Le travail de thèse consiste à étudier des composants de la filière nitrure de gallium à partir d’études électriques et d’un modèle physico-thermique. Les dispositifs de cette filière sont très prometteurs pour des applications de puissance en hyperfréquence. Cependant, leurs performances électriques sont limitées par deux causes principales : La première cause est liée à la réalisation des contacts. Dans ce travail, nous avons étudié des contacts Schottky TiN sur hétérostructures AlGaN/GaN sur substrats Si (111) réalisés par pulvérisation magnétron. Une analyse détaillée des paramètres obtenus, tels que la hauteur de barrière, le coefficient d’idéalité et le courant de fuite en polarisation inverse, permet d’optimiser la topologie et le procédé technologique, tels que la température et la durée de recuit, la passivation et le prétraitement de surface. La théorie relative aux mécanismes de conduction à travers le contact est aussi rappelée, montrant que l’effet tunnel assisté par champ électrique et le courant limité par charge d’espace sont les mécanismes dominants. La seconde cause est liée à l’effet d’auto-échauffement important dans les composants de la filière GaNcompte tenu des fortes puissances dissipées, ce qui dégrade leurs performances électriques ainsi que la fiabilité. Dans ce cadre, un modèle physico-thermique basé sur le couplage d’un modèle énergie-balance avec un modèle thermique a été développé. Ce modèle prend en compte la température de réseau en tout point du composant et décrit bien les performances électriques et thermiques des composants de cette filière. Grâce au modèle développé, nous avons d’abord analysé les hétérostructures AlGaN/GaN et InAlN/GaN sur différents substrats à partir de structures TLM, afin d’évaluer leurs performances électriques et thermiques, et ainsi d’optimiser le choix des substrats. Nous avons également étudié les diodes Gunn de la filière GaN avec différentes topologies, ce qui a permis d’optimiser une structure en termes de fréquence d’oscillations et de conversion de puissance, en prenant en compte les effets thermiques. Après une comparaison entre les résultats de simulation et ceux mesurés, il s’avère que le modèle physico-thermique est un outil de prédiction précis et fiable, extrêmement utile pour les technologues et qui permet en outre une meilleure compréhension des phénomènes physiques observés. / The work of this thesis is dedicated to study gallium nitride based components by means of electric studies and a physical-thermal model. The GaN based devices are very promising for high-frequency microwave power applications. However, their electric performances are limited by two principal causes: The first cause is related to the contacts realization. In this work, we studied TiN Schottky contacts on AlGaN/GaN heterostructures on Si (111) substrates realized by magnetron spray. A detailed analysis of the obtained parameters, such as the barrier height, the ideality factor and the reverse leakage current, permits optimizing the topology and the technological processes, such as the annealing temperature and time, thepassivation and the surface pre-etching. The theory related to the conduction mechanisms through the contact is also recalled, showing that the electric field assisted tunnel effect and the space charge limited current are the dominant mechanisms. The second cause is related to the important self-heating effect in the GaN based components inconsideration of the high dissipated power, which degrades the electric performances and the reliability as well. In this framework, a physical-thermal model based on the coupling of an energy-balance model with a thermal model was developed. Such a model takes into account the lattice temperature everywhere in the device and describes the electric and thermal performances of GaN based components. Thanks to the developed model, firstly the AlGaN/GaN and InAlN/GaN heterostructures were analyzed on different substrates by means of TLM patterns, in order to evaluate their electric and thermal performances so as to optimize the substrate choice. The GaN based Gunn diodes with different topologies were also studied with the goal to optimize a structure in terms of frequency oscillation and power conversion, taking into account the thermal effects. After a comparison between the simulation results and the measured ones, it is proved that the physical-thermal model is an accurate and reliable predictive tool, which is extremely useful for the technologists and furthermore, permits a better understanding of the observed physical phenomena.

No description available.

Texte remanié de : Thèse de doctorat : Études anglaises : Brest : 1998 : Quête d'identité et histoire dans l'oeuvre de Neil M. Gunn. / Bibliogr. p. [195]-209. Index.

The thesis describes measurements of the velocity-field characteristic of indium phosphide, below threshold by a direct method and above threshold by domain measurements. Comparisons, with good agreement, are made with microwave measurements, below threshold on material from the same slices, and above threshold on material with very similar properties. A historical description of the Gunn effect and domain theory precedes a description of the structure of indium phosphide and a survey of published velocity-field calculations and measurements, showing the difference between 2- and 3-level conduction band models. A value for the T valley deformation potential has previously been deduced from the temperature variation of low-field mobility by adding reciprocal mobilities due to different scattering processes. This method is examined and experimental results of other workers are shown consistent with a lower deformation potential than supposed. Sub-threshold results agree closely with other, published, measurements, using various techniques. Extended to 77 K, the subthreshold method gave results agreeing reasonably with predictions for this temperature, and very well with extrapolations from other calculated and measured data. Published high-field domain measurements made with a pointcontact probe differ greatly from others. Experiments to produce a point-contact probe and associated differentiator with a known performance, and analysis of a simple circuit model indicate that the specimen resistivity controls the probe response, necessitating different probe resistances for different material resistivities. The probe was still found fundamentally difficult to use and is suspected of causing specimen damage due to localised heating under the point. Domain shapes measured agreed with published measurements from capacitive probes, but domain velocity was higher, giving a higher valley velocity (0.76 - 0.99 x 1O ⁷ cm/s). Comparison of the deduced velocity-field curve with published calculations strongly supports a 2-level transfer mechanism.

Thesis (Ph.D.) - University of Glasgow, 1982. / Includes bibliographical references. Print version also available.

Thesis (Ph.D.) - University of Glasgow, 2007. / Ph.D. thesis submitted to the Department of Scottish Literature, University of Glasgow, 2007. Includes bibliographical references. Print version also available.

My thesis examines the early and middle periods of Neil M. Gunn’s writing career in the context of contemporaneous debates and discourses emergent in Scottish political and cultural nationalism. I locate my thesis within a new, broad development in Scottish Studies which is adopting more rigorously analytical, interdisciplinary and theorised models of interpretation. The first chapter examines Gunn’s own nationalism in the light of other contemporaneous Scottish nationalisms and assert that it is moderate in tone but radical, being based on a model of cultural repression / resistance. I examine current theoretical approaches to the study of nationalism and adopt the analytical methods of Anthony D. Smith’s ethno-symbolism. The second chapter examines Gunn’s used of racial figures of speech and concludes that he carefully constructs a politicised account of Scotland’s early history. This account is predicated on a theory of racial essentialism communicated through the visual clue of race. The third chapter examines Gunn’s racial tropes alongside those of D. H. Lawrence and fellow Scottish novelist James Leslie Mitchell (Lewis Grassic Gibbon). I demonstrate how they share an interest in aesthetic primitivism. All three writers adopt radical political positions based on the rejection of ‘whiteness’ and modernity. The last chapter examines Gunn from the perspective of current landscape theory, and analyses how his use of what Denis E, Cosgrove calls ‘inside’ and ‘outside’ positions is figured in his novels, and in his contribution to the Highland Hydro-Electric debates of the 1930s and 1940s. I conclude that Gunn is a profoundly political writer and urge a reassessment of his oeuvre in this light.

Self-heating can have a detrimental effect on the performance and reliability of high power microwave devices. In this work, the thermal performance of the gallium arsenide (GaAs) Gunn diode was studied. Infrared (IR) thermal microscopy was used to measure the peak operating temperature of the graded-gap structured device. Temperature measurements were experimentally validated using micro-thermocouple probing and compared to values obtained from a standard 1D thermal resistance model. Thermal analysis of the conventionally structured Gunn diode was also undertaken using high resolution micro-Raman temperature profiling, IR thermal microscopy and electro/thermal finite element modeling. The accuracy of conventional IR temperature measurements, made on semiconductor devices, was investigated in detail. Significant temperature errors were shown to occur in IR temperature measurements made on IR transparent semiconductors layers and low emissivity/highly reflective metals. A new technique, employing spherical carbon microparticles, was developed to improve the measurement accuracy on such surfaces. The new ‘IR microparticle’ technique can be used with existing IR microscopes and potentially removes the need to coat a device with a high emissivity layer, which causes damage and heat spreading.

No description available.

Monte Carlo modelling is a common technique in numerous fields, and is widely used in semiconductor device simulation. This thesis describes the application of Monte Carlo modelling to the simulation of Gunn diode devices, focusing on devices composed of Gallium Arsenide (GaAs) and Gallium Nitride (GaN). Gunn diodes are simple structures that take advantage of negative differential resistance to act as a source of high frequency radiation, from 10 GHz to over 100 GHz in GaAs devices. It has been theorised that GaN should exhibit negative differential resistance and a GaN Gunn diode could produce radiation of even higher frequency, within the terahertz band. Gunn diodes have the advantage of being cheap and portable, and so are worth exploring as such a source. Unfortunately, GaN devices have a high electron density and so they tend to generate heat quickly. It therefore becomes important to include modelling of heat generation and flow in simulations of these devices. This is uncommon in Monte Carlo models of Gunn diodes, as in less highly doped devices thermal effects can usually be assumed to result in the device reaching an equilibrium temperature of about 100 K above the ambient. This thesis describes the creation of a model to track the generation and distribution of heat during operation of a GaN device. Simulations found that thermal effects within the device were significant. Heat generation occurred to the extent that the device could only be operated in pulsed mode, with on pulses of 2 ns requiring 50 ns of cooling for sustainable operation. The increased temperature within the device also lead to deleterious changes in the Gunn diode's operating frequency. In the simulated device, a 150 K change in temperature lead to a decrease in operating frequency of 40 GHz, from an initial frequency of 280 GHz. At the end of 2 ns of operation, the mean temperature within the device had increased by 120 K. The high accidental doping level in GaN also means the use of a doping notch to act as a nucleation point for dipoles within a Gunn diode, a common technique in other materials, becomes less feasible. As an alternative to a notch, a device was simulated incorporating a doping spike to nucleate the dipole. The use of a doping spike is not novel, however its use in GaN has not been previously explored. Simulations found that a fully-depleted p-type doping notch of length 2.1 nm, doped at 1x1024 m-3 would act as a nucleation point for dipole operation. The device was compared to a simulated device incorporating a doping notch of width 0.25 µm doped at 0.5x1023 m-3 and found to operate at a similar frequency and RF efficiency, making it a viable substitute. One limitation of Gunn diodes is that when operated in transit-time mode, the operating frequency is determined by the length of the diode's transit region and so is well-defined and fixed. This means that traditional Gunn diodes are not as useful a source of radiation for spectroscopic applications as might be desirable. Recent experimental results for planar devices have shown a broadening in operation frequency and even multiple frequencies. This thesis explores the hypothesis that such a broadening might be achieved in a vertical structure via the incorporation of an additional notch into the Gunn diode's transit region, effectively incorporating two transit regions into the device. Results showed that this novel device structure did show multiple modes of operation. Under a DC applied voltage, the device showed spontaneous switching behaviour, oscillating between dipole and accumulation layer operation from the second notch. Changes in the frequency of an applied RF voltage would shift the device from operating from the first or second notch, in dipole and accumulation layer mode respectively.