Resonant Tunneling Diode Oscillator Research Articles

Terahertz Resistor-coupled Arrayed Resonant-tunneling-diode Oscillators with High DC-to-RF Efficiency using Metal-insulator-metal Capacitor

Resistor-coupled arrayed resonant-tunneling-diode (RTD) oscillators have emerged as promising candidates for high-performance Terahertz sources. Despite various advantages, the DC-to-RF efficiency of such oscillators is still low. The reason is the loss caused by resistors, which form the ends of the slot antennas. This paper presents a method to increase the DC-to-RF efficiency of resistor-coupled arrayed oscillators. Edged resistors at the ends of slot antennas are covered by metal-insulator-metal capacitors, which shunt the currents flowing through those resistors. Thus, the conduction loss caused by edged resistors is reduced. Owing to low conduction loss, high output power is obtained, resulting in a high DC-to-RF efficiency. It is estimated that the proposed oscillator’s output power and DC-to-RF efficiency reach 1.53 mW and 0.44% at 450 GHz, respectively. A high power density of up to 62 mW/mm2 is another advantage of the proposed RTD oscillator. With the improved characteristics, we believe the proposed oscillators could promote various Terahertz applications.

Gallium nitride-based resonant tunneling diode oscillators

We demonstrated GaN-based resonant tunneling diode (RTD) oscillators employing monolithic microwave integrated circuits. The GaN-based RTDs with a GaN quantum well and AlN double barriers were grown on freestanding c-plane semi-insulating GaN substrates using metal–organic chemical vapor deposition. The circuit components, including an RTD, a coplanar waveguide, a metal–insulator–metal capacitor, and shunt resistors, were monolithically fabricated on the GaN substrate. The circuits oscillated at a fundamental frequency of 17 GHz, which closely matched an estimated frequency using a three-dimensional electromagnetic simulator and a circuit simulator. This study contributes to the advancement of semiconductor high-frequency devices for millimeter wave and terahertz applications.

High-performance negative differential resistance characteristics in homoepitaxial AlN/GaN double-barrier resonant tunneling diodes

In this work, high-performance negative differential resistance (NDR) characteristics are demonstrated in homoepitaxial AlN/GaN double-barrier resonant tunneling diodes (RTDs). The devices are grown by plasma-assisted MBE on bulk GaN substrates and exhibit robust and repeatable NDR at RT. A high peak current density of 183 kA cm−2 is simultaneously demonstrated with a large peak-to-valley current ratio of 2.07, mainly benefiting from the significantly reduced dislocation density and improved hyper-abrupt heterointerfaces in the active region, which boosts the electron quantum transport in the resonant tunneling cavity. The achievement shows the promising potential to enhance the oscillation frequency and output power of GaN-based RTD oscillators, imperative for next-generation high-power solid-state compact terahertz oscillators application.

High-power in-phase and anti-phase mode emission from linear arrays of resonant-tunneling-diode oscillators in the 0.4-to-0.8-THz frequency range

Oscillators based on resonant tunneling diodes (RTDs) are able to reach the highest oscillation frequency among all electronic THz emitters. However, the emitted power from RTDs remains limited. Here, we propose linear RTD oscillator arrays capable of supporting coherent emission from both in-phase and anti-phase coupled modes. The oscillation modes can be selected by adjusting the mesa areas of the RTDs. Both the modes exhibit constructive interference at different angles in the far field, enabling high-power emission. Experimental demonstrations of coherent emission from linear arrays containing 11 RTDs are presented. The anti-phase mode oscillates at ∼450 GHz, emitting about 0.7 mW, while the in-phase mode oscillates at around 750 GHz, emitting about 1 mW. Moreover, certain RTD oscillator arrays exhibit dual-band operation: changing the bias voltage allows for controllable switching between the anti-phase and in-phase modes. Upon bias sweeping in both directions, a notable hysteresis feature is observed. Our linear RTD oscillator array represents a significant step forward in the realization of large arrays for applications requiring continuous-wave THz radiation with substantial power.

Strong nonlinear effects of a transmission line stub on resonant tunneling diode oscillators

Strong nonlinear effects of a transmission line stub on resonant tunneling diode oscillators

Analysis of terahertz double dielectric structure patch antenna using nitride semiconductors

AbstractRecently, gallium arsenide (GaAs)‐based resonant tunneling diode (RTD) oscillators and indium phosphorus (InP)‐based Schottky barrier diode (SBD) receivers have been studied in the terahertz (THz) band. The THz devices for practical use should operate at room temperature, be small in size, and have high output power. Therefore, this study was focused on gallium nitride (GaN), which possesses excellent material properties, such as wide bandgap characteristics and heteroepitaxy on Si substrates. The GaN‐based oscillators and receivers are expected to be compact, operate at room‐temperature, and act as a high‐power device for the THz‐band devices. However, GaN has crystal defects, which can cause instability in device operations. A double dielectric structure patch antenna composed of Silicon Nitride (SiN) and Benzo Cyclo Butene (BCB) with different dielectric constants was proposed to realize a GaN‐based THz transmitter and receiver. The antenna characteristics were investigated using the Finite Difference Time Domain (FDTD) method. The results showed that the SiN has little effect on the radiation, whereas the BCB is strongly responsible for the radiation. Comparing the absolute gain between the double dielectric structure and the conventional structure using the SiN, it was confirmed that the double dielectric structure can improve the absolute gain.

Large-signal dynamics of resonant-tunneling diodes

A model for analyzing dynamic large-signal characteristics of double-barrier resonant-tunneling diodes (RTDs) is presented. The model is based on the analysis of dynamical trajectories in phase space, defined by the RTD bias and electron density in the RTD quantum well. We show that an accurate dynamic model can be reformulated in an approximate way, relying only on a directly measurable DC I–V curve and on few other RTD parameters, which could be easily estimated with simple DC calculations. We further demonstrate that a simple equivalent circuit, composed of a capacitor, inductor, and two resistors (RLRC), accurately describes the large-signal admittance of RTDs. The circuit elements can be described in terms of relaxation time, geometrical RTD capacitance, and low- and high-frequency resistors. The circuit has the very same structure as that previously derived for small-signal RTD admittance, although with deviating parameters, which are now dependent on the AC-signal amplitude. We show that the large-signal RTD relaxation time can be shorter and longer than the small-signal one. In the context of RTD oscillators, a shorter RTD relaxation time allows one to get higher output power at high frequencies. The availability of an accurate, general, but rather simple, physics-based model for analyzing large-signal RTD dynamics removes one of the major hindrances to the further development of sub-THz and THz RTD oscillators.

Limitations of Output Power and Efficiency of Simple Resonant-Tunneling-Diode Oscillators

We show that the maximum output power of a simple resonant-tunneling-diode (RTD) oscillator is fundamentally limited by the radiation conductance of its antenna and by a maximum RTD voltage swing. For stand-alone RTD oscillators with common simple antenna types, this power level most likely cannot exceed 1 mW at sub-THz and THz frequencies. The RTD current density, conductance, and capacitance have no direct influence on the maximum power level but those RTD parameters determine how close one can get to the maximum. We show that the maxima of the output power and dc-to-RF conversion efficiency are achieved for different sets of the parameters of RTD oscillators and not simultaneously. We demonstrate slot-antenna RTD oscillators with up to 0.5-mW output power at <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\approx \!100}\,$</tex-math></inline-formula> GHz and with 5 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\%}$</tex-math></inline-formula> conversion efficiency, which could be increased further up to 8 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\%}$</tex-math></inline-formula> , although at the expense of reduced output power. Composite RTD oscillators with separate resonators and radiators can possibly overcome the limitations of simple oscillators.

Demonstration of Sub-THz Traveling-Wave Resonant-Tunneling-Diode Oscillators

As a proof of principle, operation of traveling-wave resonant-tunneling-diodes (RTDs) oscillators has been demonstrated experimentally in the frequency range 100-400 GHz. A theoretical model describing a realistic coupled system, consisting of a traveling-wave RTD and a patch antenna, has been developed. The model describes the eigen resonances and the eigen modes, which have a mixed character of both subsystems. The model and the experimental results are in agreement with each other. We also demonstrate that some oscillators were switching between different resonant modes of the system with the change of bias. The phenomenon could be used for an extended frequency control of the RTD oscillators.

Analysis of Terahertz Double Dielectric Structure Patch Antenna Using Nitride Semiconductors

Recently, gallium arsenide (GaAs)-based resonant tunneling diode (RTD) oscillators and indium phosphorus (InP)-based Schottky barrier diode (SBD) receivers have been studied in the terahertz (THz) band. The THz devices for practical use should operate at room temperature, be small in size, and have high output power. Therefore, this study was focused on gallium nitride (GaN), which possesses excellent material properties, such as wide bandgap characteristics and heteroepitaxy on Si substrates. The GaN-based oscillators and receivers are expected to be compact, operate at room-temperature, and act as a high-power device for the THz-band devices. However, GaN has crystal defects, which can cause instability in device operations. A double dielectric structure patch antenna composed of Silicon Nitride (SiN) and Benzo Cyclo Butene (BCB) with different dielectric constants was proposed to realize a GaN-based THz transmitter and receiver. The antenna characteristics were investigated using the Finite Difference Time Domain (FDTD) method. The results showed that the SiN has little effect on the radiation, whereas the BCB is strongly responsible for the radiation. Comparing the absolute gain between the double dielectric structure and the conventional structure using the SiN, it was confirmed that the double dielectric structure can improve the absolute gain.

Simulation of traveling-wave resonant tunneling diode oscillator waveguides

In a traveling-wave resonant tunneling diode oscillator, the gain medium is encapsulated in a metallic waveguide. The geometrical parameters of the system and the skin penetration depth in the metal layers are of similar length scales. It confirms the need for a full-wave simulation, where the impedance boundary conditions can not be applied in a straightforward manner. In this work, a method of moments-based electromagnetic wave solver was developed and used to illustrate different traveling-wave RTD oscillator structures.

Phase Control of Terahertz Waves Using Injection-Locked Resonant Tunneling Diode Oscillator

Controlling the phase of terahertz (THz) waves is necessary for various applications, particularly beam steering with phased arrays. Resonant tunneling diodes (RTDs) are promising candidates for compact THz sources, and injection-locking phenomenon can be used to control the phase of RTD oscillators. In this study, we experimentally demonstrate the phase control of an injection-locked RTD oscillator. We fabricated an RTD oscillator with a slot antenna and developed a device package that reduces external THz feedback to obtain continuous frequency tuning. The free-running frequency varied from 380.5 to 386.7 GHz over a bias range of 0.61 to 0.65 V. To measure the phase of RTD oscillator, we constructed a heterodyne detection system in which all signals were well synchronized to achieve good phase measurement accuracy. The frequency of the RTD oscillator was locked to an injection signal of 383.2 GHz from an external source, and the phase was controlled by the bias voltage under the injection locking. The measured phase changed from −81° to +73° within the locking range. The bias-voltage dependence of the phase change reasonably agreed with the theoretical expectation.

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.

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.

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.

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 (&amp;gt;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.

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&#x00A0;are solved simultaneously and consistently with time-domain electric field integral equations&#x00A0;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.

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.

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.

Broadband terahertz resonant tunnelling diode transmitter integrated with coplanar‐waveguide‐fed slot‐ring antenna

Resonant tunnelling diode (RTD) oscillators used for high-speed terahertz wireless communication systems have garnered significant attention in recent years. In these systems, RTD devices are directly on-off modulated through baseband (BB) circuits, which limit the data rate. This letter investigates an efficient approach to increase the bandwidth of the BB circuits, by addressing the broadband connection between the two-RTD oscillator chip and the radiofrequency circuit board. As a result, an error-free wireless communication has been achieved with a data rate of 25 Gbit/s in the 300-GHz band using the RTD transmitter and receiver.