Tese (Doutorado em Engenharia Elétrica) - Centro Universitário FEI, São Bernardo do Campo, 2016.

Dissertação (Mestrado em Engenharia Elétrica) - Centro Universitário FEI, São Bernardo do Campo, 2018.

High performance optoelectronic applications require simultaneously high mobility ($\mu$) and high quantum efficiency of fluorescence ($\Phi$). While this has been realised for organic small molecule semiconductors, applications such as high efficiency organic photovoltaics and bright organic light-emitting diodes towards electrically driven lasing are hampered by an apparent trade-off between $\mu$ and $\Phi$ in disordered systems. Recent reports of state-of-the-art device performance often optimise $\mu$ and $\Phi$ in disordered organic materials separately, and employ multi-layer architectures. In this work, we investigate materials in a class of donor-acceptor polymer materials; the indacenodithiophene-$\textit{alt}$-benzothiadiazole family, which demonstrate high $\mu$ in spite of a low long-range structural order, to understand the interplay between these two important device figures-of-merit. In the first section, we evaluate the effect of various tuneable parameters on $\mu$ and device performance in organic field-effect transistors. Using chemical modifications to the solubilising side chains, we observe that the substitution of bulky groups leads to a reduction of the hole mobility $\mu_h$ > 2 cm$^{2}$/Vs to ~ 0.5 cm$^{2}$/Vs in the benchmark polymer of this family, indacenodithiophene-$\textit{alt}$-benzothiadiazole. Crystallographic and exciton-quenching based experiments confirm this observation is closely related to the degree of polymer backbone aggregation, and this leads to a different temperature evolution of the transport behaviour. In order to reliably improve $\mu$ in these systems, an elongation of the donor subunit is required. This increases the $\pi$-electron density on the donor, and can lead to an improvement in $\mu$ where the side chain density is decreasing. This chemical design, leading to a more highly aggregated structural motif is much more potent in determining $\mu$, it seems, than design strategies to further improve the energetic disorder in the joint density of states and the potential barrier to torsion, which may be near optimised in these low-disorder systems. In the second section, we unpick the precise relationship between the degree of aggregation apparently linking high $\mu$ to low $\Phi$. With a prototype system, we compare the photophysics of two indacenodithiophene-$\textit{alt}$-benzothiadiazole polymers differing by side chain bulkiness. Despite the aforementioned suppression of $\mu$, we observe an improvement to $\Phi$ of $< 0.02$ to $\sim 0.18$ upon backbone separation. This derivative has the highest $\Phi$ reported for any polymer with $\mu$ exceeding that of amorphous-Si. However, the $\Phi$ in the more aggregated derivative is not limited by the formation of non-emissive excitons, but rather by an additional internal conversion pathway which is strongly temperature dependent, and mediated by Raman-active vibrations and close chain coupling. Extending this study, we analyse additional materials in this family with the highest $\Phi \cdot \mu$ values reported for conjugated polymers. We find that increasing the energy gap leads to an increase in $\Phi$, and secondary emission pathways via weakly luminescent inter-chain charge transfer species. By solving the rate equations for exciton recombination, we use the radiative rate of inter-chain luminescence as a probe to show strong wavefunction mixing at close-contact points for some polymers, and suggest this as the origin for a superior $\mu$ in dithiopheneindenofluorene-$\textit{alt}$-benzothiadiazole compared to indacenodithiophene-$\textit{alt}$-benzothiadiazole. We demonstrate how low $\mu$ can be decoupled from the energy gap ($E_g$), and propose backbone elongation leading to increased inter-chain wavefunction overlap and a higher $E_g$ as a design rule to increase $\Phi$ and $\mu$ together. Finally, we assess the role of low-frequency vibrations in organic semiconductors displaying thermally activated delayed fluorescence (TADF). In the low-aggregation limit where $\Phi$ is maximised, we show that non-radiative triplet recombination is strongly related to low frequency torsional motion, and both are reduced in the presence of a rigid polymer host matrix for various TADF materials across different classes. However, we also explore the importance of rotational freedom in determining the oscillator strength, exchange energy, and spin-orbit coupling matrix elements which mediate luminescence in the absence of a rigid host. We demonstrate that suppressing dynamic motion is a powerful tool to modulate the photophysical properties of these emitters, and can lead to improved $\Phi$ particularly for low $E_g$ emitters.

abstract: Space exploration is a large field that requires high performing circuitry due to the harsh environment. Within a space environment one of the biggest factors leading to circuit failure is radiation. Circuits must be robust enough to continue operation after being exposed to the high doses of radiation. Bandgap reference (BGR) circuits are designed to be voltage references that stay stable across a wide range of supply voltages and temperatures. A bandgap reference is a piece of a large circuit that supplies critical elements of the large circuit with a constant voltage. When used in a space environment with large amounts of radiation a BGR needs to maintain its output voltage to enable the rest of the circuit to operate under proper conditions. Since a BGR is not a standalone circuit it is difficult and expensive to test if a BGR is maintaining its reference voltage. This thesis describes a methodology of isolating and simulating bandgap references. Both NPN and PNP bandgap references are simulated over a variety of radiation doses and dose rates. This methodology will allow the degradation due to radiation of a BGR to be modeled easily and affordably. It can be observed that many circuits experience enhanced low dose rate sensitivity (ELDRS) which can lead to failure at low total ionizing doses (TID) of radiation. A compact model library demonstrating degradation of transistors at both high and low dose rates (HDR and LDR) will be used to show bandgap references reliability. Specifically, two bandgap references being utilized in commercial off the shelf low dropout regulators (LDO) will be evaluated. The LDOs are reverse engineered in a simulation program with integrated circuit emphasis (SPICE). Within the two LDOs the bandgaps will be the points of interest. Of the LDOs one has a positive regulated voltage and one has a negative regulated voltage. This requires an NPN and a PNP based BGR respectively. This simulation methodology will draw conclusions about the above bandgap references, and how they operate under radiation at different doses and dose rates. / Dissertation/Thesis / Masters Thesis Electrical Engineering 2019

Les transistors à haute mobilité électronique à base de Nitrure de Gallium sur substrat de silicium (GaN-sur-Si) sont des candidats prometteurs pour les futures générations de convertisseur de puissance. Aujourd’hui, plusieurs verrous techniques ralentissent la commercialisation de cette technologie au niveau industriel, en particulier pour les applications requérant de haute tension (≥ 600 V). Dans ce contexte, ces travaux constituent une contribution au développement de composants innovants à base de GaN-sur-Si fonctionnant au-delà de 1kV. Nous nous sommes principalement focalisés sur l’amélioration de la tenue en tension de ce type de transistors au travers du développement d’un procédé de gravure localisée du substrat en face arrière permettant de supprimer le phénomène de conduction parasite localisé entre les couches tampons et le substrat. Ce procédé ainsi que l’utilisation de structures d’épitaxie innovantes nous ont permis d’observer une amélioration drastique des performances électriques des transistors à haute tension. Nous avons pu notamment démontrer pour la première fois la possibilité de délivrer des tenues en tension de plus de 3 kV sur cette filière émergente. Ces résultats obtenus, supérieurs à l’état de l’art, laissent envisager l’utilisation des technologies GaN-sur-Si pour les moyennes et hautes gammes de tension (>1000V). / GaN-based High Electron Mobility Transistors (HEMTs) on Silicon substrate (GaN-on-Si) are promising candidates for future generations of power converters. Today, technical limitations need to be overcome in order to allow the industrial commercialization of this technology, particularly for high-voltage applications (≥ 600 V). In this frame, this work constitutes a contribution to the development of innovative GaN-on-Si devices operating above 1kV. We mainly focused on the improvement of the blocking voltage of the transistors with the realization of a local substrate removal process with the aim of suppressing the parasitic conduction phenomena between the buffer layer and the substrate. Owing to an improved technological process and innovative epitaxial structures, we observed a drastic improvement of the electrical performances of the transistor under high voltages. In particular, we have been able to demonstrate for the first time a blocking voltage above 3kV for this emerging technology. These results, well beyond the state of the art, pave the way for higher voltage operation GaN-on-Si power devices.

This research effort advances millimeter-wave transistor modeling in a current RF/Microwave circuit simulator (Agilent's Advanced Design System-ADS) for small-signal noise and large signal simulations. The device modeled is a metamorphic High Electron Mobility Transistor (mHEMT) supplied by Raytheon RF components. Because of their structure, these new low noise devices are used in this work to test the abilities to accurately model in the sub 0.5dB noise figure territory and to study model prediction into W-band (75-110 GHz). New modeling issues discussed in this thesis involve the effects of noise modeling in relation to the small-signal model parameters. The noise modeling identifies two methods of extraction and how to determine good noise data. Other modeling topics addressed are the use of an advanced nonlinear model, and the ability to optimize for gain compression in the nonlinear model. Several measurement systems were used in the extraction and validation of this modeling effort. They consist of the ATN NP5 noise system, Maury Automated Tuner System, Agilent's IC-CAP, and Gateway's Special. The concept behind using these systems was to construct a complete modeling reference for a transistor and validate it against noise parameter and nonlinear measured data. Since the modeling work for this thesis is built on previous work, one goal has been to bring past USF field-effect transistor (FET) modeling efforts up to date and refine them for future use. The noise measurements were compared to results from Raytheon to validate the USF ATN noise parameter measurement system. Also the IC-CAP modeling system has been validated in measuring the test devices using the Maury load-pull system. Small-signal and noise modeling were accomplished using techniques standardized from several technical papers and prior USF Ph.D. work relative to the model extraction. The IC-CAP modeling software also provided a straightforward platform for large-signal model extraction that is documented in this thesis. Using optimization in ADS, a final nonlinear was created. Measured DC, S-parameter, noise parameters, harmonic power, TOI, load-pull, and efficiency measurements were shown to compare well with model data simulated in ADS. Temperature scaling was also executed using a linear approximation of model values over measured temperatures in the noise model. The results presented show that the models developed illustrate good fitting of the behavior of the mHEMT device.

This thesis investigates the use of single Cooper-pair transistor (SCPT) for fast and sensitive detection of quasiparticle dynamics. This investigation is motivated by the possibility of quantum information processing using superconducting nanoscale circuits, such as the SCPT and the Cooper-pair-box (CPB). In the SCPT coherent charge transport can be temporarily halted due to quasiparticle tunnelling, known as quasiparticle poisoning. Quasiparticle poisoning can be reduced by the use of engineered island and lead gap energies. The thesis begins by reporting measurements of the superconducting gap in aluminium - aluminium-oxide - aluminium tunnel junctions, as a function of film thickness. We have observed an increase in the superconducting energy gap of aluminium with decreasing film thickness. This method is used to engineer the island and gap energies in a SCPT and consequently we observe reduced poisoning and a modification of the thresholds for finite bias transport processes. Radio-frequency reflectometry is used to perform high-bandwidth measurements of quasiparticle tunnelling in a gap engineered SCPT. A model for the radio-frequency (rf) operation of the SCPT is presented and shows close agreement with experiment. Thermal activation of the quasiparticle dynamics is investigated, and consequently, we are able to determine energetics of the poisoning and unpoisoning processes. This enables an effective quasiparticle temperature to be determined, allowing the poisoning to be parametrised. An investigation of the use of normal metal quasiparticle traps for suppression of quasiparticle poisoning in SCPT devices is performed. To date, there has been little quantitative information about the behaviour of quasiparticle traps even though they have been used extensively. The work presented serves to clarify the nature of quasiparticle trap performance. Finally the single-quasiparticle sensitivity of the SCPT is employed to directly probe a few quasiparticle gas in a small superconducting volume. The quasiparticle population is monitored both in the steady-state and under non-equilibrium conditions of injection. In the non-equilibrium regime the quasiparticle recombination time is accessed from the response of the SCPT to pulsed injection. Agreement to previous experimental studies of recombination times in aluminium is found.

Includes bibliographical references (leaves 272-279.)

The emergence of organic semiconductors is exciting since it promises to open up for straightforward and low-cost fabrication of a wide range of efficient and novel electronic devices. However, in order for this promise to become reality it is critical that new and functional fabrication techniques are developed. This thesis demonstrates the conceptualization, development, realization and implementation of a particularly straightforward and scalable fabrication process: the photo-induced and resist-free imprint patterning technique.Initial experiments revealed that some members of a group of carbon-cage molecular semiconductors – termed fullerenes – can be photochemically modified into dimeric or polymeric structures during exposure to laser light, and, importantly, that the exposed fullerene material retains its good electron-transport property while its solubility in common organic solvents is drastically lowered. With this information at hand, it was possible to design and create well-defined patterns in a solution-deposited fullerene film by exposing selected film areas to laser light and then developing the entire film in a tuned developer solution. An electronically active fullerene pattern emerges at the locations defined by the incident laser beam, and the patterning technique was successfully utilized for the fabrication of arrays of efficient field-effect transistors.In a later stage, the capacity of the photo-induced and resist-free imprint technique was demonstrated to encompass the fabrication of ubiquitous and useful CMOS circuits. These are based on a combination of p-type and n-type transistors, and a blend between a p-type organic semiconductor and an n-type fullerene compound was designed so that the latter dominated. By solution-depositing the blend film on an array of transistor structures, exposing selected transistors to laser light, and then developing the entire transistor array in a developer solution, it was possible to establish a desired combination of (non-exposed) p-type transistors and (exposed) n-type transistors. We finally utilized this combination of transistors for the fabrication of a CMOS circuit in the form of well a-functional organic inverter stage.

Traditionally, the design of continuous time active filters usually has a trade offbetween low-voltage and high dynamic range. One way to solve this problem is companding technology. There are two methods for companding filters. The first method utilizes the exponential I-V characteristics of BJT in the saturation region. In order to reduce the cost andintegrate the analog and digital circuits, the other method was exploited using CMOS process. In this project, a new first-order low pass log-domain filter based on CMOS parasitic vertical BJTwill be proposed. This filter has higher frequency response than previous circuits. We will first employ Hspice to simulate the log-domain filter to ensure the correctness of the circuit and make it a reliable reference with the circuit layout. After summarizing all the simulations and analyses, the chip will be fabricated with 0.35um CMOS technology.