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.

Operation of PCB-embedded, high-voltage multilevel-converter GaN-IC

Abstract: This work presents the operation of a PCB-embedded diode-clamped multilevel-converter integrated circuit (IC) fabricated in a lateral, high-voltage AlGaN/GaN-on-Si heterojunction technology. It is demonstrated, that PCB-embedding is an appropriate assembly technique for lateral power ICs with high-integration levels. By placing DC-link capacitors onto the IC-package, parasitc inductances in the power loop can be reduced to the sub-nH range. Considerations regarding common substrate biasing issues in lateral GaN-ICs enable the inverter operation of the 2 × 3 mm2-IC at input voltages up to 300V and output power levels of 45W at a switching frequency of 100 kHz.

A 243 GHz low-noise amplifier module for use in next-generation direct detection radiometers

Abstract: A compact H-band (220-325 GHz) low-noise amplifier circuit has been developed, based on a grounded coplanar waveguide (GCPW) technology utilizing 50 nm metamorphic high electron mobility transistors (mHEMTs). The realized four-stage cascode LNA achieved a small-signal gain of 31 dB at 243 GHz and more than 28 dB in the frequency range from 218 to 280 GHz. Coplanar topology in combination with cascode transistors resulted in a very compact die size of only 0.5 × 1.5 mm2. For low-loss packaging of the circuit, a set of waveguide-to-microstrip transitions has been realized on 50 μm thick GaAs substrates demonstrating an insertion loss of less than 0.5 dB at 243 GHz. The realized LNA module achieved a small-signal gain of 30.6 dB and a room temperature (T = 293 K) noise figure of 5.6 dB at the frequency of operation.

Analysis of composition fluctuations in AlxGa1−xN

Abstract: Composition fluctuations in Al x Ga 1 −x N-layers ( x =0.25 and 0.35) are investigated on an atomic scale by high-resolution transmission electron microscopy (HRTEM). The samples were grown by plasma induced molecular beam epitaxy on Al 2 O 3 (0001). A strain state analysis of the cross-sectional HRTEM micrographs is performed with the digital analysis of lattice images (DALI) program package. Composition profiles on an atomic scale are derived by the measurement of the distances between intensity maxima positions in the HRTEM image. The analyses revealed different areas in the Al x Ga 1 −x N-layers with either homogeneous or ‘striped’ contrast. In the striped areas the analyses indicate a strong decomposition that leads to the formation of self-organized superlattice structures.

GaN‐based high‐frequency devices and circuits: A Fraunhofer perspective

Abstract: We present the current status of our technology for GaN-based HEMTs and MMICs as well as results ranging from the L-band up to the W-band. Epitaxial growth is carried out on 4H-SiC(0001) substrates by both MOCVD and MBE. Processing is done using standard III–V equipment including both frontside and backside processing. For L-band power bars we arrive at output powers, efficiencies and gains beyond 100 W, 60% and 17 dB, all measured under cw conditions at 50 V drain bias. The X-band MMICs are characterized by a high efficiency above 40% for two-stage amplifiers. Towards mm-wave applications we have fabricated HEMTs with transit frequencies above 100 GHz and W-band MMICs delivering 0.5 W/mm at 94 GHz with 7% PAE. First quaternary InAlGaN barriers show promising results for this new materials system. Reliability tests return a very good long-term stability of our devices even at an elevated channel temperature of 200 °C with an extrapolated lifetime of 5 × 105. Initial space capability tests including total ion dose radiation insensitivity, radiation displacement damage, hydrogen poisoning and single event effect are successfully passed.

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.