Thermal effects in Ni/Au and Mo/Au gate metallization AlGaN/GaN HEMT's reliability

AlGaN/GaN high electron mobility transistors (HEMT) are key devices for the next generation of high-power, high-frequency and high-temperature electronics applications. Although significant progress has been recently achieved [1], stability and reliability are still some of the main issues under investigation, particularly at high temperatures [2-3]. Taking into account that the gate contact metallization is one of the weakest points in AlGaN/GaN HEMTs, the reliability of Ni, Mo, Pt and refractory metal gates is crucial [4-6]. This work has been focused on the thermal stress and reliability assessment of AlGaN/GaN HEMTs. After an unbiased storage at 350 o C for 2000 hours, devices with Ni/Au gates exhibited detrimental IDS-VDS degradation in pulsed mode. In contrast, devices with Mo/Au gates showed no degradation after similar storage conditions. Further capacitance-voltage characterization as a function of temperature and frequency revealed two distinct trap-related effects in both kinds of devices. At low frequency (< 1MHz), increased capacitance near the threshold voltage was present at high temperatures and more pronounced for the Ni/Au gate HEMT and as the frequency is lower. Such an anomalous “bump” has been previously related to H-related surface polar charges [7]. This anomalous behavior in the C-V characteristics was also observed in Mo/Au gate HEMTs after 1000 h at a calculated channel temperatures of around from 250 o C (T2) up to 320 ºC (T4), under a DC bias (VDS= 25 V, IDS= 420 mA/mm) (DC-life test). The devices showed a higher “bump” as the channel temperature is higher (Fig. 1). At 1 MHz, the higher C-V curve slope of the Ni/Au gated HEMTs indicated higher trap density than Mo/Au metallization (Fig. 2). These results highlight that temperature is an acceleration factor in the device degradation, in good agreement with [3]. Interface state density analysis is being performed in order to estimate the trap density and activation energy.

Predictor paramétrico de glucosa subcutánea

La diabetes mellitus es una enfermedad que se caracteriza por la nula o insuficiente producción de insulina, o la resistencia del organismo a la misma. La insulina es una hormona que ayuda a que la glucosa (por ejemplo la obtenida a partir de los alimentos ingeridos) llegue a los tejidos periféricos y al sistema nervioso para suministrar energía. Hoy en día la tecnología actual permite abordar el desarrollo del llamado “páncreas endocrino artificial”, que consta de un sensor continuo de glucosa subcutánea, una bomba de infusión subcutánea de insulina y un algoritmo de control en lazo cerrado que calcule la dosis de insulina requerida por el paciente en cada momento, según la medida de glucosa obtenida por el sensor y según unos objetivos. El mayor problema que presentan los sistemas de control en lazo cerrado son los retardos, el sensor de glucosa subcutánea mide la glucosa del líquido intersticial, que representa la que hubo en la sangre un tiempo atrás, por tanto, un cambio en los niveles de glucosa en la sangre, debidos por ejemplo, a una ingesta, tardaría un tiempo en ser detectado por el sensor. Además, una dosis de insulina suministrada al paciente, tarda un tiempo aproximado de 20-30 minutos para la llegar a la sangre. Para evitar trabajar en la medida que sea posible con estos retardos, se intenta predecir cuál será el nivel de glucosa en un futuro próximo, para ello se utilizara un predictor de glucosa subcutánea, con la información disponible de glucosa e insulina. El objetivo del proyecto es diseñar una metodología para estimar el valor futuro de los niveles de glucosa obtenida a partir de un sensor subcutáneo, basada en la identificación recursiva del sistema glucorregulatorio a través de modelos lineales y determinando un horizonte de predicción óptimo de trabajo y analizando la influencia de la insulina en los resultados de la predicción. Se ha implementado un predictor paramétrico basado en un modelo autorregresivo ARX que predice con mejor precisión y con menor RMSE que un predictor ZOH a un horizonte de predicción de treinta minutos. Utilizar información relativa a la insulina no tiene efecto en la predicción. El preprocesado, postprocesado y el tratamiento de la estabilidad tienen un efecto muy beneficioso en la predicción. Diabetes mellitusis a group of metabolic diseases in which a person has high blood sugar, either because the body does not produce enough insulin, or because cells do not respond to the insulin produced. The insulin is a hormone that helps the glucose to reach to outlying tissues and the nervous system to supply energy. Nowadays, the actual technology allows raising the development of the “artificial endocrine pancreas”. It involves a continuous glucose sensor, an insulin bump, and a full closed loop algorithm that calculate the insulin units required by patient at any time, according to the glucose measure obtained by the sensor and any target. The main problem of the full closed loop systems is the delays, the glucose sensor measures the glucose in the interstitial fluid that represents the glucose was in the blood some time ago. Because of this, a change in the glucose in blood would take some time to be detected by the sensor. In addition, insulin units administered by a patient take about 20-30 minutes to reach the blood stream. In order to avoid this effect, it will try to predict the glucose level in the near future. To do that, a subcutaneous glucose predictor is used to predict the future glucose with the information about insulin and glucose. The goal of the proyect is to design a method in order to estimate the future valor of glucose obtained by a subcutaneous sensor. It is based on the recursive identification of the regulatory system through the linear models, determining optimal prediction horizon and analyzing the influence of insuline on the prediction results. A parametric predictor based in ARX autoregressive model predicts with better precision and with lesser RMSE than ZOH predictor in a thirty minutes prediction horizon. Using the relative insulin information has no effect in the prediction. The preprocessing, the postprocessing and the stability treatment have many advantages in the prediction.

Theory of a Probe in a Strong Magnetic Field. AT(30-1)-1238-MATT 599

A kinetic approach is used to develop a theory of electrostatic probes in a fully ionized plasma in the presence of a magnetic field. A consistent asymptotic expansion is obtained assuming that the electron Larmor radius is small compared to the radius of the probe. The order of magnitude of neglected terms is given. It is found that the electric potential within the tube of force defined by the cross section of the probe decays non-mono tonic ally from the probe; this bump disappears at a certain probe voltage and the theory is valid up to this voltage. The transition region, which extends beyond plasma potential, is not exponential. The possible saturation of the electron current is discussed. Restricted numerical results are given; they seem to be useful for weaker magnetic fields down to the zero-field limit. Extensions of the theory a r e considered.

Las casualidades controladas : la primera visibilidad del cambio del paradigma moderno

Como no pocos proyectos, el origen de esta tesis es fruto de una casualidad. Hace unos años me topé en Londres, en la librería Walden Books del 38 de Harmood St., con una primera edición de la conocida monografía de Mies a cargo de Philip Johnson. El librito, en realidad catálogo de la exposición que en 1947 el MoMA de Nueva York dedicara a la obra de Mies Van der Rohe a los diez años del desembarco del arquitecto en Estados Unidos, tiene un tamaño de 10 x 7,5 pulgadas, es decir, la mitad del formato americano Crown (20x15 pulgadas), equivalente a 508 x 381 mm. Se imprimieron, en su primera tirada, editada por The Plantin Press, 12.000 ejemplares. Ese mismo año, con edición al cuidado de Reynal y Hitchcock, se publicaría la primera traducción al inglés de Cuando las catedrales eran blancas de Le Corbusier y una selección de poemas de Lorca, siete años después de su Poeta en Nueva York. En la monografía, en la página 109, aparece el conocido croquis de Mies Sketch for a glass house on a hillside. c. 1934, escasamente unas líneas, aunque precisas y llenas de matices, de la casa en una ladera que rápidamente nos remite a aquella primera propuesta de Saarinen para una casa en el aire, primero en 1941 en Pensilvania y después, en 1945, con Charles Eames, en Los Angeles, que nunca llegarían a construirse, sino en su aliteraciones posteriores realizadas por Harry Seidler (Julian Rose House, Wahroonga, Sydney, 1949), Philip Johnson (Leonhardt house, Long Island, Nueva York, 1956) o Craig Ellwood (Smith House, Crestwood Hills, 1958; Frank & Polly Pierson House, Malibú, 1962; Chamorro House, Hollywood Hills, 1963, o la serie Weekend House, con Gerald Horn, entre 1964 y 1970, hasta el magnífico Art Center College of Design de Pasadera, su puente habitado de 1977). El relato que da origen al texto discurre en un estricto período de tiempo, desde los primeros dibujos de la Case Study House nº8, dentro del programa promovido por John Entenza y su revista Arts & Architecture en California, realizados en el estudio de Saarinen en Bloomfield Hills, Michigan, hasta que el proyecto de la casa Eames finaliza cinco años después de acabar la obra en 1955, en la versión conocida, radicalmente distinta al proyecto original, cuando la pareja Charles y Ray Eames edita el corto House After Five Years of Living. La discusión original en torno a esta casita, o mejor, a las circunstancias, casualidades controladas, que rodean su construcción, se produce estrictamente cuando rastreamos aquellos invariantes que se mantienen en las dos versiones y las reconcilian. En este corto período de tiempo se producen en el proyecto una serie de decisiones que permiten descubrir, tanto en la forma como en el mismo proceso transformador del proyecto, en su obsesivo registro, en los nuevos referentes asumidos y propuestos, la primera visibilidad del cambio del paradigma moderno. Pero este momento germinal que cristaliza el paso a la postmodernidad no es inédito en la historia de la arquitectura americana. Así, el relato abre su ámbito temporal hasta un nuevo período de cincuenta años que arranca en 1893, año de la celebración en Chicago de la Exposición Internacional Colombina. En la White City de Hunt & McKim y del traidor Burham, Louis Sullivan construye su Golden Doorway en el pabellón de los Transportes. Aquella que visitará Adolf Loos antes de volver, renovado, a Viena; la misma que admirará André Bouillet, representante de la Union Centrale des Arts Decoratifs de Paris, y que deslumbrará en los museos de toda Europa, de París a Moscú en grandes fotografías y maquetas. Hasta que en Finlandia alguien muestra una de esas fotografías a un joven estudiante de diecinueve años en el Instituto Politécnico. Eliel Saarinen queda fascinado por la poderosa novedad de la imagen. Cuelga la fotografía frente a su tablero de dibujo, consciente de que la Golden Doorway, esa puerta de la aventura y la catarsis que Sullivan acuñaría como distintivo y que resolvería como único elemento complejo sus proyectos más maduros, desprovistos de todo ornamento; la misma que repetirían más tarde, con profundo reconocimiento, Ladovsky, Wright, Scarpa o Moneo, puerta dentro de puerta, fuelle y umbral, contenía, en sus propias palabras emocionadas, todo el futuro de la arquitectura. Desde ahí, pasando por el año 1910, momento de la huida de Wright a La Toscana y el descubrimiento de su arquitectura en Europa, entre otros por un joven Mies van der Rohe, meritorio en el estudio de Peter Behrens, a través del Wasmuth Portfolio; y así como algo después, en 1914, Schindler y en 1923 Neutra, harán el camino inverso, hasta que Mies les siga en 1937, animado por un Philip Johnson que había viajado a Europa en 1930, volviendo a 1945 y el inicio del programa Case Study House en California, hasta 1949, momento de la construcción de la CSH#8, y, por fin, al año 1955, after 5 years of living, en el que Julius Shulman fotografía la casa de Ray y Charles Eames en el prado de Chautauqua sobre las Pacific Palisades de Los Angeles, lanzando sus finas líneas amarillas hasta Alison y Peter Smithson y su tardomoderno heroico, hasta el primer Foster y su poético hightech y hasta el O-riginal Ghery, deconstruyendo el espacio esencial de su casa desde el espacio mismo, abiertas ya las puertas al nuevo siglo. Y en estos cambios de paradigma, desde el rígido eclecticismo de los estilos al lirismo moderno en el gozne secular y de ahí a la frivolidad, ligereza, exhibicionismo y oportunismo cultos del hecho postmoderno, hay algo que se mantiene invariante en los bandazos de la relación del hombre contemporáneo con su entorno. Como la escultura, que según Barnett Newman no es sino aquello contra lo que uno choca cuando retrocede para mirar un cuadro, en estos prístinos lugares, comunes y corrientes, recorrido, puerta, umbral, recinto y vacío, te topas con la arquitectura antes de darte cuenta de que es arquitectura. ABSTRACT As with many other projects, the origin of this doctoral thesis is the result of a chance. A few years ago I found in a bookstore in London, 38 Harmood st., Walden Books, a first edition of the well-known monograph about Mies by Philip Johnson. The tiny book, in fact a catalog of the exhibition that the MoMA of New York devoted to the work of Mies van der Rohe in 1947, ten years after his landing in the United States, has a size of 10 x 7.5 inches, that is, half of Crown American format (20 x 15 inches), equivalent to 508 x 381 mm. In the first printing, published by The Plantin Press, 12,000 copies were released. That same year, produced by Reynal and Hitchcock, both the first English translation of When the cathedrals were white by Le Corbusier and a selection of poems by Lorca were published, seven years after his Poet in New York. Inside the book, the famous drawing from Mies Sketch for a glass house on a hillside c. 1934 appears on page 109, barely a few lines, precise and nuanced though, the house on a hillside that quickly reminds us of the proposals of Eero Saarinen for a house in the air, first in 1941, in Pennsylvania, and later, in 1945, with Charles Eames, in Los Angeles, that would never be built, but in their later alliterations made by Harry Seidler (Julian Rose House, Wahroonga, Sydney, 1949), Philip Johnson (Leonhardt house, Long Island, New York, 1956) or Craig Ellwood (Smith House, Crestwood Hills, 1958; Frank Pierson & Polly House, Malibu, 1962, Chamorro House, Hollywood Hills, 1963, or the Weekend House series, with Gerald Horn, between 1964 and 1970, to the magnificent Art Center College of Design Pasadena, the inhabited bridge, in 1977). The story that gives rise to the text flows in a short amount of time, from the first drawings of the Case Study House No. 8, within the program promoted by John Entenza and his magazine Arts & Architecture in California, made in the study of Saarinen in Bloomfield Hills, Michigan, until the project of the Eames house is completed five years after finishing the construction in 1955, in the final version we know, radically different from the initial state, when the couple, Charles and Ray, published the film House after Five Years of Living. The original discussion around this house, or better, about the circumstances, controlled coincidences, regarding its construction, appears when one takes account of those that remain, the invariants, in the two versions, drawn and built, which precisely allow the reconciliation between both projects. In this short period of time a series of decisions made in the transformation process of the project reveal, in the obsessive record made by Charles Eames and in the new proposed references, the first visibility of the changing of the modern paradigm. But this germinal moment that crystallizes the transition to postmodernism is not unprecedented in the history of American architecture. So, the story opens its temporal scope to a fifty-year period that started in 1893, date of the celebration of the Chicago World´s Columbian Exposition. In the White City by Hunt & McKim and Burnham, the traitor, Louis Sullivan builds his Golden Doorway in the Transportation Building. That visited by a renovated Adolf Loos before his coming back to Vienna; the same that André Bouillet, Head of the Union Centrale des Arts Decoratifs in Paris, admired and dazzled in museums all over Europe, from Paris to Moscow, in large photographs and models. Until someone in Finland showed one of those photos to a young nineteen-years-old student at the Polytechnic Institute. Eliel Saarinen became fascinated by the powerful new image: he hanged the picture in front of his drawing board, aware that the Golden Doorway, that door of adventure and catharsis Sullivan coined as distinctive and as a single complex element which would solve their most mature projects, devoid of all ornament; the same that would repeat later, with deep appreciation, Ladovsky, Wright, Scarpa, or Moneo, a door inside a door, a threshold, a gap that contained, in its own moving words, the whole future of architecture. From there, through 1910, when Wright's flight to Tuscany allows Europe to discover his architecture, including a young Mies van der Rohe, meritorious in the studio of Peter Behrens, via the Wasmuth Portfolio; and as a little bit later, in 1914, Schindler and Neutra in 1923, made the travel in opposite direction, until Mies follows them in 1937, led by a Philip Johnson who had traveled to Europe in 1930, we return to 1945 and the beginning of the program Case Study House in California, and from 1949, when construction of the CSH # 8 begins, and finally, to 1955, after five years of living, when Julius Shulman photographs the inside of the house with Ray and Charles Eames, and all their belongins, at the Chautauqua meadows on Pacific Palisades in Los Angeles, launching its fine yellow lines to Alison and Peter Smithson and his heroic late modern, up to the first Foster and his poetic hightech and even the O-riginal Gehry, deconstructing the essential space of his home from the space itself, opening the doors to the new century. And these paradigm shifts, from the hard eclectic styles to modern secular lyricism in the hinge and then overcoming the cultured frivolity, lightness, exhibitionism, and opportunism of the postmodern skeptical focus, something remains intense, invariant in the lurching relationship of contemporary man and his environment. As the sculpture, which according to Barnett Newman is what you bump into when you back up to see a painting, in these pristine, ordinary places, promenade, door, threshold, enclosure and emptiness, you stumble upon the architecture even before realizing that it is architecture.