz sensing and wireless communications.

Spike propagation in a nanolaser-based optoelectronic neuron

With the recent development of artificial intelligence and deep neural networks, alternatives to the Von Neumann architecture are in demand to run these algorithms efficiently in terms of speed, power and component size. In this theoretical study, a neuromorphic, optoelectronic nanopillar metal-cavity consisting of a resonant tunneling diode (RTD) and a nanolaser diode (LD) is demonstrated as an excitable pulse generator. With the proper configuration, the RTD behaves as an excitable system while the LD translates its electronic output into optical pulses, which can be interpreted as bits of information. The optical pulses are characterized in terms of their width, amplitude, response delay, distortion and jitter times. Finally, two RTD-LD units are integrated via a photodetector and their feasibility to generate and propagate optical pulses is demonstrated. Given its low energy consumption per pulse and high spiking rate, this device has potential applications as building blocks in neuromorphic processors and spiking neural networks.

Design and Technological Optimization of Electrical Parameters of the Wideband Balanced Microwave Frequency Mixer Based on Resonant-Tunneling Diodes

Design and Technological Optimization of Electrical Parameters of the Wideband Balanced Microwave Frequency Mixer Based on Resonant-Tunneling Diodes

Mathematical model of a frequency pressure transducer based on a resonant tunneling diode

The paper presents the design of pressure sensor with a frequency output signal that is based on the physical processes in a resonant tunnel diode under the action of pressure. The use of devices with negative differential resistance can significantly simplify the design of pressure sensors in the entire radio frequency range. Depending on the operating modes of the sensor, an output signal can be obtained in the form of harmonic oscillations. Pressure sensor characteristics researches based on complete equivalent circuit diagram resonant tunnel diode, which takes into account its capacitive and inductive properties. A mathematical model of the pressure sensor was developed, upon which the analytical dependences of the change in the elements tunnel resonance diode from pressure have been determined, as well as conversion functions and sensor sensitivity. It is shown that the main contribution to changes in the conversion function and sensor sensitivity is made by the change in the negative differential resistance with the change in pressure. This, in turn, results in different readings of the instrument's output frequency. The sensitivity of the sensor varied from 1.15 kHz/Pa·105 to 14.16 kHz/Pa·105 in the pressure range from 50·105 Pa up to 350·105 Pa.

Study of Electronic and Transport Properties in Double-Barrier Resonant Tunneling Systems.

Resonant tunneling devices are still under study today due to their multiple applications in optoelectronics or logic circuits. In this work, we review an out-of-equilibrium GaAs/AlGaAs double-barrier resonant tunneling diode system, including the effect of donor density and external potentials in a self-consistent way. The calculation method uses the finite-element approach and the Landauer formalism. Quasi-stationary states, transmission probability, current density, cut-off frequency, and conductance are discussed considering variations in the donor density and the width of the central well. For all arrangements, the appearance of negative differential resistance (NDR) is evident, which is a fundamental characteristic of practical applications in devices. Finally, a comparison of the simulation with an experimental double-barrier system based on InGaAs with AlAs barriers reported in the literature has been obtained, evidencing the position and magnitude of the resonance peak in the current correctly.

Hybrid Integration Between Resonant Tunneling Diodes and Unclad Microphotonic Diplexer for Dual-Channel Coherent Terahertz Receiver

Recent advances in substrateless all-silicon microphotonics are leading towards a much-needed general-purpose platform for compact integrated devices and systems for terahertz waves. Devices of this sort are innately efficient, as high-resistivity intrinsic silicon exhibits near-negligible absorption in the terahertz range. On the other hand, this platform offers no capacity or potential to directly implement active devices such as sources and detectors of terahertz signals. Hybrid integration presents a viable means to address this shortcoming, by incorporating a separately-fabricated InP integrated circuit together with silicon microphotonics. In this work, we present a two-channel coherent receiver for terahertz waves that is composed of a pair of self-oscillating-mixer-based resonant tunneling diode (RTD) detector chips and a silicon photonic diplexer. This device is deployed in terahertz-range communications experiments, where it is shown that aggregate data rates in excess of 50 Gbit/s may be supported. Finally, a viable approach to physically package the hybrid-integrated terahertz receiver, and electrically interface the RTDs with standard coaxial ports, is presented.

Resonant Tunneling Diodes High-Speed Terahertz Wireless Communications - A Review

Resonant tunneling diode (RTD) technology is emerging as one of the promising semiconductor-based solid-state technologies for terahertz (THz) wireless communications. This article provides a review of the state of the art, with a focus on the THz RTD oscillator, which is the key component of RTD-based THz transmitters and coherent receivers. A brief summary on the device principle of operation, technology, modeling, as well as an overview of oscillator design and implementation approaches for THz emitters, is provided. A new insight to device evaluation and to the reported oscillator performance levels is also given, together with brief remarks on RTD-based THz detectors. Thereafter, an overview of the reported wireless links which utilize an RTD in either transmission or reception, or in both roles, is given. Highlight results include the record single-channel wireless data rate of 56 Gb/s employing an all RTD-based transceiver, which demonstrates the potential of the technology for future short-range communications. The article concludes with a discussion of the current technical challenges and possible strategies for future progress.

Cylindrical Cavity Optimization for Resonant-Tunneling Diode Oscillators

In this work, the theoretical shapes of cavities were investigated as suited for resonant-tunneling diode (RTD) lumped-element oscillators to achieve the highest possible operational frequency. The scope of this work is restricted to cylindrical cavities that are in their dimensions much smaller or similar to the free-space wavelength. The idea is to find a shape that has the highest quality factor possible. The proposed analysis is based on functional variation principles coupled with electric-field integral equation (EFIE) method of moments solver. Simple design parameters were found covering the cavities that are much smaller than the free-space wavelength. The maximum reachable frequency was calculated with these types of new cavities using some theoretical RTD parameters. The theoretical limitation of frequency was also defined for given RTD parameters and fabrication limitations originating from the contact resistance and RTD radius. In order to demonstrate the significance of the findings, a theoretical geometry was compared with a practical resonator shape.

Physics Based RTD Model Accounting for Space Charge and Phonon Scattering Effects

This paper presents a fully analytical model for the current-voltage (I–V) characteristics of Resonant Tunneling Diodes. Based on Tsu-Esaki formalism, we consider the full electrical potential distribution in the structure, including the space charge regions at the emitter and collector layers. In addition, we account for the scattering suffered by carriers when tunneling through the double-barrier region, as a function of the applied bias voltage. These considerations improve the accuracy of the proposed model when compared with other approaches while keeping it physics based and fully analytical. Finally, the model is validated with experimental and numericaldata, demonstrating its feasibility for applications in circuit simulation environments.

Statistical device simulations of III-V nanowire resonant tunneling diodes as physical unclonable functions source

In this paper, utilising the non-equilibrium Green’s function (NEGF) formalism within the new device simulator NESS (Nano-Electronic Software Simulator) developed at the University of Glasgow’s Device Modelling Group, we present quantum mechanical simulations of current flow in double-barrier III-V GaAs-AlGaAs nanowire resonant tunneling diodes (RTDs). NESS is a fast and modular Technology Computer Aided Design (TCAD) tool with flexible architecture which can take into account various sources of statistical variability in nanodevices. The aim of this work is to show that, in the RTD devices with nano-scale dimensions, there is a direct correlation between the position and the numbers of random dopants and the key device parameters, e.g., position of the resonant-peak (VR) variations as well as the shape and number of peaks in the output current–voltage (I-V) characteristics. Such VR variability can be used as a quantum fingerprint which can provide robust security and hence can be used to deliver Physical Unclonable Functions (PUFs).

Hydrogenation, width and strain effect in Me-graphene devices

Recently, Me-Graphene (Me-G) was proposed as a graphene (G) allotrope with sp2-sp3 hybridization, near zero Poisson’s ratio, sizeable band gap, and high carrier mobility. In this work, we replicate three partially hydrogenated Me-Graphene NanoRibbon (Me-GNR) relaxed unit cells (C13H4, C26H4 and C39H4) of different widths (Me-GNR1, Me-GNR2 and Me-GNR3) and investigate how these molecular devices behave under strain (compression and distension) via DFT/NEGF. Our results show that edge hydrogenation present: (i) metallic behavior for three devices under equilibrium or strain through band structure (BS) and density of states (DOS) at Ve = 0 V which is confirmed by charge density and liquid charge; (ii) negative differential resistance (NDR) for three devices under compression and plateau (resonance) to six other devices; (iii) larger widths increase current and ohmic region; (iv) Molecular Projected Self-Consistent Hamiltonian (MPSH) states that participate of electronic transport are from HOMO − 1 up to LUMO + 1; (v) how applied external bias voltage in the device (Ve) affects energy levels distribution through projected density of state (PDOS) and transmittance. These results make Me-GNR be highly promising in nanoelectronics applications such as resonant tunnel diode (RTD) and field effect transistors (FET).

An Overview of Terahertz Imaging with Resonant Tunneling Diodes

Terahertz (THz) imaging is a rapidly growing application motivated by industrial demands including harmless (non-ionizing) security imaging, multilayer paint quality control within the automotive industry, insulating foam non-invasive testing in aerospace, and biomedical diagnostics. One of the key components in the imaging system is the source and detector. This paper gives a brief overview of room temperature THz transceiver technology for imaging applications based on the emerging resonant tunneling diode (RTD) devices. The reported results demonstrate that RTD technology is a very promising candidate to realize compact, low-cost THz imaging systems.

Influence of the active region structure of the resonant tunneling diode on the critical points of its current-voltage characteristic

The article proposes a study of the effect of the structure of the active region of a resonant tunneling diode on the critical points of its current-voltage characteristic. The basic configuration of a resonant tunneling diode, which is a structure of a quantum well with a nanometer-sized double barrier including two contacts, and a region with strongly doped contacts made of a semiconductor with a relatively small band gap, is disclosed and illustrated. It is emphasized that since the characteristic dimensions of the structure of the quantum well with a double barrier are comparable to the wavelengths of electrons, the wave nature of electrons leads to such quantum phenomena as interference, tunneling, energy quantization, etc. the double barrier causes resonant tunneling phenomena, which form the basis for the operation of the resonant-tunneling diode. It is emphasized that repeated reflection causes destructive or constructive interference depending on the wavelength of a particular electron. For electrons with a certain wavelength that promotes constructive interference, a transfer probability close to unity can be found at energies corresponding to these wavelengths. The modification of the active region of the resonant tunnel diode with a barrier height of 0.3 - 0.4 eV is mathematically substantiated. The dependence of the transmission coefficient is found by solving the Schrödinger equation in one electron approximation without taking into account the scattering effects. The calculation of the volt-ampere characteristic of the resonant-tunnel diode was performed at temperatures of 100 and 300 K. The given volt-ampere characteristics were obtained without taking into account the effects of electron scattering. However, it should be noted that the main influencing factor is the resonant tunneling through the second level, for which the peak of the transmission coefficient is much wider and higher. However, in gallium doped arsenide, the fact of electron scattering can significantly affect the value of the transmission coefficient and the value of current. It is established that an increase in the width of quantum wells leads to a significant decrease in the densities of peak currents and valley currents, and an increase in the width of potential barriers leads to a slight decrease in the current density of the first peak and current densities of the second peak and valley.

Resonant Tunneling Diodes: Mid-Infrared Sensing at Room Temperature.

Resonant tunneling diode photodetectors appear to be promising architectures with a simple design for mid-infrared sensing operations at room temperature. We fabricated resonant tunneling devices with GaInAsSb absorbers that allow operation in the μm range with significant electrical responsivity of 0.97 A/W at 2004 nm to optical readout. This paper characterizes the photosensor response contrasting different operational regimes and offering a comprehensive theoretical analysis of the main physical ingredients that rule the sensor functionalities and affect its performance. We demonstrate how the drift, accumulation, and escape efficiencies of photogenerated carriers influence the electrostatic modulation of the sensor’s electrical response and how they allow controlling the device’s sensing abilities.

Structure dependence of oscillation characteristics of structure-simplified resonant-tunneling-diode terahertz oscillator

A structure-simplified resonant tunneling diode (RTD) oscillator eliminates metal–insulator–metal capacitors, resulting in a simple and brief fabrication process. However, the structure dependence of oscillation characteristics has not been identified. We revealed the structure dependence and obtained an oscillation frequency of up to ∼1 THz using a short antenna. We found that an increase in radiation conductance using offset-fed structure combined with coplanar stripline antennas is effective for high output power, and achieved up to ∼220 μW of output power at 500 GHz. We also clarified the dependence of the oscillation frequency on the stabilization resistor.

Analytical Modeling and Band-Structure Engineering of Resonant Tunneling Diode for Biased Terahertz Detector

A new strategy to significantly enhance the nonlinearity of double-barrier resonant tunneling diode (RTD) in the positive differential resistance region is theoretically proposed and experimentally demonstrated. An analytical model containing physical insight into the resonant tunneling process is established to bridge the nonlinearity of the current–voltage curve and the band-structure information of RTD. It intuitively shows that the intrinsic current responsivity of RTD as well as its nonlinearity can be greatly improved via band-structure engineering, as enhancing the resonant energy level of the well and reducing the current before the resonant tunneling process. The validation of the proposed model and the associated band-structure engineering strategy are verified by both device simulation and experiments. A RTD device with intrinsic current responsivity of 6.74 A/W is realized, while the peak-to-valley current ratio (PVCR) is also as large as 8.92. This method provides a new pathway to design high-performance terahertz (THz) detectors using double-barrier RTD and may share new perspective on the potential of RTD-based THz transceivers.

Resonant Tunneling Diode Nano-Optoelectronic Excitable Nodes for Neuromorphic Spike-Based Information Processing

In this work, we introduce an interconnected nano-optoelectronic spiking artificial neuron emitter-receiver system capable of operating at ultrafast rates (about $100\phantom{\rule{0.2em}{0ex}}\mathrm{ps}/$optical spike) and with low-energy consumption ( pJ/spike). The proposed system combines an excitable resonant tunneling diode (RTD) element exhibiting negative differential conductance, coupled to a nanoscale light source (forming a master node) or a photodetector (forming a receiver node). We study numerically the spiking dynamical responses and information propagation functionality of an interconnected master-receiver RTD node system. Using the key functionality of pulse thresholding and integration, we utilize a single node to classify sequential pulse patterns and perform convolutional functionality for image feature (edge) recognition. We also demonstrate an optically interconnected spiking neural network model for processing of spatiotemporal data at over 10 Gbit/s with high inference accuracy. Finally, we demonstrate an off-chip supervised learning approach utilizing spike-timing-dependent plasticity for the RTD-enabled photonic spiking neural network. These results demonstrate the potential and viability of RTD spiking nodes for low footprint, low-energy, high-speed optoelectronic realization of spike-based neuromorphic hardware.

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.