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Tecnologie e Dispositivi a Semiconduttore Avanzati per Applicazioni Optoelettroniche
No full textL'optoelettronica è uno dei settori dell'ingegneria in cui diversi dispositivi e tecnologie sono combinati insieme, integrando trasduttori, sensori, amplificatori e processori in sistemi eterogenei. L'industria dei semiconduttori gioca chiaramente un ruolo chiave, dovendo definire il modo più veloce ed efficace per trasformare i più recenti risultati della ricerca scientifica in processi di produzione su larga scala.
L'attività di ricerca riportata in questo lavoro riguarda due diverse tecnologie di dispositivo a semiconduttore con applicazioni nel campo dell'optoelettronica: l'High Electron Mobility Transistor (HEMT) basato su fosfuro d'indio (InP) e il Light-Emitting Diode (LED) basato su nitruro di gallio (GaN). Due diverse strutture di dispositivo, incentrate su diversi materiali, con diverse funzionalità, che sfruttano tuttavia la stessa tecnologia di base (le eterostrutture a semiconduttori composti) e, per questo motivo, condividono problematiche di materiale/dispositivo, modelli fisici e approcci di simulazione.
L'HEMT in InP è il miglior dispositivo elettronico per ciò che riguarda prestazioni di velocità e/o frequenza, e trova applicazioni nelle comunicazioni wireless e ottiche, imaging passivo e analisi atmosferiche. Grazie alla compatibilità a livello wafer con laser in InP, questi HEMT sono i transistor che permetteranno la realizzazione di circuiti optoelettronici monolitici integrati. La prima parte del mio dottorato è stata dedicata alla progettazione, caratterizzazione e ottimizzazione di InP HEMTs ad alta tensione. La capacità in tensione di questi dispositivi è stata migliorata introducendo un cosiddetto "field-plate", ottimizzato e inserito nella struttura del dispositivo. Questi HEMT dotati di field-plate sono stati progettati attraverso simulazioni numeriche di dispositivo, per poi essere fabbricati e caratterizzati. Questa nuova tecnologia sviluppata ha il potenziale necessario per rendere possibile la fabbricazione di amplificatori di potenza ad alta efficienza, ricetrasmettitori integrati e amplificatori a basso rumore (LNAs) più robusti.
La seconda parte del mio dottorato si è invece incentrata su LED in GaN. Questi emettitori di luce hanno praticamente saturato il mercato della retroilluminazione LCD (cellulari, PC e display) e hanno già iniziato la penetrazione in quello molto più ampio dell'illuminazione, spiazzando le lampade ad incandescenza e fluorescenza grazie all'efficienza superiore. Per essere utilizzati per l'illuminazione, tuttavia, i LED devono funzionare ad alte correnti. Sotto queste condizioni, i LED in GaN sono sfortunatamente affetti dal cosiddetto "efficiency droop", un fenomeno penalizzante che diminuisce l'efficienza quantica interna dei LED all'aumentare della corrente di funzionamento, riducendo pertanto significativamente il vantaggio competitivo di questi dispositivi nei confronti delle lampade a fluorescenza. L'origine fisica del droop è ancora incerta e oggetto di intenso dibattito all'interno della comunità scientifica. Inoltre, sono state proposte diverse modifiche tecnologiche in modo da mitigare l'effetto, ma una soluzione definitiva non è stata ancora individuata. La mia ricerca in questo ambito si è basata su di una analisi sistematica, incentrata su simulazione, dei meccanismi che possono generare il droop e delle relative modifiche tecnologiche. L'analisi ha indicato che un droop che ha l'intensità tipica osservata nei dispositivi reali può essere difficilmente giustificato da un punto di vista fisico (nell'ipotesi che un solo meccanismo domini sugli altri), suggerendo quindi un'origine multi-fattore per questo fenomeno e giustificando le difficoltà che gli esperti di tecnologia stanno incontrando nel produrre LED liberi dal droop.Optoelectronics is one of the fields of engineering in which many different technologies and devices merge together, integrating transducers, sensors, amplifiers, processors into heterogeneous systems. Semiconductor industry evidently plays a key role, having to define the quickest and most effective way to transform the most recent advances of the scientific community into large-scale production processes.
The research reported in this dissertation has addressed two different semiconductor device technologies with applications in the optoelectronics field: the High Electron Mobility Transistor (HEMT) based on indium phosphide (InP) and the Light-Emitting Diode (LED) based on gallium nitride (GaN). Two different device structures, relying on different material systems, and having different functionalities, that exploit however the same enabling technology, i.e. the compound-semiconductor heterostructures, and, for this reason, share basic material/device issues, physical models, and simulation approaches.
The InP HEMT is the best electronic device as far as frequency and/or speed performance is concerned, finding applications in both wireless and optical communications, passive imaging and atmospheric sounding. Thanks to the wafer-level compatibility with InP-based lasers, InP HEMTs are the transistors that will enable monolithic optoelectronic circuits to be realized. The first part of my PhD program has been devoted to the design, characterization and optimization of high-voltage InP HEMTs. The voltage capability of InP HEMTs has been extended by introducing an optimized “field-plate” into the device structure. These field-plated HEMTs have been designed by means of numerical device simulations, have successfully been fabricated and characterized. This newly developed technology has the potentials for enabling the fabrication of high-power, high-efficiency power amplifiers, integrated transceivers and more robust LNAs.
The second part of my PhD program has dealt with GaN LEDs. These light emitters have almost completely saturated the LCD backlighting market (phones, PCs and TV displays) and have already started to penetrate the general illumination one, displacing both incandescent and fluorescent lamps thanks to the superior power efficiency. General illumination requires, however, the LED to be operated at a high driving current. Under these operating conditions, GaN LEDs are unfortunately affected by the efficiency droop, a detrimental phenomenon that makes the LED internal quantum efficiency to decline at increasing driving current, thus reducing significantly the competitive advantage of GaN LEDs over fluorescent lamps. The physical origin of the droop is still unclear and under intense debate within the scientific community. Moreover, many technological modifications to the LED structure have been proposed as a means to mitigate the droop but a definitive antidote has not been achieved. My research in this field has mostly been devoted to a systematic, simulation-based analysis of the possible droop mechanisms and the related technological remedies. This analysis pointed out that a droop having the amplitude that is typically observed in actual devices can hardly be justified from a physical standpoint, under the hypothesis of only one droop mechanism dominating over the others, thus suggesting a multi-factor origin for this phenomenon and explaining the difficulties technologists are experiencing in producing droop-free LEDs
Dispositivo e metodologia per la misura di concentrazione di gas radon
No full textUn dispositivo di misura della concentrazione di gas Radon comprende una camera di misura (20) dotata di un dispositivo rivelatore (23), atto a rivelare particelle alfa da gas Radon; il dispositivo dell’invenzione comprende inoltre un elettrodo di raccolta (22) esterno alla camera di misura(20), atto a raccogliere i prodotti di decadimento del Radon elettricamente carichi provenienti dalla camera di misura (20).Un’unità di controllo (30) temporizza il passaggio tra due fasi operative (I, II) in cui il dispositivo rivelatore (23) e l’elettrodo di raccolta (22) sono attivi non contemporaneamente. [FIG. 1
Improvement of breakdown and DC-to-pulse dispersion properties in field-plated InGaAs-InAlAs pHEMTs
No full textWe report on novel, high voltage InGaAs-InAlAs pHEMTs, which have been processed on an optimized epilayer and incorporate field-plate structures of different dimensions. Fabricated devices demonstrate great improvements in break-down voltage and gate leakage, while keeping the same DC and RF behaviour with respect to baseline devices, i.e. with no field-plate implemented. In addition, the field plate strongly attenuates DC-to-pulse dispersion, making these devices suitable for high-power-density RF power amplifiers
TCAD optimization of field-plated InAlAs-InGaAs HEMTs
No full textHigh-voltage InGaAs-InAlAs HEMTs featuring optimized field-plate structures are being developed. A TCAD approach has been adopted for their design. Two-dimensional device simulations have preliminarily been calibrated by comparison with DC and RF measurements from the baseline InP HEMT technology into which the field plate is being incorporated. Simulations have then been used to design field-plate structures with optimal length andpassivation thickness
Fabrication of novel high frequency and high breakdown InAlAs-InGaAs pHEMTs
No full textThis paper presents a Novel low noise, high breakdown InAlAs/InGaAspseudomorphic High Electron Mobility Transistors (pHEMTs). Theimprovements in breakdown voltage are brought about by a judiciouscombination of epitaxial layer design and field plate techniques. No significant degradations of DC and RF characteristics are observed for devices with field plate structures. An outstanding improvement in breakdown voltages of >30% is attained by field plate devices which should allow their usage in efficient high-added power efficiency amplifiers design
Design and Characterization of Current-Assisted Photonic Demodulators in 0.18-micron CMOS Technology
No full textWe report on the design of a current-assisted photonic
demodulator (CAPD) using standard 0.18-micron complementary
metal–oxide–semiconductor technology and its electrooptical
characterization. The device can perform both light detection
and demodulation in the charge domain, owing to a drift field
generated in the silicon substrate by a majority carrier flow.
Minimum-sized 10 × 10 micron2 CAPDs exhibit a direct-current
charge–transfer efficiency larger than 80% (corresponding to
demodulation contrast larger than 40% under sine-wave modulation)
at the modest power consumption of 10 μW and a 3-dB
bandwidth of>45 MHz. An excellent linearity value with an error
lower than 0.11% is obtained in phase measurements. CAPDs with
optimized modulation electrode geometries are finally designed,
aiming at an improved contrast-versus-power tradeoff
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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