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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
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TL;DR: An experimental method is introduced that improves the measurement accuracy of millimeterwave (mmW) ellipsometry for characterization of homogeneous polymers and dielectric composite materials based on using a metal substrate on which the samples are placed.
Abstract: In this paper, we introduce an experimental method that improves the measurement accuracy of millimeter-wave (mmW) ellipsometry for characterization of homogeneous polymers and dielectric composite materials. Our approach is based on using a metal substrate on which the samples are placed. This enhances the amount of Fabry–Perot reflections within the material under investigation, and therefore reinforces a stronger phase rotation of the electromagnetic field. We discuss the achieved measurement accuracy with respect to the critical measurands, the ellipsometer angles $\Psi $ and $\Delta $ , and show that it is significantly improved by the presented method. In order to overcome ambiguities in the material parameters, we measure the ellipsometer angles over a frequency range from 80 to 110 GHz. In addition, we present an approximation method that reduces the measurement time of mmW ellipsometry in the case of low-loss samples. We demonstrate that it is sufficient to determine the maximum and minimum power of the measured ellipsometer curve and discuss the limits of this approximation. We performed numerous test series on different plastic materials over a frequency range of 20 GHz, and will discuss the measurement accuracy with respect to the refractive index and the thickness of our samples.

5 citations

Journal ArticleDOI
TL;DR: In this paper, the ideality factor and the barrier height of Pt and Pd AlGaN Schottky diodes were determined from I-V measurements and a barrier height was inferred from the determination of ideality factors versus barrier height to be 1.1 and 1.15 eV respectively.
Abstract: Ultraviolet light detectors based on Pt, Ni, Pd, and Au n-type AlGaN Schottky diodes were grown by plasma induced molecular beam epitaxy on sapphire substrates. The ideality factor and the barrier height can be determined from I–V measurements and a barrier height of Pt and Pd AlGaN Schottky diodes is inferred from the determination of ideality factor versus barrier height to be 1.1 and 1.15 eV respectively. The rejection of visible light response with respect to the peak UV responsivity was three orders of magnitude for an AlGaN Schottky diode with 20% Al content exhibiting a peak energy of 3.8 eV, corresponding to its bandgap. A broad defect shoulder seen in subgap absorption around 2.9 eV can be correlated with surface defects leading to enhanced recombination of strongly absorbed light. The responsivity shows a linear dependence with the applied reverse bias voltage.

5 citations

Journal ArticleDOI
TL;DR: This module achieves the highest bandwidth in $G$ -band and the highest output power when comparing it with modules based on similar technologies.
Abstract: The first full $G$ -band power amplifier module demonstrated in GaAs metamorphic high electron mobility transistor (mHEMT) technology has been developed for use as a driver amplifier in instrumentation. The circuit integrated in this module uses a compact balanced cascode topology and is based on a 35-nm mHEMT technology in a grounded coplanar waveguide environment. High compactness is achieved due to the use of a small ground-to-ground spacing of $14~\mu {\mathrm{ m}}$ and an advanced process with three metallization layers. The millimeter-wave integrated circuit exhibits a linear gain higher than 15.3 dB and return losses better than 10 dB from 118 to 236 GHz, which represents an ultrabroad relative bandwidth (RBW) of 67%. A peak output power of 10 dBm is achieved at 200 GHz. The WR-5 module uses broadband transitions, which show insertion losses of less than 1.5 dB. It demonstrates a small-signal gain that exceeds 13.4 dB in the whole $G$ -band (RBW ≥ 54%). Large-signal characterization exhibits power levels higher than 3.6 dBm from 130 to 210 GHz (RBW = 47%). This module achieves the highest bandwidth in $G$ -band and the highest output power when comparing it with modules based on similar technologies.

5 citations

20 Mar 2018
TL;DR: In this article, a printed circuit board (PCB)-embedding technology was used as packaging technology for 600 V class GaN-on-Si power devices and power integrated circuits (ICs).
Abstract: This paper investigates a printed circuit board (PCB)-embedding technology as packaging technology for 600 V class GaN-on-Si power devices and power integrated circuits (ICs). The electrical and thermal characteristics of PCB-embedding are investigated and compared to conventional assemblies based on wire-bonding and soldering.

5 citations

Journal ArticleDOI
Abstract: Combined electron beam induced current and transmission electron microscopy measurements have been performed on both undoped and Si-doped AlGaN epitaxial films with aluminium contents x ranging from x=0 to x=0.79. TEM analysis shows the distribution and type of structural defects, the content of cubic phase and the decomposition phenomena strongly to depend on the Al content of the layers. The influence of the cubic phase on the electronic properties of the samples is discussed taking into account the high piezoelectric effect present in these alloys. The diffusion length of minority carriers has been determined by EBIC measurements and a lower limit of the lifetime has been estimated.

5 citations


Cited by
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[...]

08 Dec 2001-BMJ
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

Journal ArticleDOI
06 Jun 1986-JAMA
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

Journal ArticleDOI
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

Journal ArticleDOI
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

Journal ArticleDOI
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