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.

Magnetic resonance investigations of defects in Ga14N and Ga15N

Abstract: The influence of the nitrogen nuclear spin on the optically detected magnetic resonance and electron spin resonance signatures of the intrinsic shallow donor and a deep defect causing the characteristic yellow luminescence have been studied on wurtzite GaN epitaxial layers grown by plasma induced molecular beam epitaxy with isotopically pure 14N and 15N. In particular, the linewidth of the deep defect signal is observed to be independent of the nitrogen isotope. The missing effect of the different nuclear spin properties of the 14N and 15N isotopes is discussed in view of current microscopic models for the yellow luminescence in GaN.

High-Q Anti-Series AlGaN/GaN High Electron-Mobility Varactor

Abstract: This paper gives a first presentation of an anti-series AlGaN/GaN high electron-mobility varactor with a tuning ratio of 4.7 and a Q factor above 100. The losses could be decreased by 30 % by omitting the ohmic junctions in the signal path. The devices are characterized and a large-signal model is extracted. Comparing the anti-series varactor to a similar single varactor device shows an increased Q factor and superior linearity. The second harmonic could be decreased by 50 dB.

A 200 GHz driver amplifier in metamorphic HEMT technology

Abstract: A driver amplifier operating at 200 GHz has been design and manufactured with compact size, high gain, broadband performance and high power added efficiency (PAE). This monolithic microwave integrated circuit (MMIC) is realized in grounded coplanar waveguides (GCPW) technology in conjunction with a 35 nm gate length metamorphic high electron mobility transistor technology (mHEMT). The four-stage driver amplifier provides more than 20 dB linear gain between 180 GHz and 270 GHz (40 % bandwidth) and PAE higher than 3.3% with more than 7.4 dBm saturated output power at 200 GHz.

Power limits of polarization-induced AlGaN/GaN HEMT's

Abstract: The present and predicted limits on microwave power performance of undoped AlGaN/GaN HEMTs are presented, based on measured frequency response and drain-source breakdown voltage, both as functions of gate length. The spontaneous and piezoelectric polarization that induce the 2DEG in these HEMTs are covered. Process methods, including Si/sub 3/N/sub 4/ passivation are included. Thermal simulation results are shown for heat dissipation that limits channel temperature to 300/spl deg/C. Microwave cw power density limits of 12.5 W/mm at 10 GHz are predicted for class A operation on thick SiC substrates.

Sensing applications of micro- and nanoelectromechanical resonators

Abstract: The sensitivity of micro- and nanoscale resonator beams for sensing applications in ambient conditions was investigated. Micro-electromechanical (MEMS) and nanoelectromechanical systems (NEMS) were realized using silicon carbide (SiC) and polycrystalline aluminium nitride (AlN) as active layers on silicon substrates. Resonant frequencies and quality factors in vacuum as well as in air were measured. The sensitivity behaviour under ambient conditions with a mass loading in the range of picogram (pg) was verified and measurements with biological mass loading were performed. In addition, the sensitivity to pressure variations was analysed.

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.