scispace - formally typeset
Search or ask a question
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
More filters
Journal ArticleDOI
TL;DR: In this article, the growth of AlInGaN/GaN heterostructures on sapphire substrates was achieved by plasma induced molecular beam epitaxy, and different alloy compositions were obtained by varying the growth temperature with constant Al, In, Ga and N fluxes.
Abstract: Growth of AlInGaN/GaN heterostructures on sapphire substrates was achieved by plasma induced molecular beam epitaxy. Different alloy compositions were obtained by varying the growth temperature with constant Al, In, Ga and N fluxes. The In content in the alloy, measured by Rutherford Backscattering Spectroscopy, increased from 0.4% to 14.5% when the substrate temperature was decreased from 775 to 665°C. X-Ray reciprocal space maps of asymmetric AlInGaN (2.05) reflexes were used to measure the lattice constants and to verify the lattice match between the quaternary alloy and the GaN buffer layers.

2 citations

Proceedings Article
M. Muser1, Rudiger Quay1, F. van Raay1, Michael Mikulla1, Oliver Ambacher1 
15 Dec 2011
TL;DR: In this paper, the analysis of GaN HEMT power cells for broadband high-power amplifier design is presented, where the authors compare different power cell layouts, different Al contents in the barrier layer, and different fieldplate configurations.
Abstract: In this paper we present the analysis of GaN HEMT power cells for broadband high-power amplifier design. We compare different power cell layouts, different Al contents in the barrier layer, and different fieldplate configurations. With the results of the analysis we can specifically optimize the epitaxial choice, fieldplate configuration, and the layout of the power cell for broadband high-power amplifiers. The main findings include the challenging interdependence of the parameters with the variation of the technology parameters.

2 citations

Journal ArticleDOI
TL;DR: In this article, the impact of different pseudosubstrates as AlN, GaN or AlN/GaN on the crystalline quality of InN layers by transmission electron microscopy is reported.
Abstract: A study of the impact of different pseudosubstrates as AlN, GaN or AlN/GaN on the crystalline quality of InN layers by transmission electron microscopy is reported. The temperature of the substrate was found to play an important role: for higher temperatures, low quality InN layers are obtained, poorly supported on the buffers and consequently almost free of dislocations; whereas for lower temperatures, higher qualities, with smooth surfaces and continuous interfaces are observed. In these cases, high resolution electron microscopy showed that the high lattice mismatch existing between the pseudosubstrate and the InN epilayer is mainly accommodated by a network of geometrical misfit dislocations at the heterointerface. Additionally, low densities of threading dislocations, also decaying exponentially with the increasing thickness were observed. The introduction of a GaN buffer layer on top of AlN promotes the reduction in one order of magnitude the density of threading dislocations in the epilayer, partially due to the reduction of dislocations propagating from the pseudosubstrate. However, other relevant factors as the domains coalescence or the generation of secondary misfit dislocation must be taken into account. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

1 citations

Proceedings ArticleDOI
12 Mar 2018
TL;DR: A 4-way Wilkinson and a 4:1 Dolph-Chebyshev divider MMIC based on grounded coplanar waveguide technology on GaAs are presented, analyzed and compared.
Abstract: A 4-way Wilkinson and a 4:1 Dolph-Chebyshev divider MMIC based on grounded coplanar waveguide technology on GaAs are presented, analyzed and compared. The Dolph-Chebyshev structure was simulated by using electromagnetic software, whereas the 4-way Wilkinson divider was simulated by a schematic CAD tool. Both dividers are processed and experimentally evaluated for operation in H-band (220–330 GHz). The 4-way Wilkinson MMIC achieves the best insertion losses, which are lower than 107 dB between 235 and 255 GHz (8 2 % of relative bandwidth, RBW). Input return losses better than 12 dB, an amplitude imbalance between outer and inner branches lower than 0.7 dB and a phase difference of (1.5 ± 0.1)° is achieved by the Dolph-Chebyshev from 235 to 257 GHz (RBW=8.9 %). The 4-way Wilkinson divider is very compact with a size of 0.09 × 0.22 mm2.

1 citations

Proceedings ArticleDOI
01 Aug 2020
TL;DR: In this paper, the authors presented an RF-power digital-to-analog converter (DAC) with voltage swings up to 8.32 V, suitable to drive subsequent single-stage microwave gallium nitride (GaN) power amplifer for sub-six frequencies.
Abstract: The RF-power digital-to-analog converter (DAC) presented here provides RF-signals in the gigabit regime with voltage swings up to 8.32 V, suitable to drive subsequent single-stage microwave gallium nitride (GaN) power amplifer for sub-six frequencies. A current-steering architecture is driven by a custom algorithm to provide a programmable high output current, up to 250 mA, to a capacitive load such as the capacitive input impedance of an single-stage GaN power amplifier. This architecture provides data rates up to 32 Gb/s with an custom encoding, while the output voltage swing at the load capacitance is higher than 5 Vpp. Therefore, slew rates of up to 76 V/ns can be established.

1 citations


Cited by
More filters
Journal ArticleDOI

[...]

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