Oliver Ambacher

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.

Piezoelectric actuation of all-nitride MEMS

Abstract: We present a MEMS technology based on (GaN/)AlGaN/GaN–heterostructures. Thereby the lower GaN layer represents the mechanical active layer, while the upper GaN and AlGaN layers supply the piezoelectrically active layers for actuation and the confinement of a 2D electron gas (at the lower interface). The 2DEG serves as back electrode for the piezoelectric actuation and as read–out, since it is modulated by the mechanical oscillation. The upper AlGaN and GaN layer both contribute to the total piezoelectric response, which was determined by piezoelectric force microscopy. The electrical field distribution throughout the heterostructure was determined by means of electroreflectance. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

MMIC based wireless data transmission of a 12.5 Gbit/s signal using a 220 GHz carrier

Abstract: In this paper wireless data transmission at 220 GHz with up to 12.5 Gbit/s data rate and over a distance of 2 meters is presented. At 12.5 Gbit/s, the measured eye diagram quality factor is 3.59. At 10 Gbit/s, the 215 − 1 PRBS signal is received with a measured bit error rate better than 3.0 · 10−10. The signal is modulated onto a carrier frequency at 220 GHz using fully integrated 50 nm metamorphic HEMT-based transmit and receive MMICs, packaged into waveguide modules. Horn antennas with attached lenses are used to collimate the beam.

Finite Element Analysis of SAW Propagation Characteristics in c-plane (0001) and a-plane (11-20) AlScN Thin Films

Abstract: Al 1−x Sc x N (AlScN) is known for its large elastic and piezoelectric constants and thus is a favorable material for applications in novel radio frequency (RF) components. We investigated a-plane AlScN on r-plane Al 2 O 3 (AlScN(11-20)/Al 2 O 3 (1-102)). The surface acoustic wave (SAW) propagation properties of AlScN(11-20)/Al 2 O 3 (1-102) and AlScN(11-20)/Al 2 O 3 (1-102) were analyzed using finite element method (FEM) simulations and the results were compared. Rayleigh-type and Sezawa-type wave modes were identified and the acoustic parameters such as phase velocity, electromechanical coupling coefficient, and reflectivity were evaluated. An increased effective coupling of 5.5 % was detected for Rayleigh-type waves on AlScN(11-20)/Al 2 O 3 (1-102). The Sezawa-type modes show an even higher coupling up to 6.2 %. Furthermore, we detected increased reflectivity of AlScN(11-20) films compared to c-plane AlScN(0001). Our results reveal the potential of using a-plane AlScN for increasing the electromechanical coupling, which is needed for the upcoming piezo-acoustic filter requirements.

I and i

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 …

Principles of Neural Science

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

Phonons and related crystal properties from density-functional perturbation theory

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.

Band parameters for III–V compound semiconductors and their alloys

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.

Conjugated polymer-based organic 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.