Author
Oliver Ambacher
Other affiliations: Osram, Siemens, Cornell University ...read more
Bio: Oliver Ambacher is an academic researcher from Fraunhofer Society. The author has contributed to research in topics: Amplifier & High-electron-mobility transistor. The author has an hindex of 64, co-authored 848 publications receiving 26256 citations. Previous affiliations of Oliver Ambacher include Osram & Siemens.
Papers published on a yearly basis
Papers
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07 Sep 2020TL;DR: In this paper, the phase velocity dispersion for Pt metallized epitaxial AlScN films on sapphire (Al2O3) was computed for two different propagation directions.
Abstract: Partial substitution of Al atoms with Sc in wurtzite AlN crystals increases the piezoelectric constants. This leads to an increased electromechanical coupling, which is required for high bandwidths in piezo-acoustic filters. The crystal bonds in Ah-xScxN (AlScN) are softened as function of Sc atomic percentage x, leading to reduction of phase velocity in the film. Combining high Sc content AlScN films with high velocity substrates favors higher order guided surface acoustic wave (SAW) modes [1]. This study investigates higher order SAW modes in epitaxial AlScN on sapphire (Al2O3). Their dispersion for Pt metallized epitaxial AlScN films on Al2O3was computed for two different propagation directions. Computed phase velocity dispersion branches were experimentally verified by the characterization of fabricated SAW resonators. The results indicated four wave modes for the propagation direction (0°, 0°, 0°), featuring 3D polarized displacement fields. The sensitivity of the wave modes to the elastic constants of AlScN was investigated. It was shown that due to the 3D polarization of the waves, all elastic constants have an influence on the phase velocity and can be measured by suitable weighting functions in material constant extraction procedures.
3 citations
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TL;DR: In this article, five different versions of grounded coplanar waveguide (GCPW) branch-line couplers on GaAs were EM simulated, processed and investigated for operation in G-band (140-220 GHz), based on different layer structures (including 2 or 3 metallization layers) and layouts.
Abstract: Five different versions of grounded coplanar waveguide (GCPW) branch-line couplers on GaAs were EM simulated, processed and investigated for operation in G-band (140–220 GHz), based on different layer structures (including 2 or 3 metallization layers) and layouts. The best results have been obtained with the structures based on a continuous galvanic metal in the central conductor line and ohmic connections between top ground planes, reducing the insertion losses by 0.8 dB. A measured amplitude imbalance less than 0.5 dB from 160 to 200 GHz (22%), with a coupler insertion loss lower than 1.3 dB was achieved for a 3 metallization layer branch-line coupler.
3 citations
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TL;DR: In this article, the development of advanced millimeter-wave monolithic integrated circuits for application in active and passive high-resolution imaging systems operating beyond 200 GHz is presented, where a wideband 210 GHz Low Noise Amplifier has been successfully realized using one of their three metamorphic high electron mobility transistor (mHEMT) technologies in combination with grounded coplanar circuit topology (GCPW).
Abstract: The development of advanced millimeter-wave monolithic integrated circuits for application in active and passive high-resolution imaging systems operating beyond 200 GHz is presented. A wideband 210 GHz Low Noise Amplifier has been successfully realized using one of our three metamorphic high electron mobility transistor (mHEMT) technologies in combination with grounded coplanar circuit topology (GCPW). Additionally, a 200 GHz voltage controlled oscillator (VCO) MMIC demonstrating good output power over a wide bandwidth was fabricated, using our 100 nm mHEMT technology. Finally, a high resolution 220 GHz radiometer was realized and shows very promising performance. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
3 citations
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TL;DR: In this paper, the potential of diamond-based spherical resonators to be used in modern communication devices was evaluated by laser vibrometry in order to obtain mechanical quality factors and identify the resonant frequencies and eigenmodes of the system.
Abstract: In this work, we report on the electro-mechanical studies of high-Q spherical oscillators designed to operate in radio-frequency circuits. Resonating composite spheres, consisting of a silicon core and a thick nanodiamond shell, were studied by laser vibrometry in order to obtain mechanical quality factors and identify the resonant frequencies and eigenmodes of the system. Finite element method simulations were used to analyze and confirm the experimental data. Additionally, reflection/transmission measurements were carried out on capacitively coupled spheres in order to evaluate the electrical parameters of the system. The main aim of these investigations was to evaluate the potential of diamond-based spherical resonators to be used in modern communication devices.
3 citations
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01 Jun 2014TL;DR: In this paper, an inverter-based digital switchmode driver circuit, highly integrated with a GaN power transistor, is presented, which enables a switchmode operation from DC to 3 GHz.
Abstract: An inverter-based digital switch-mode driver circuit, highly-integrated with a GaN power transistor is presented. The layout is fully scalable in output power and therefore suitable for amplifiers with power levels of higher than 10 W. The circuit enables a switch-mode operation from DC to 3 GHz. A consistent analysis of digital switching measurements and active harmonic load-pull results is performed. Measurements at 2 GHz and 30 V yield a drain efficiency of 82 % and 6 W of output power, while a gain up to 35 dB and more than 76 % overall circuit efficiency were achieved.
3 citations
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TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.
Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …
33,785 citations
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TL;DR: The editors have done a masterful job of weaving together the biologic, the behavioral, and the clinical sciences into a single tapestry in which everyone from the molecular biologist to the practicing psychiatrist can find and appreciate his or her own research.
Abstract: I have developed "tennis elbow" from lugging this book around the past four weeks, but it is worth the pain, the effort, and the aspirin. It is also worth the (relatively speaking) bargain price. Including appendixes, this book contains 894 pages of text. The entire panorama of the neural sciences is surveyed and examined, and it is comprehensive in its scope, from genomes to social behaviors. The editors explicitly state that the book is designed as "an introductory text for students of biology, behavior, and medicine," but it is hard to imagine any audience, interested in any fragment of neuroscience at any level of sophistication, that would not enjoy this book. The editors have done a masterful job of weaving together the biologic, the behavioral, and the clinical sciences into a single tapestry in which everyone from the molecular biologist to the practicing psychiatrist can find and appreciate his or
7,563 citations
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TL;DR: In this paper, the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method, is reviewed.
Abstract: This article reviews the current status of lattice-dynamical calculations in crystals, using density-functional perturbation theory, with emphasis on the plane-wave pseudopotential method. Several specialized topics are treated, including the implementation for metals, the calculation of the response to macroscopic electric fields and their relevance to long-wavelength vibrations in polar materials, the response to strain deformations, and higher-order responses. The success of this methodology is demonstrated with a number of applications existing in the literature.
6,917 citations
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TL;DR: In this article, the authors present a comprehensive, up-to-date compilation of band parameters for the technologically important III-V zinc blende and wurtzite compound semiconductors.
Abstract: We present a comprehensive, up-to-date compilation of band parameters for the technologically important III–V zinc blende and wurtzite compound semiconductors: GaAs, GaSb, GaP, GaN, AlAs, AlSb, AlP, AlN, InAs, InSb, InP, and InN, along with their ternary and quaternary alloys. Based on a review of the existing literature, complete and consistent parameter sets are given for all materials. Emphasizing the quantities required for band structure calculations, we tabulate the direct and indirect energy gaps, spin-orbit, and crystal-field splittings, alloy bowing parameters, effective masses for electrons, heavy, light, and split-off holes, Luttinger parameters, interband momentum matrix elements, and deformation potentials, including temperature and alloy-composition dependences where available. Heterostructure band offsets are also given, on an absolute scale that allows any material to be aligned relative to any other.
6,349 citations
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TL;DR: This review gives a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells, and discusses the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells.
Abstract: The need to develop inexpensive renewable energy sources stimulates scientific research for efficient, low-cost photovoltaic devices.1 The organic, polymer-based photovoltaic elements have introduced at least the potential of obtaining cheap and easy methods to produce energy from light.2 The possibility of chemically manipulating the material properties of polymers (plastics) combined with a variety of easy and cheap processing techniques has made polymer-based materials present in almost every aspect of modern society.3 Organic semiconductors have several advantages: (a) lowcost synthesis, and (b) easy manufacture of thin film devices by vacuum evaporation/sublimation or solution cast or printing technologies. Furthermore, organic semiconductor thin films may show high absorption coefficients4 exceeding 105 cm-1, which makes them good chromophores for optoelectronic applications. The electronic band gap of organic semiconductors can be engineered by chemical synthesis for simple color changing of light emitting diodes (LEDs).5 Charge carrier mobilities as high as 10 cm2/V‚s6 made them competitive with amorphous silicon.7 This review is organized as follows. In the first part, we will give a general introduction to the materials, production techniques, working principles, critical parameters, and stability of the organic solar cells. In the second part, we will focus on conjugated polymer/fullerene bulk heterojunction solar cells, mainly on polyphenylenevinylene (PPV) derivatives/(1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C61) (PCBM) fullerene derivatives and poly(3-hexylthiophene) (P3HT)/PCBM systems. In the third part, we will discuss the alternative approaches such as polymer/polymer solar cells and organic/inorganic hybrid solar cells. In the fourth part, we will suggest possible routes for further improvements and finish with some conclusions. The different papers mentioned in the text have been chosen for didactical purposes and cannot reflect the chronology of the research field nor have a claim of completeness. The further interested reader is referred to the vast amount of quality papers published in this field during the past decade.
6,059 citations