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.

Design of Low-Resistance and Area-Efficient GaN-HEMTs for Low-Voltage Power Applications

Abstract: This work analyzes different layouts for low-resistance and area-efficient GaN-HEMT devices for lowvoltage power applications. The current distribution in interdigital comb and matrix structures is investigated and geometry parameters are optimized to achieve the lowest possible area-specific onstate resistance for given technology limits. A new model is analytically derived to investigate the static on-state behavior in matrix structures. Fabricated large area power transistors feature low specific onstate resistances RON∙A of 0.61 mOmega∙cm2 for the comb structure, a reduced specific on-state resistance of 0.37 mOmega∙cm2 for the matrix structure, and a further reduced specific onstate resistance of 0.23 mOmega∙cm2 for a high-density, fully symmetrical matrix structure.

Power Module Design for GaN Transistors Enabling High Switching Speed in Multi‐Kilowatt Applications

Abstract: The advantages of gallium nitride (GaN) and silicon carbide (SiC) transistors over silicon (Si) devices are highlighted. The design criteria for power modules in multi-kilowatt applications to effectively leverage these advantages are described. Various concepts to overcome limitations associated with state-of-the-art power module packaging presented in the literature are summarized and evaluated. A novel power module design comprising a 650, 300 half-bridge with integrated DC-link and gate drivers is proposed. The results of a finite element analysis (FEA) of its parasitic elements and subsequent double pulse test simulation are presented. This article is protected by copyright. All rights reserved.

Group III-Nitride based Gas Sensors for Exhaust Gas Monitoring

Abstract: Pt-GaN Schottky devices with catalytically active platinum (Pt) electrodes were investigated. Some measurements were also performed on high-electron-mobility transistors based on GaN/AlGaN heterostructures. The response of these devices towards a number of relevant exhaust gas components such as H2, HC, CO, NO and NO2 was tested. Sensitivity and cross sensitivity profiles were found to depend on temperature, the porosity of the Pt gates as well as on the polarity of the underlying GaN material.

Experimental determination of Al 1-x Sc x N thin film thermo-electro-acoustic properties up to 140°C by using SAW resonators

Abstract: 1 µm thick sputtered Al1-xScxN films were used to fabricate SAW resonators with 2-20 µm wavelength and the frequency response was measured in the range of 20 °C to 140 °C. Extracted phase velocity dispersion curve was in good agreement with the simulated dispersion curve by FEM model. Moreover, for each investigated temperature, the corresponding phase velocity shift dispersion curves as a function of normalized thickness were shown in detail for each resonator layer (Si substrate, AlScN piezoelectric layer, and Pt electrode), and the independent thermal influence of the Si, AlScN, and Pt were evaluated. The temperature coefficients of frequency (TCF) of Al1-xScxN (x = 0, 0.14, 0.32) layer was calculated independently and compared with the combined TCFs of the resonators.

Pt Schottky contacts on Ga- and N-face surfaces of free-standing GaN

Abstract: Thick GaN films, grown by hydride vapor phase epitaxy (HVPE), were separated from their sapphire substrate with a laser-induced lift-off process. After cleaning and polishing, these films offer the most direct way to investigate and compare the influence of crystal polarity on the electronic properties of Ga-face and N-face surfaces, respectively. Different barrier heights for Pt Schottky diodes evaporated onto Ga- and N-face GaN are determined from the dependence of the effective barrier height versus ideality factor by I-V measurements to 1.15 eV and 0.80 eV, respectively. The charge neutrality condition at the surface is modified by the spontaneous polarization due to the polarization induced bound sheet charge. This effect has to be included in the electronegativity concept of metal induced gap states (MIGS) and can also be illustrated by different band bending of the conduction and valence band, inferred from the self-consistent solution of the Schrodinger-Poisson equation. Furthermore, temperature dependent I-V characteristics are compared to simulated behavior of Schottky diodes, exhibiting excellent agreement in forward direction, but showing deviations in the reverse current.

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.