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.

An ultra-broadband low-noise traveling-wave amplifier based on 50nm InGaAs mHEMT technology

Abstract: A monolithic integrated eight-stage traveling-wave amplifier (TWA) is presented that has been developed and fabricated using a 50nm InGaAs metamorphic HEMT technology. High-impedance coplanar waveguides (CPW) are used as compensation for the input and output capacitances of the stages, consisting of two transistors in a cascode configuration. A small signal gain of 11dB with a ripple of around ±1dB and a 3dB bandwidth of more than 110GHz is achieved. The noise figure (NF) is as low as 2.5dB at the best and less than 5dB for frequencies up to 90GHz. Furthermore, the amplifier provides an 1-dB-compression-point of 7dBm and a saturated output power of about 11dBm at 75GHz.

Residual strain effects on the two-dimensional electron gas concentration of AlGaN/GaN heterostructures

Abstract: Ga-face AlGaN/GaN heterostructures with different sheet carrier concentrations have been studied by photoluminescence and Raman spectroscopy. Compared to bulk GaN, an energy shift of the excitonic emission lines towards higher energies was observed, indicating the presence of residual compressive strain in the GaN layer. This strain was confirmed by the shift of the E2 Raman line, from which biaxial compressive stresses ranging between 0.34 and 1.7 GPa were deduced. The spontaneous and piezoelectric polarizations for each layer of the heterostructures have been also calculated. The analysis of these quantities clarified the influence of the residual stress on the sheet electron concentration (ns). Possible causes for the discrepancies between the calculated and experimentally determined sheet carrier densities are briefly discussed.

Mobility edge in hydrogenated amorphous carbon

Abstract: Optical and electronic properties of plasma enhanced chemical vapor deposited diamondlike amorphous carbon (a-C:H and a-C1−xSix:H) films were studied by photothermal deflection spectroscopy and the spectral photocurrent method. Two different regimes of photoconductivity in the diamondlike carbon films have been detected. The carrier mobility due to the σ-σ* bands is found to be 1–2 orders of magnitude higher than that due to the π-π* transitions. The σ-σ* mobility edge of diamondlike hydrogenated amorphous carbon is equal to 5.3±0.1eV and appears to be closely correlated with the band gap of diamond. We have also shown that the mobility edge in diamondlike amorphous carbon is not associated with its band gap. The optical band gap of diamondlike amorphous carbon films depends on the sp2 phase content as already reported by Robertson [Phys. Rev. B 53, 16302 (1996)]. However, the mobility edge is not influenced by either the Si doping level, the film thickness, or the ion impact energy during deposition.

A 67 GHz GaN Voltage-Controlled Oscillator MMIC With High Output Power

Abstract: This letter describes the design and the realization of a fixed-frequency oscillator and voltage-controlled oscillator (VCO) MMIC realized in an AlGaN/GaN HEMT technology with 100 nm gate length. Both oscillators achieve output power levels of almost 20 dBm without post-amplification. The oscillation frequency of the fixed-frequency oscillator is 65.6 GHz, while the VCO can be tuned from 65.6 to 68.8 GHz, which leads to a relative bandwidth of 5%. The phase noise of the VCO is -83 dBc/Hz at 1 MHz frequency offset.

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.