Terahertz Bloch oscillator with a modulated bias

Electrons performing Bloch oscillations in an energy band of a dc-biased superlattice in the presence of weak dissipation can potentially generate THz fields at room temperature. The realization of such Bloch oscillator is a long-standing problem due to the instability of a homogeneous electric field in conditions of negative differential conductivity. We establish the theoretical feasibility of stable THz gain in a long superlattice device in which the bias is quasistatically modulated by microwave fields. The modulation waveforms must have at least two harmonics in their spectra.Comment: 5 page

Resonant tunnelling diode optoelectronic receivers and transmitters

This thesis describes the research work on double barrier quantum well (DBQW) resonant tunneling diode (RTD) based optoelectronic transmitters and receivers, focused on the design and characterization of resonant tunneling diode photodetectors (RTD-PD) implemented in the In53Ga47As/InP material system for operation at 1.55 μm and 1.31 μm wavelengths, and evaluate numerically the merits of the integration of an RTD/RTD-PD with a laser diode (LDs) to act as simple optoelectronic transmitters. The aim of the work was to investigate simple, low-cost, high-speed transmitter and receiver architectures taking advantage of RTDs properties such as the structural simplicity, high frequency (up to terahertz), and wide-bandwidth built-in electrical gain (roughly, from dc to terahertz). Also described are the preliminary studies of RTD-PDs operation as single photon detector at room temperature utilizing the excitability property. In this work, we evaluate which factors affect the bandwidth of RTD-PDs. Knowing the answer to this, we propose rules and optimizations necessary to achieving high bandwidth (>10 GHz) RTD-PDs. Furthermore, we show how to utilize the built-in amplification, arising from the RTD non-linear current-voltage (IV) curve and the presence of a negative differential resistance region (NDR) to building high responsivity photodetectors that can outperform current commercial technologies, particularly PIN photodiodes, in novel applications. The design and modeling work relied on numerical simulations utilizing the nonequilibrium Green’s function formalism (NEGF), which we implement using Silvaco ATLAS. We briefly introduce the NEGF method and Silvaco ATLAS and utilize them to do the design of the epitaxial structure of novel devices. The results of which are novel models which allow us to predict the effect that the RTD structural parameters (doping concentration and the lengths of both the emitter and collector) have on the peak voltage of the RTD. We study experimentally the factors affecting the bandwidth by optical characterization of several epitaxial layer stacks and propose hypotheses that help to explain the measured bandwidths. We show that for high-speed RTD-PDs (sub nanosecond), the light absorption layers should be confined to the locations where the electric field is sufficiently high and avoiding highly doped thick contact layers with band gap energies below the energy of the photons being detected. Additionally, we outline a set of rules for the design of RTD-PD detectors based on ni-n and p-i-n heterostructures, where the length, location, and doping level of the absorption regions are the relevant parameters to be considered in determining the bandwidth and responsivity of the devices. Moreover, we measure and report on the responsivity of RTDPDs under both DC and AC optical excitation. We show that RTD-PDs can have very high responsivity values reaching up to 1×107 A/W, and electrical bandwidth of around 1.26 GHz (1.75 GHz optical) that is limited by the lifetime of the photo-generated minority carriers (the holes). The last part of the thesis is dedicated to the study of RTD-PD circuits, where the integration between an RTD-PD and a laser diode (LD) is thoroughly examined. The LD acts as a load that is driven by the RTD-PD current. We derive and investigate the equivalent circuit for such a system incorporating the Schulman function for the RTD-PD IV, using the solution to study several operation regimes using MATLAB code. These regimes include the RTD-PD biased in the positive differential resistance region (PDR), when it is biased in the NDR region, and when induced to switch between the PDR and NDR regions. We also show how the excitability property of the RTD-PD can be used for detecting very low signal intensity levels, and the ability of RTDs to operate as voltage-controlled oscillators while biased in the NDR region

Graphene Tunneling Transit-Time Terahertz Oscillator Based on Electrically Induced p-i-n Junction

We propose and analize a graphene tunneling transit time device based on a heterostructure with a lateral p-i-n junction electrically induced in the graphene layer by the applied gate voltages of different polarity. The depleted i-section of the graphene layer (between the gates) serves as both the tunneling injector and the transit region. Using the developed device model, we demonstrate that the ballistic transit of electrons and holes generated due to interband tunneling in the i-section results in the negative ac conductance in the terahertz frequency range, so that the device can serve as a terahertz oscillator.Comment: 5 pages, 3 figure

The Third International Symposium on Space Terahertz Technology: Symposium proceedings

Papers from the symposium are presented that are relevant to the generation, detection, and use of the terahertz spectral region for space astronomy and remote sensing of the Earth's upper atmosphere. The program included thirteen sessions covering a wide variety of topics including solid-state oscillators, power-combining techniques, mixers, harmonic multipliers, antennas and antenna arrays, submillimeter receivers, and measurement techniques

High efficiency and high frequency resonant tunneling diode sources

Terahertz (THz) technology has been generating a lot of interest due to the numerous potential applications for systems working in this previously unexplored frequency range. THz radiation has unique properties suited for high capacity communication systems and non-invasive, non-ionizing properties that when coupled with a fairly good spatial resolution are unparalleled in its sensing capabilities for use in biomedical, industrial and security fields. However, in order to achieve this potential, effective and efficient ways of generating THz radiation are required. Devices which exhibit negative differential resistance (NDR) in their current-voltage (I – V) characteristics can be used for the generation of these radio frequency (RF) signals. Among them, the resonant tunnelling diode (RTD) is considered to be one of the most promising solid-state sources for millimeter and submillimeter wave radiation, which can operate at room temperature. However, the main limitations of RTD oscillators are producing high output power and increasing the DC-to-RF conversion efficiency. Although oscillation frequencies of up to 1.98 THz have been already reported, the output power is in the range of micro-Watts and conversion efficiencies are under 1 %. This thesis describes the systematic work done on the design, fabrication, and characterization of RTD-based oscillators in monolithic microwave/millimeter-wave integrated circuits (MMIC) that can produce high output power and have a high conversion efficiency at the same time. At the device level, parasitic oscillations caused by the biasing line inductance when the diode is biased in the NDR region prevents accurate characterization and compromises the maximum RF power output. In order to stabilise the NDR devices, a common method is the use of a suitable resistor connected across the device, to make the differential resistance in the NDR region positive. However, this approach severely hinders the diode’s performance in terms of DC-to-RF conversion efficiency. In this work, a new DC bias decoupling circuit topology has been developed to enable accurate, direct measurements of the device’s NDR characteristic and when implemented in an oscillator design provides over a 10-fold improvement in DC-to-RF conversion efficiency. The proposed method can be adapted for higher frequency and higher power devices and could have a major impact with regards to the adoption of RTD technology, especially for portable devices where power consumption must be taken into consideration. RF and DC characterization of the device were used in the realization on an accurate large-signal model of the RTD. S-parameter measurements were used to determine an accurate small-signal model for the device’s capacitance and inductance, while the extracted DC characteristics where used to replicate the I-V characteristics. The model is able to replicate the non-stable behavior of RTD devices when biased in the NDR region and the RF characteristics seen in oscillator circuits. It is expected that the developed model will serve in future optimization processes of RTD devices in millimeter and submillimeter wave applications. Finally, a wireless data transmission link operating in the Ka-band (26.5 GHz – – 40 GHz) using two RTDs operating as a transmitter and receiver is presented in this thesis. Wireless error-free data transfer of up to 2 gigabits per second (Gbit/s) was achieved at a transmission distance of 15 cm. In summary, this work makes important contributions to the accurate characterization, and modeling of RTDs and demonstrates the feasibility of this technology for use in future portable wireless communication systems and imaging setups

Components and circuits for tunneling diode based high frequency sources

Terahertz (THz) technology has been generating a lot of interest because of the potential applications for systems working in this frequency range. However, to fully achieve this potential, effective and efficient ways of generating controlled signals in the terahertz range are required. Devices that exhibit negative differential resistance (NDR) in a region of their current-voltage (I-V ) characteristics have been used in circuits for the generation of radio frequency signals. Of all of these NDR devices, resonant tunneling diode (RTD) oscillators, with their ability to oscillate in the THz range are considered as one of the most promising solid-state sources for terahertz signal generation at room temperature. There are however limitations and challenges with these devices, from inherent low output power usually in the range of micro-watts (uW) for RTD oscillators when milli-watts (mW) are desired. At device level, parasitic oscillations caused by the biasing line inductance when the device is biased in the NDR region prevent accurate device characterisation, which in turn prevents device modelling for computer simulations. This thesis describes work on I-V characterisation of tunnel diode (TD) and RTD (fabricated by Dr. Jue Wang) devices, and the radio frequency (RF) characterisation and small signal modelling of RTDs. The thesis also describes the design and measurement of hybrid TD oscillators for higher output power and the design and measurement of a planar Yagi antenna (fabricated by Khalid Alharbi) for THz applications. To enable oscillation free current-voltage characterisation of tunnel diodes, a commonly employed method is the use of a suitable resistor connected across the device to make the total differential resistance in the NDR region positive. However, this approach is not without problems as the value of the resistor has to satisfy certain conditions or else bias oscillations would still be present in the NDR region of the measured I-V characteristics. This method is difficult to use for RTDs which are fabricated on wafer due to the discrepancies in designed and actual resistance values of fabricated resistors using thin film technology. In this work, using pulsed DC rather than static DC measurements during device characterisation were shown to give accurate characteristics in the NDR region without the need for a stabilisation resistor. This approach allows for direct oscillation free characterisation for devices. Experimental results show that the I-V characterisation of tunnel diodes and RTD devices free of bias oscillations in the NDR region can be made. In this work, a new power-combining topology to address the limitations of low output power of TD and RTD oscillators is presented. The design employs the use of two oscillators biased separately, but with the combined output power from both collected at a single load. Compared to previous approaches, this method keeps the frequency of oscillation of the combined oscillators the same as for one of the oscillators. Experimental results with a hybrid circuit using two tunnel diode oscillators compared with a single oscillator design with similar values shows that the coupled oscillators produce double the output RF power of the single oscillator. This topology can be scaled for higher (up to terahertz) frequencies in the future by using RTD oscillators. Finally, a broadband Yagi antenna suitable for wireless communication at terahertz frequencies is presented in this thesis. The return loss of the antenna showed that the bandwidth is larger than the measured range (140-220 GHz). A new method was used to characterise the radiation pattern of the antenna in the E-plane. This was carried out on-wafer and the measured radiation pattern showed good agreement with the simulated pattern. In summary, this work makes important contributions to the accurate characterisation and modelling of TDs and RTDs, circuit-based techniques for power combining of high frequency TD or RTD oscillators, and to antennas suitable for on chip integration with high frequency oscillators

Dynamical Analysis of Terahertz Quantum Cascade Lasers

This thesis focuses on various aspects in terahertz quantum cascade lasers such as high temperature performances, electrical instabilities and chaos. In the first part of the thesis, an introduction to the topic and the non equilibrium Green's function package applied is given with particular emphasise on the inhomogeneous domain model and the chaos theory. In addition, new results regarding the high temperature operation of low frequency terahertz quantum cascade lasers are presented. In the second part, the related research articles are shown. Paper I shows numerical simulations demonstrating the first autonomous terahertz quantum cascade laser exhibiting chaos. Paper II provides a detailed analysis of the current record-holding terahertz quantum cascade lasers operating around 4 terahertz. A new design is suggested which should operates at an even higher temperature of 265 K.Paper III analyses the spectral behaviour of the output light generated by oscillating field domains in direct cooperation with the experiment. The overall behaviour is related to the phenomenological boundary conductivity and the external capacitance