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.

Switching frequency modulation for GaN-based power converters

Abstract: The effects on the EMI spectrum for various switching frequency modulation (SFM) scenarios in a high frequency boost converter are investigated in this paper. A GaN-device and a Si-device are compared with respect to their EMI behavior, which results from different gate charges and therefore different voltage gradients dv/dt on the power lines. First, the dynamic characteristics of the GaN-HEMT are demonstrated in detail. Then the behavior in the time domain and the frequency domain for switching operations at 300 kHz with various frequency modulation settings and an output power of 250 W are presented.

High linearity active GaN-HEMT down-converter MMIC for E-band radar applications

Abstract: A down-converter MMIC in 100 nm gate length AlGaN/GaN HEMT technology achieves an input-related 1-dB compression point of 13 dBm at a center frequency of 77 GHz, providing high linearity for radar applications. The single-ended fundamental mixer without preor post-amplification shows 8 dB conversion loss when driven with 13 dBm of LO power within an RF frequency range exceeding 75 to 81 GHz. The high linearity is achieved by operating the GaN transistor as active transconductance mixer, allowing for a high voltage swing of the RF signal even when using a relatively small transistor size as required by the high operating frequency.

Photoluminescence of Ga-face AlGaN/GaN single heterostructures

Abstract: The radiative recombination in Ga-face Al 0.30 Ga 0.70 N/GaN single heterostructures (SHs) was studied by photoluminescence (PL) measurements. An energy shift of the excitonic transitions toward higher energies was observed, indicating the presence of residual compressive strain in the GaN layer. In addition to these exciton lines, a broad band energetically localized between the exciton lines and the LO-phonon replica was noticed in the undoped SH. From its energy position, excitation power dependence, as well as temperature behaviour, we have attributed this luminescence to the H -band (HB), which is representative of the two-dimensional electron gas (2DEG) recombination.

Enhanced AlScN/GaN Heterostructures Grown with a Novel Precursor by Metal–Organic Chemical Vapor Deposition

Abstract: Growth of AlScN high‐electron‐mobility transistor (HEMT) structures by metal–organic chemical vapor deposition (MOCVD) is challenging due to the low vapor pressure of the conventionally used precursor tris‐cyclopentadienyl‐scandium (Cp3Sc). It is shown that the electrical and structural characteristics of the AlScN/GaN heterostructure improve significantly by using bis‐methylcyclopentadienyl‐scandiumchloride ((MCp)2ScCl), which has a higher vapor pressure and allows for an increased molar flow and thus higher growth rate (GR). AlScN/GaN HEMT heterostructures with superior electrical characteristics deposited at different barrier growth temperatures are presented. The sheet resistance Rsh of 172 Ω sq−1 obtained at 900 °C barrier growth temperature is among the lowest reported so far for AlScN/GaN HEMT structures. The sheet charge carrier density ns is 3.23×1013 cm−2 and the electron mobility μ is 1124 cm2 Vs−1.

Investigation of GaN-HEMTs in Reverse Conduction

Abstract: This work investigates the reverse conduction characteristics of 600 V-class GaN-HEMTs. The behavior of a conventional HEMT is analyzed and compared to the reverse conduction of an improved HEMT structure with integrated freewheeling diode. The characteristics of both structures are measured on fabricated test samples. Furthermore, the electrical behavior is analyzed with regards to the intrinsic layouts.

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.