1,720,972 research outputs found
Investigation of High-Electric-Field Degradation Effects in AlGaN/GaN HEMTs
No full textHigh-electric-field degradation phenomena are investigated in GaN-capped AlGaN-GaN HEMTs by comparing experimental data with numerical device simulations. 150-hour DC stresses were carried out under power-state and off-state conditions. Degradations effects characterizing both stress experiments were: a drop in the DC drain current, the amplification of gate-lag effects, and a decrease in the reverse gate leakage current. Numerical simulations indicate that the simultaneous generation of surface (and/or barrier) traps and of buffer traps can account for all of the above degradation modes. Experiments showed also that the power-state stress induced a drop in the transconductance at high gate-source voltages only, whereas the off-state stress led to a uniform transconductance drop over the entire gate-source-voltage range. This behavior can be reproduced by simulations provided that, under power-state stress, traps are assumed to accumulate over a wide region extending laterally from the gate edge towards the drain contact, whereas, under off-state stress, trap generation is supposed to take place in a narrower portion of the drain access region close to the gate edge and to be accompanied by a significant degradation of the channel transport parameters
Physical investigation of high-field degradation mechanisms in GaN/AlGaN/GaN hemts
No full textHigh-electric-field degradation phenomena are investigated in GaN-capped AlGaN-GaN HEMTs by comparing experimental data with numerical device simulations. Simulations indicate that the stress-induced amplification of gate-lag effects and the correlated gate-leakage-current reduction can be ascribed to the generation of acceptor traps at the gate-drain surface. The drop in DC drain current observed after stress should rather be attributed to trap accumulation within the GaN buffer region. Only the simultaneous generation of surface and buffer traps can account for all of the observed degradation modes. © 2006 JEDEC
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Analysis of the physical mechanisms limiting performance and reliability of GaN based HEMTs
No full textCe manuscrit présente les résultats d’une analyse exhaustive des mécanismes physiques qui limitent les performances et la fiabilité des transistors à haute mobilité d’électrons (HEMT) sur nitrure de gallium (GaN). En particulier : • Les phénomènes de dégradation à fort champ électrique des HEMT sur GaN sont analysés en comparant les données expérimentales avec les résultats de simulations physiques. Des stresses DC de 150 heures ont été effectués en conditions de canal ouvert et de pincement. Les effets des dégradations qui ont caractérisé ces deux types de stresses sont les suivants: une chute de courant DC de drain, une amplification des effets de gate-lag, et une diminution du courant inverse de grille. Les simulations physiques indiquent que la génération simultanée de piéges de surface (et/ou barrière) et de volume peut expliquer tous les modes de dégradation décrits plus haut. Les mesures expérimentales ont également montré que le stress en canal ouvert a causé une chute de la transconductance seulement pour de fortes valeurs de la tension VGS, alors que le stress au pincement a provoqué une chute de transconductance uniforme pour toutes les valeurs de VGS. Ce comportement peut être reproduit par la simulation physique pourvu que, dans le cas de stress a canal ouvert, on considère que les piéges s’accumulent au long d’une vaste région qui s’étend latéralement du bord de la grille vers le contact de drain, tandis que, dans le cas du stress au pincement, on considère que la génération des pièges ait lieu dans une portion plus petite de la zone d’accès à proximité de la grille et qu’elle soit accompagnée par une grande dégradation des paramètres de transport du canal. Enfin on propose que les électrons chauds et l’augmentation de la contrainte par le champ électrique soient à l’origine des dégradations observées après les stresses a canal ouvert et au pincement respectivement. • Les piéges dans les HEMT sur GaN ont été caractérisés en utilisant les techniques de DLTS et leur comportement associé de charge/décharge est interprété à l’aide des simulations physiques. Sous certaines conditions de polarisation, les piéges du buffer peuvent produire de faux signaux de piéges de surface, c'est-à-dire, le même type de signaux I-DLTS et ICTS attribués généralement aux piéges de surface. Clarifier cet aspect est très important à la fois pour les tests de fiabilité et pour l’optimisation des dispositifs, car il peut provoquer une identification erronée du mécanisme de dégradation, et par conséquent induire une mauvaise correction des procédés technologiques. • Les mécanismes physiques qui provoquent l’effondrement du courant RF dans les HEMT sur GaN sont analysés par le biais de mesures expérimentales et de simulations physiques. Ce travail propose les conditions suivantes : i) les piéges du buffer aussi bien que ceux de surface peuvent contribuer à l’effondrement du courant RF à travers un mécanisme identique qui impliquerait la capture et l’émission des électrons provenant de la grille; ii) la passivation de la surface diminue considérablement l’effondrement du courant RF par la réduction du champ électrique en surface et la diminution qui en découle de l’injection d’ électrons de la grille vers les pièges ; iii) pour des densités de piéges de surface inférieures à 9 × 1012 cm-2 , des barrières de potentiel superficiels dans l’ordre de 1-2 eV peuvent coexister avec des piéges de surface ayant des énergies plus faibles et qui causent l’effondrement du courant RF caractérisé par des constantes de temps relativement courtes. • Les effets de l’effondrement du courant dans les HEMT sur GaN sont étudiés en utilisant les résultats de mesures expérimentales et de simulations physiques. D’après les mesures pulsées, les dispositifs employés montrent un gate-lag considérable et un drain-lag négligeable qui peuvent être attribués à la présence de piéges de surface et de buffer respectivement.This thesis reports the results of an extensive analysis of the physical mechanisms that limit the performance and reliability of gallium nitride (GaN) based High Electron Mobility Transistors (HEMT). In particular: • High electric field degradation phenomena are investigated in GaN-capped AlGaN/GaN HEMTs by comparing experimental data with numerical device simulations. Under power- and OFF-state conditions, 150-h DC stresses were carried out. Degradation effects characterizing both stress experiments were as follows: a drop in the dc drain current, the amplification of gate-lag effects, and a decrease in the reverse gate leakage current. Numerical simulations indicate that the simultaneous generation of surface (and/or barrier) and buffer traps can account for all of the aforementioned degradation modes. Experiments also showed that the power-state stress induced a drop in the transconductance at high gate–source voltages only, whereas the OFF-state stress led to a uniform transconductance drop over the entire gate-source-voltage range. This behavior can be reproduced by simulations provided that, under the power-state stress, traps are assumed to accumulate over a wide region extending laterally from the gate edge toward the drain contact, whereas, under the OFF-state stress, trap generation is supposed to take place in a narrower portion of the drain-access region close to the gate edge and to be accompanied by a significant degradation of the channel transport parameters. Channel hot electrons and electric-field-induced strain-enhancement are finally suggested to play major roles in power-state and off-state degradation, respectively. • Traps are characterized in AlGaN-GaN HEMTs by means of DLTS techniques and the associated charge/discharge behavior is interpreted with the aid of numerical device simulations. Under specific bias conditions, buffer traps can produce ‘‘false’’ surface-trap signals, i.e. the same type of current-mode DLTS (I DLTS) or gate-lag signals that are generally attributed to surface traps. Clarifying this aspect is important for both reliability testing and device optimization, as it can lead to erroneous identification of the degradation mechanism, thus resulting in wrong correction actions on the technological process. • The physical mechanisms underlying RF current collapse effects in AlGaN-GaN high electron mobility transistors are studied by means of measurements and numerical device simulations. This work suggests the following conclusions: i) both surface and buffer traps can contribute to RF current collapse through a similar physical mechanism involving capture and emission of electrons tunneling from the gate; ii) surface passivation strongly mitigates RF current collapse by reducing the surface electric field and inhibiting electron injection into traps; iii) for surface-trap densities lower than 9 × 1012 cm-2, surface-potential barriers in the 1–2 eV range can coexist with surface traps having much a shallower energy and, therefore, inducing RF current-collapse effects characterized by relatively short time constants. • Current collapse effects are investigated in AlGaN/GaN HEMTs by means of measurements and numerical device simulations. According to pulsed measurements, the adopted devices exhibit a significant gate-lag and a negligible drain-lag ascribed to the presence of surface and buffer traps, respectively. Furthermore, illumination of the devices with two specific wavelengths can result in either a recovering of current collapse or a decrease in the gate current. On the other hand, numerical device simulations suggest that the kink effect can be explained by electron trapping into barrier traps and the subsequent electron emission after a critical electric-field value is reached
Analyse des mécanismes physiques qui limitent les performances et la fiabilité des HEMTs sur GaN
No full textCe manuscrit présente les résultats d’une analyse exhaustive des mécanismes physiques qui limitent les performances et la fiabilité des transistors à haute mobilité d’électrons (HEMT) sur nitrure de gallium (GaN). En particulier : • Les phénomènes de dégradation à fort champ électrique des HEMT sur GaN sont analysés en comparant les données expérimentales avec les résultats de simulations physiques. Des stresses DC de 150 heures ont été effectués en conditions de canal ouvert et de pincement. Les effets des dégradations qui ont caractérisé ces deux types de stresses sont les suivants: une chute de courant DC de drain, une amplification des effets de gate-lag, et une diminution du courant inverse de grille. Les simulations physiques indiquent que la génération simultanée de piéges de surface (et/ou barrière) et de volume peut expliquer tous les modes de dégradation décrits plus haut. Les mesures expérimentales ont également montré que le stress en canal ouvert a causé une chute de la transconductance seulement pour de fortes valeurs de la tension VGS, alors que le stress au pincement a provoqué une chute de transconductance uniforme pour toutes les valeurs de VGS. Ce comportement peut être reproduit par la simulation physique pourvu que, dans le cas de stress a canal ouvert, on considère que les piéges s’accumulent au long d’une vaste région qui s’étend latéralement du bord de la grille vers le contact de drain, tandis que, dans le cas du stress au pincement, on considère que la génération des pièges ait lieu dans une portion plus petite de la zone d’accès à proximité de la grille et qu’elle soit accompagnée par une grande dégradation des paramètres de transport du canal. Enfin on propose que les électrons chauds et l’augmentation de la contrainte par le champ électrique soient à l’origine des dégradations observées après les stresses a canal ouvert et au pincement respectivement. • Les piéges dans les HEMT sur GaN ont été caractérisés en utilisant les techniques de DLTS et leur comportement associé de charge/décharge est interprété à l’aide des simulations physiques. Sous certaines conditions de polarisation, les piéges du buffer peuvent produire de faux signaux de piéges de surface, c'est-à-dire, le même type de signaux I-DLTS et ICTS attribués généralement aux piéges de surface. Clarifier cet aspect est très important à la fois pour les tests de fiabilité et pour l’optimisation des dispositifs, car il peut provoquer une identification erronée du mécanisme de dégradation, et par conséquent induire une mauvaise correction des procédés technologiques. • Les mécanismes physiques qui provoquent l’effondrement du courant RF dans les HEMT sur GaN sont analysés par le biais de mesures expérimentales et de simulations physiques. Ce travail propose les conditions suivantes : i) les piéges du buffer aussi bien que ceux de surface peuvent contribuer à l’effondrement du courant RF à travers un mécanisme identique qui impliquerait la capture et l’émission des électrons provenant de la grille; ii) la passivation de la surface diminue considérablement l’effondrement du courant RF par la réduction du champ électrique en surface et la diminution qui en découle de l’injection d’ électrons de la grille vers les pièges ; iii) pour des densités de piéges de surface inférieures à 9 × 1012 cm-2 , des barrières de potentiel superficiels dans l’ordre de 1-2 eV peuvent coexister avec des piéges de surface ayant des énergies plus faibles et qui causent l’effondrement du courant RF caractérisé par des constantes de temps relativement courtes. • Les effets de l’effondrement du courant dans les HEMT sur GaN sont étudiés en utilisant les résultats de mesures expérimentales et de simulations physiques. D’après les mesures pulsées, les dispositifs employés montrent un gate-lag considérable et un drain-lag négligeable qui peuvent être attribués à la présence de piéges de surface et de buffer respectivement.This thesis reports the results of an extensive analysis of the physical mechanisms that limit the performance and reliability of gallium nitride (GaN) based High Electron Mobility Transistors (HEMT). In particular: • High electric field degradation phenomena are investigated in GaN-capped AlGaN/GaN HEMTs by comparing experimental data with numerical device simulations. Under power- and OFF-state conditions, 150-h DC stresses were carried out. Degradation effects characterizing both stress experiments were as follows: a drop in the dc drain current, the amplification of gate-lag effects, and a decrease in the reverse gate leakage current. Numerical simulations indicate that the simultaneous generation of surface (and/or barrier) and buffer traps can account for all of the aforementioned degradation modes. Experiments also showed that the power-state stress induced a drop in the transconductance at high gate–source voltages only, whereas the OFF-state stress led to a uniform transconductance drop over the entire gate-source-voltage range. This behavior can be reproduced by simulations provided that, under the power-state stress, traps are assumed to accumulate over a wide region extending laterally from the gate edge toward the drain contact, whereas, under the OFF-state stress, trap generation is supposed to take place in a narrower portion of the drain-access region close to the gate edge and to be accompanied by a significant degradation of the channel transport parameters. Channel hot electrons and electric-field-induced strain-enhancement are finally suggested to play major roles in power-state and off-state degradation, respectively. • Traps are characterized in AlGaN-GaN HEMTs by means of DLTS techniques and the associated charge/discharge behavior is interpreted with the aid of numerical device simulations. Under specific bias conditions, buffer traps can produce ‘‘false’’ surface-trap signals, i.e. the same type of current-mode DLTS (I DLTS) or gate-lag signals that are generally attributed to surface traps. Clarifying this aspect is important for both reliability testing and device optimization, as it can lead to erroneous identification of the degradation mechanism, thus resulting in wrong correction actions on the technological process. • The physical mechanisms underlying RF current collapse effects in AlGaN-GaN high electron mobility transistors are studied by means of measurements and numerical device simulations. This work suggests the following conclusions: i) both surface and buffer traps can contribute to RF current collapse through a similar physical mechanism involving capture and emission of electrons tunneling from the gate; ii) surface passivation strongly mitigates RF current collapse by reducing the surface electric field and inhibiting electron injection into traps; iii) for surface-trap densities lower than 9 × 1012 cm-2, surface-potential barriers in the 1–2 eV range can coexist with surface traps having much a shallower energy and, therefore, inducing RF current-collapse effects characterized by relatively short time constants. • Current collapse effects are investigated in AlGaN/GaN HEMTs by means of measurements and numerical device simulations. According to pulsed measurements, the adopted devices exhibit a significant gate-lag and a negligible drain-lag ascribed to the presence of surface and buffer traps, respectively. Furthermore, illumination of the devices with two specific wavelengths can result in either a recovering of current collapse or a decrease in the gate current. On the other hand, numerical device simulations suggest that the kink effect can be explained by electron trapping into barrier traps and the subsequent electron emission after a critical electric-field value is reached.Questa tesi riporta i risultati ottenuti da un’ampia analisi dei meccanismi fisici che limitano le prestazioni e l’affidabilità dei transistor ad alta mobilità di elettroni (HEMT) al nitruro di gallio (GaN). In particolare: • I fenomeni di degradazione ad alto campo elettrico nei GaN/AlGaN/GaN HEMT sono analizzati confrontando i dati sperimentali con i risultati delle simulazioni numeriche. Sono stati effettuati stress DC di 150 ore in condizioni di canale aperto e chiuso. Gli effetti di degradazione che hanno caratterizzato entrambi i tipi di stress sono i seguenti: una caduta nella corrente DC di drain, un’amplificazione degli effetti di gate lag, e una diminuzione della corrente inversa di gate. Le simulazioni numeriche indicano che la generazione simultanea di trappole in superficie (e/o barriera) e buffer può spiegare tutti i suddetti modi di degradazione. Le misure sperimentali hanno mostrato inoltre che lo stress a canale aperto ha causato una caduta della tranconduttanza solo ad alte tensioni VGS, mentre lo stress a canale chiuso ha provocato una caduta della transconduttanza uniforme a tutte le tensioni VGS. Questo comportamento può essere riprodotto con le simulazioni se, nel caso di stress a canale aperto, si assume che le trappole si accumulano lungo un’ampia regione che si estende lateralmente dal bordo di gate verso il contatto di drain, mentre, nel caso di stress a canale chiuso, si suppone che la generazione delle trappole abbia luogo in una più stretta porzione della zona di accesso vicino al bordo di gate e che sia accompagnata da una degradazione significativa dei parametri di trasporto del canale. Infine si propone che gli elettroni caldi del canale e l’aumento di strain indotto dal campo elettrico siano alla base delle degradazioni osservate dopo gli stress a canale aperto e chiuso rispettivamente. • Le trappole in AlGaN-GaN HEMTs sono caratterizzate usando le tecniche di DLTS e il relativo comportamento di carica/scarica é interpretato con l’aiuto delle simulazioni numeriche. Sotto particolari condizioni di polarizzazione, le trappole di buffer possono produrre falsi segnali da trappole di superficie, ossia lo stesso tipo di segnali I-DLTS e forma d’onda di gate lag attribuiti generalmente alle trappole di superficie. Chiarire questo aspetto è molto importante sia per le prove di affidabilità che per l’ottimizzazione dei dispositivi, in quanto può provocare una errata identificazione del meccanismo di degradazione, portando ad azioni correttive sbagliate nell’ottimizzazione del processo tecnologico. • I meccanismi fisici che originano il collasso di corrente RF negli HEMT AlGaN-GaN sono analizzati usando misure sperimentali e simulazioni numeriche. Questo lavoro suggerisce le seguenti condizioni: i) sia le trappole di superficie che quelle di buffer possono contribuire al collasso di corrente RF tramite un simile meccanismo fisico che coinvolge la cattura e l’emissione di elettroni provenienti dal gate; ii) la passivazione della superficie diminuisce fortemente il collasso della corrente RF tramite la riduzione del campo elettrico in superficie e la conseguente diminuzione dell’iniezione di elettroni dal gate alle trappole; iii) per densità di trappole di superficie minori di 9 × 1012 cm-2 , barriere di potenziale superficiale di 1-2 eV possono coesistere con trappole di superficie aventi energie relativamente basse e che provocano effetti di collasso di corrente RF caratterizzati da costanti di tempo relativamente corte. • Gli effetti di collasso di corrente negli HEMT AlGaN-GaN sono studiati usando i risultati delle misure sperimentali e delle simulazioni numeriche. Basandosi sulle misure delle caratteristiche d’uscita impulsate, i dispositivi utilizzati mostrano un evidente gate-lag e un trascurabile drain-lag, attribuiti alla presenza di trappole di superficie e buffer rispettivamente
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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