Ces travaux de recherche se rapportent à l’évaluation des potentialités des transistors à base de graphène ainsi que la mise en évidence des effets dispersifs sur les transistors HEMTs en technologie Nitrure de Gallium. Les principaux résultats issus de ces travaux sont obtenus suite au développement d’un banc de caractérisation spécifique. L’objectif principal pour la caractérisation des transistors en technologie AlGaN/GaN a été de développer des techniques innovantes de caractérisation. Des mesures IV et RF impulsionnelles ont été réalisées afin de caractériser et modéliser les phénomènes de pièges. Cette méthode fournit un moyen efficace d’évaluer les constantes de temps thermiques et de pièges pour la modélisation non linéaire. Ces mesures illustrent ainsi l’impact des effets de pièges sur le comportement dynamique grand signal des transistors GaN. Le second aspect de ces travaux de recherche est axé sur la caractérisation de différents composants à base de graphène afin d’extraire un modèle non linéaire. Des caractérisations DC et HF ont été réalisées. Des structures spécifiques de test ont été fabriquées pour la technique de « de-embedding » permettant l’extraction du modèle non linéaire. La cohérence du modèle électrique a été vérifiée via une comparaison des paramètres S mesurés et simulés. Il est primordial de construire des modèles performants prenant en compte les nouvelles caractéristiques de ces dispositifs dans le but d’établir un lien fort entre les aspects technologiques et systèmes afin d’améliorer les performances HF des transistors à base de graphène. / The aim of this work is to assess the potentialities of Graphene Field Effect Transistors (G-FET) as well as to put in evidence dispersive effects of AlGaN/GaN High Electron Mobility Transistors (HEMTs). The main experimental results of this study have been obtained through the development of an advanced characterization set-up. The main objective for characterization of AlGaN/GaN HEMTs was to develop innovative characterization techniques such as very short pulses and electrical history measurements. Dedicated time-domain pulsed I-V measurements have been performed in order to characterize and model the time dependent trapping phenomena in such devices. The current collapse (Kink effect) and drain lag are directly related to quiescent and instantaneous bias points as well as thermal effects which play a prominent role. This method provides an efficient way to assess the different thermal and trapping time constants for the nonlinear modeling. The second aspect of this research work was the characterization of several graphene-based devices in order to assess the potentialities of such transistors and to derive a nonlinear device model. DC and high frequency characterization were performed. Specific test structures fabricated for accurate de-embedding at high frequencies along with the nonlinear model extraction were detailed in this work. This electrical model consistency has been checked through the comparison of measured and simulated multi-bias S-parameters. For this new material with outstanding electrical properties and promising capabilities, material and technological process are still subject to intensive research activities to improve high frequency graphene FET performances.

Les systèmes de transmission de l’information sans fil connaissent un essor considérable et sont intégrés dans la plupart des systèmes électroniques modernes. De manière plus spécifique, la consommation énergétique de la fonction amplification de puissance RF, qui constitue le cœur de ce travail de recherche, est un enjeu économique et écologique de premier plan. Dans ce sens, ce travail présente une architecture de polarisation de drain dynamique permettant de maintenir un rendement énergétique élevé sur une large dynamique de puissance de sortie. La conception et la réalisation d’un amplificateur de puissance RF large bande, d’un modulateur de polarisation de drain haute fréquence et d’un pilote de grille en technologie GaN sont présentés. L’architecture proposée démontre une amélioration du rendement énergétique global. Une focalisation sur la problématique de couplage non-linéaire entre l’amplificateur de puissance RF et le module d’alimentation agile met en évidence les répercussions de cette méthode sur l’intégrité du signal. Une étroite impulsion de polarisation de grille est appliquée afin d'atténuer l’impact de la polarisation dynamique de drain sur les formes d'onde de l'enveloppe du signal RF amplifié. Une validation expérimentale du démonstrateur proposée est effectuée pour un signal impulsionnel RF multi-niveaux de test. Cette méthode permet de maintenir un facteur de forme de l’enveloppe du signal de sortie RF quasi-rectangulaire sans impact majeur sur les performances globales énergétiques. / Wireless communications are experiencing tremendous growth and are integrated into most modern electronic systems. More precisely, saving energy consumption of RF power amplifier is the core of this thesis work. This work presents a dynamic drain bias architecture used to keep a high efficiency over a large output power range. Design and implementation of a wideband RF power amplifier, a drain supply modulator and a gate driver circuit in GaN technology are presented. The built-in prototype demonstrates an overall efficiency improvement. A specific focus on non-linear interaction between the RF power amplifier and the drain supply modulator highlights the effects of this technique on the output envelope signal shape. A narrow pulse gate bias peaking preceding drain bias voltage variations is applied in order to mitigate drain bias current, voltage overshoot and power droop, thus improving pulse envelope waveforms of the RF output signal. An experimental validation of the proposed demonstrator is performed for a RF pulsed test sequence having different power levels. This way enables to keep rectangular pulse envelope shape at the RF output signal without any major impact on overall efficiency performances.

GaN-HEMTs (Gallium Nitride-based High Electron Mobility Transistors) have, thanks to the large band gap of GaN, electrical properties that are suitable for applications of high electrical voltages, high currents, and fast switching. The large band gap also gives GaN-HEMTs a high resistance to radiation. In this degree project, the effects of 2 MeV proton irradiation of GaN-HEMTs constructed on both silicon carbide and silicon substrates are investigated. 20 transistors per substrate were irradiated in the particle accelerator 5 MV NEC Pelletron in the Ångström laboratory at Uppsala University. These transistors were exposed to radiation doses in the range of 10^11 to 10^15 protons/cm^2. The analysis shows that both transistors on silicon, as well as silicon carbide, are unaffected by proton irradiation up to a dose of 10^14 protons/cm^2. GaN-on-Si transistors show less influence of radiation than GaN-on-SiC transistors. The capacitances between gate and drain as well as drain and source for both GaN-on-SiC and GaN-on-Si HEMTs show hysteresis as a function of forward and backward gate voltage sweeps for the radiation dose of 10^15 protons/cm^2. / GaN-HEMTs (Galliumnitridbaserade High Electron Mobility Transistors) har tack vare det stora bandgapet i GaN goda elektriska egenskaper som lämpar sig för höga elektriska spänningar, höga strömmar och snabb växling mellan av- och på-tillstånd. Det stora bandgapet ger även GaN-HEMTs ett stort motstånd mot strålning.I detta examensarbete undersöks effekterna av 2 MeV protonbestrålning av GaN-HEMTs. Dessa HEMTs är konstruerade på både kiselkarbid- och kiselsubstrat.20 transistorer per transistorsubstrat bestrålades i partikelacceleratorn 5 MV NEC Pelletron i Ångströmslaboratoriet vid Uppsala Universitet. Dessa transistorer utsattes för strålningsdoser inom intervallet 10^11 till 10^15 protoner/cm^2. Resultaten visar att både tranisistorer på kisel såsom kiselkarbid är opåverkade av strålning upp till en dos av 10^14 protoner/cm^2. GaN-på-Si-transistorer visar en mindre påverkan av protonstrålning än GaN-på-SiC-transistorer. Ytterligare uppstod hysteresis för kapacitanser mellan gate och drain och mellan gate och source som en funktion av fram- och bakriktad gate-spänning efter en strålningsdos av 10^15 protoner/cm^2.

International Telemetering Conference Proceedings / October 26-29, 1987 / Town and Country Hotel, San Diego, California / The Very Large Array (VLA) radio telescope, located in west-central New Mexico, obtains high-resolution radio images of astronomical objects by using Fourier aperture synthesis with 27 antennas. With the addition of X-band to its receiving capabilities by 1989, and when arrayed with the Goldstone Deep Space Communications Complex (GDSCC), the VLA will double the Deep Space Network (DSN) receiving aperture in the U. S. longitude for signals from Voyager 2 at Neptune. This paper describes the VLA and the installation of the X-band system, its operation and performance for Voyager data reception, and its capabilities for other science at X-band.

AlGaN/GaN high electron mobility transistors (HEMTs) have been widely used for high power and high frequency RF communications due to their fast switching and large current handling capabilities. The reliability of such devices is strongly affected by the junction temperature where the highest magnitude occurs in a local region on the drain side edge of the gate called the hotspot. Thus, thermal management of these devices remains a major concern in the design and reliability of systems employing AlGaN/GaN HEMTs. Due to the large power densities induced in these devices locally near the drain side edge of the gate, it is clear that moving thermal management solutions closer to the heat generation region is critical in order to reduce the overall junction temperature of the device. In this work, we explore the use of embedded microchannel cooling in the substrate of AlGaN/GaN HEMTs made on Si and SiC substrates and compare them to passive cooling techniques using Si, SiC, and diamond substrates. In addition, the impact of cooling fluids and harsh environmental conditions were considered. The study was performed using a combination of CFD and finite volume analysis on packaged AlGaN/GaN HEMTs. Active cooling using embedded microchannels were shown to have a significant impact on the heat dissipation over the passive cooling methods, approaching or exceeding that of diamond cooled devices. For vertical power devices (IGBT), embedded microchannels in the power electronics substrates were explored. In both the power devices and lateral AlGaN/GaN HEMTs, the use of embedded microchannels with nonlinear channel geometries was shown to be the most effective in terms of reducing the device junction temperature while minimizing the pumping power required.

L’avènement des technologies itinérantes s’est accompagné de l’accroissement des besoins en autonomie des appareils électroniques mobiles. De plus la forte densité d’intégration des dispositifs conduit aux limites de dissipation thermique des systèmes de refroidissement classiques. Ainsi dans cette thèse le régime de fonctionnement bas Vds a été investigué dans les HEMTs à base de matériaux faibles gaps afin de réduire les puissances consommées. Cette étude est réalisée à 300K et 77K. Concernant les HEMTs à base d’antimoine des performances records ont été publiées à 300K (fT = 144GHz à 100mV pour Lg=120nm). En termes de performances en bruit à la fois pour les HEMTs industriels étudiés et les HEMTs à base d’antimoine à 30GHz et Vds=100mV un NFmin autour de 1.6dB et un Gass de l’ordre de 6dB sont atteints pour des puissances dissipées inférieures à 10mW/mm. Enfin des dimensionnements d’amplificateurs faibles bruit ont été réalisé afin d’évaluer la potentielle réalisation d’une électronique basse consommation à température ambiante et cryogénique. Ces dimensionnements ont permis d’obtenir à 100mV de tension drain un NF autour de 1,7dB pour un gain associé avoisinant 6dB avec une puissance dissipée inférieure à 6mW/mm à température ambiante et à basse température (77K) la valeur de NF est autour de 0,6dB pour un gain associé supérieur à 7dB. / Advent of itinerant technologies was done with the autonomy needs increase in the mobile electronics devices. In addition the high devices integration density leads to the thermal dissipation limits of the classic cooling systems. Thus in this thesis the low Vds operation mode is investigated in narrow band gap materials based HEMTs in order to reduce the power consumption. This study is done at room temperature (300K) and at cryogenic temperature (77K). Concerning antimonide based HEMTs, record performances was published at 300K (fT = 144GHz à 100mV for Lg=120nm). In terms of noise performances both of industrial studied HEMTs and antimonide based HEMTs present at 30GHz and Vds=100mV NFmin value around 1.6dB and Gass around 6dB for power dissipation lower than 10mW/mm. Finally low noise amplifier (LNA) design is done in order to evaluate the potential use of those devices in low power consumption electronics at room and cryogenic temperature. Those designs allowed to obtain at Vds=100mV NF value around 1.7dB and Gass around 6dB with power dissipation lower than 6mW/mm at room temperature and at cryogenic temperature NF value around 0,6dB for Gass value higher than 7dB.

To fully exploit the potential of gallium nitride (GaN) devices for optoelectronics and power electronic applications, the structures of device need to be investigated and optimized. In particular carrier densities, conductivities and localised charges can have a significant impact to device performances. Electrical scanning probe microscopy techniques, including scanning capacitance microscopy (SCM), conductive atomic force microscopy (C-AFM) and kelvin probe force microscopy (KPFM), were utilized to study the structures of nitride devices such as high electron mobility transistors (HEMTs), light emitting diodes (LEDs) and junction diodes. These results combine with other characterisation techniques to give an enhanced understanding about the nitride structures. Leakage currents are one of the major challenges in HEMTs, especially leakages in buffer layers which deteriorate the breakdown voltage of the devices. To achieve an insulating buffer layer, carbon doping is usually used to compensate the unintentional n-type doping of nitride materials. Here, I show that vertical leakage can originate from the formation of inverted hexagonal pyramidal defects during the low temperature growth of an AlGaN:C strain relief layer. The semi-polar facets of the defects enhanced the oxygen incorporation and led to the formation of leakage pathways which were observed using SCM. Leakage occurring at HEMT surfaces will lead to current collapses of devices. In this work, I discovered nano-cracks on a HEMT surface. C-AFM showed enhanced conductivity along these nano-cracks. A model based on stress relaxation analysis was proposed to explain the drop of surface potential along the nano-cracks. Advances in the quality of epitaxial GaN grown by MOVPE have been facilitated by understanding the formation of defects within the materials and structures. However, hillocks as a specific type of defects have not been intensively studied yet. In this work, three types of hillocks were discovered on GaN p-i-n diodes and a GaN film grown on patterned sapphire substrates. It was found that pits were always present around the centres of hillocks. Multi-microscopy results showed these pits were developed from either an inversion domain or a nano-pipe or a void under the sample surface. Formation of hillocks was usually associated with a change of growth condition, such as an increase in Mg doping or a decrease in growth temperature and gas flows, despite the formation mechanism is still unclear. GaN$_{1-x}$As$_x$ is a highly mismatched alloy semiconductor whose band-gap can be engineered across the whole visible spectrum. For this reason and the potential to achieve high p-type doping, GaN$_{1-x}$As$_x$ is a promising material for optoelectronic applications. However, the growth of GaN$_{1-x}$As$_{x}$ at intermediate As fraction while maintaining a high conductivity and uniformity of the material is still challenging. Two n-GaN/p-GaN$_{1-x}$As$_x$ diodes with different Ga flows were investigated. Both samples demonstrated that highly Mg-doped GaN$_{1-x}$As$_x$ with high As fraction is achievable. However, the samples contained both amorphous and polycrystalline regions. The electrical scanning probe microscopy results suggested the amorphous structure has a lower hole concentration and hence conductivity than the polycrystalline structure. Nevertheless, there is still a lack of understanding about the electrical properties and conduction mechanisms of the GaN$_{1-x}$As$_x$ alloy.

Les transistors à haute mobilité électronique (HEMTs) à base de nitrure de gallium constituent une filière prometteuse pour l’amplification de puissance hyperfréquence pour les applications en bande millimétrique. Les propriétés remarquables du GaN, tels que le champ de claquage , la vitesse de saturation et la densité des électrons élevés sont à l’origine des performances exceptionnelles obtenues avec les dispositifs à base de GaN. Les travaux de thèse ont été réalisés au sein du groupe Composants et Dispositifs Micro-ondes de Puissance à l’IEMN. Ce travail relate la fabrication et la caractérisation de dispositifs de type HEMT de la filière GaN pour des applications de puissance hyperfréquence. La première partie de ce travail expose les phénomènes physiques mis en jeu dans les hétérostructures à base de GaN. La suite porte sur l’optimisation des procédés technologiques ayant comme point de mire la montée en fréquence ainsi qu’en puissance hyperfréquence. Un travail a été mené en vue de la réduction de la longueur du pied de grille permettant d’atteindre des longueurs minimales de l’ordre de 60nm. De plus, des analyses sont effectuées afin d’étudier les principales limitations inhérentes aux composants HEMTs. Le dernier chapitre présente l’ensemble des caractérisations en régimes statique et hyperfréquence sur des structures HEMTs fabriquées dans le cadre de ce travail. Il en ressort notamment un résultat en terme de densité de puissance à 40GHz, à ce jour à l’état de l’art, relatif à un HEMT de topologie 2x50x0.075µm2. Celui-ci ayant permis d’obtenir une densité de puissance de 2.7W/mm associée à un gain linéaire de 6.5dB et un rendement en puissance ajoutée de 12.5%. / Gallium Nitride (GaN) based High Electron Mobility Transistors (HEMTs) have emerged as the best candidate for high temperature, high voltage and high power operation in millimeter-wave range. The unique combination of high breakdown field, high electron velocity, and large sheet electron densities of III-N material permits outstanding performance. The work was performed within IEMN laboratory in Microwave Power Devices group. It relates the fabrication and the characterization of GaN HEMT devices for microwave power applications. The first part exposes the physical and electrical properties of gallium nitride as well as a review concerning the state of the art in terms of output power density related to GaN HEMTs. The second chapter deals with the technological processes with a particular attention on the process optimization regarding the ohmic contact and the T-gate technology. Despite outstanding properties, the HEMT performance remains inherently limited by physical and electrical parasitic phenomena. Thus, the third chapter presents the whole studies performed in other to understand these limitation effects (losses, traps, thermal effect). In the last chapter DC, RF, pulsed and large signal measurements are reported for HEMTs based on different heterostructures. In particular, the capability of AlGaN/GaN transistors on Si(111) substrate grown by MBE is demonstrated for high frequency microwave power applications at 40GHz with a continuous wave output power density of 2.7W/mm associated with a power added efficiency of 12.5% and a linear gain of 6.5dB corresponding to the highest saturated power density ever reported on Si(111) substrate to date.

Des avancées significatives dans la synthèse des matériaux semiconducteurs à large bande interdite de la famille de GaN permettent aujourd'hui la réalisation de dispositifs optoélectroniques (diodes, lasers) mais aussi celle de dispositifs électroniques (transistor). L'absence de substrat natif GaN ou AlN a pour conséquence le recours à l'hétéroépitaxie sur des substrats de nature différente comme le silicium qui présente un grand intérêt de par son prix très compétitif, la taille des substrats disponibles et sa conductivité thermique. L'orientation (111) du silicium est préférée en raison d'une symétrie de surface hexagonale, compatible avec la phase wurtzite du GaN. Néanmoins, les différences de paramètres de maille et de coefficients d'expansion thermique génèrent des défauts cristallins et des contraintes dans les matériaux élaborés qui peuvent, s'ils ne sont pas maîtrisés, dégrader les performances des dispositifs. Ce travail de thèse a porté sur la croissance par épitaxie par jets moléculaires (EJM) d'hétérostructures à base de GaN sur substrat Si(111) en vue de la réalisation de transistors à haute mobilité d'électrons (Al,Ga)N/GaN. Ce travail avait pour objectif l'identification des paramètres de croissance susceptibles d'avoir un impact notable sur la qualité structurale et électrique de la structure HEMT (High Electron Mobility Transistor), et notamment sur l'isolation électrique des couches tampon et le transport des électrons dans le canal. Nous montrerons l'impact notable de certains paramètres de la croissance sur la qualité structurale et électrique de la structure HEMT. Nous verrons comment la relaxation des contraintes est liée au dessin d'empilement des couches, à leurs conditions d'élaboration et à la densité de défauts.

SiC MESFETs and GaN HEMTs have an enormous potential in high-power amplifiers at microwave frequencies due to their wide bandgap features of high electric breakdown field strength, high electron saturation velocity and high operating temperature. The high power density combined with the comparably high impedance attainable by these devices also offers new possibilities for wideband power microwave systems. In this thesis, Class C switching response of SiC MESFET in TCAD and two different generations of broadband power amplifiers have been designed, fabricated and characterized. Input and output matching networks and shunt feedback topology based on microstrip and lumped components have been designed, to increase the bandwidth and to improve the stability. The first amplifier is a single stage 26-watt using a SiC MESFET covering the frequency from 200-500 MHz is designed and fabricated. Typical results at 50 V drain bias for the whole band are, 22 dB power gain, 43 dBm output power, minimum power added efficiency at P 1dB is 47 % at 200 MHz and maximum 60 % at 500 MHz and the IMD3 level at 10 dB back-off from P 1dB is below ‑45 dBc. The results at 60 V drain bias at 500 MHz are, 24.9 dB power gain, 44.15 dBm output power (26 W) and 66 % PAE. In the second phase, two power amplifiers at 0.7-1.8 GHz without feed back for SiC MESFET and with feedback for GaN HEMT are designed and fabricated (both these transistors were of 10 W). The measured maximum output power for the SiC amplifier at Vd = 48 V was 41.3 dBm (~13.7 W), with a PAE of 32 % and a power gain above 10 dB. At a drain bias of Vd= 66 V at 700 MHz the Pmax was 42.2 dBm (~16.6 W) with a PAE of 34.4 %. The measured results for GaN amplifier are; maximum output power at Vd = 48 V is 40 dBm (~10 W), with a PAE of 34 % and a power gain above 10 dB. The SiC amplifier gives better results than for GaN amplifier for the same 10 W transistor. A comparison between the physical simulations and measured device characteristics has also been carried out. A novel and efficient way to extend the physical simulations to large signal high frequency domain was developed in our group, is further extended to study the class-C switching response of the devices. By the extended technique the switching losses, power density and PAE in the dynamics of the SiC MESFET transistor at four different frequencies of 500 MHz, 1, 2 and 3 GHz during large signal operation and the source of switching losses in the device structure was investigated. The results obtained at 500 MHz are, PAE of 78.3%, a power density of 2.5 W/mm with a switching loss of 0.69 W/mm. Typical results at 3 GHz are, PAE of 53.4 %, a power density of 1.7 W/mm with a switching loss of 1.52 W/mm. / Report code: LIU-TEK-LIC-2008:32