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.

Broadband low-noise GaN HEMT TWAs using an active distributed drain bias circuit

Abstract: Modern communication and radar systems show an increasing demand for robust ultra-broadband amplifiers for low-noise applications. A set of three different 0.5 GHz to 20 GHz MMIC LNAs using a GaN HEMT technology with a gate length of 0.25 μm was designed and fabricated, each with a noise figure between 3 dB and 7 dB over frequency. Two designs with four and five FET cells feature approx. 10 dB and 11 dB broadband gain, while a third MMIC with a chain connection of both figures more than 20 dB of gain. A distributed active drain bias circuit substitutes large area or off-chip inductor structures and enables a full-MMIC chain connection of both TWA stages.

Investigation of Temperature Characteristics and Substrate Influence on AlSeN-Based SAW Resonators

Abstract: It has been found that introducing Sc into AIN to form Al 1-x Sc x N can lead to a significant increase in piezoelectric response, making Al 1-x Sc x N a promising material for electroacoustic devices, such as frequency filters for mobile communications. 1 J.1m thick highly c-axis oriented Al 1-x Sc x N thin films were deposited on 100 mm Si(001)(x = 0, 0.14, 0.23, and 0.32)and on Al203(0001)(x = 0.14 and 0.23)substrates using reactive pulsed-DC magnetron sputtering and SAW resonators with wavelengths λ = 2–24 μm were fabricated. The effective electromechanical coupling k2eff of the resonators improved with increasing Sc concentration, for instance, the λ = 2 μm AIN (fs = 2.08 GHz)and Alo.68Sco.32N (fs = 1.83 GHz)resonators exhibited coupling of 0.6 % and 2.6 %, respectively. Furthermore, 3 % increase in resonant frequency and 9.6 % increase in coupling were observed when comparing λ = 2 μm resonators based on Al203 to those based on Si substrates. The temperature coefficient of frequency (TCF)was found to reach up to −27.4 ppm/K at x = 0.23 using Si substrates, while the equivalent resonators based on Al203 substrates exhibited lower TCF of −26.1 ppm/K. Thermal hysteresis errors were calculated to be ~1%. Employing Al203 substrates for λ = 2 μm Al 0.77 Sc 0.23 N-based resonators showed reduced TCF, higher resonant frequency, and higher coupling coefficient when compared to the same design resonators on Si substrates.

InN/In2O3 heterostructures

Abstract: Cubic (c-) InN is predicted to possess superior electronic properties for device applications, while c-In2O3 is an excellent candidate as gate material for InN based high-frequency field effect transistors. In this paper, the epitaxial growth of the InN/In2O3 and In2O3/InN heterosystems was investigated. High-quality c-InN (001) was deposited on (001) In2O3 tem- plate, while single crystalline c-In2O3 was epitaxially grown on hexagonal InN (0001). The epitaxial relationship of the heterosystems was determined. Phenomenological models of the nucleation and of an atomic arrangement at the interface are proposed. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

InP-based heterojunction bipolar transistors with InGaAs/GaAs strained-layer-superlattice

Abstract: In this paper, we report the design, fabrication, and characterization of an InP/InGaAs heterojunction bipolar transistor (HBT) employing a lattice-mismatched In0.53Ga0.47As/GaAs strained-layer-superlattice (SLS) base structure. The performance of the SLS-base device is also compared with that of an In0.53Ga0.47As uniform-base structure. The digitally graded-base devices exhibit a slightly lower gain at low bias voltage and a higher current gain at high currents. While the offset voltage remains comparable in the two structures, the graded-base InP/InGaAs HBTs, with built-in fields of ∼66 kV/cm, have typically a maximum dc current gain of 18% larger than that of transistors with uniform base.

Lateral alignment of SiC dots on Si

Abstract: An alignment of self-assembled SiC dots grown by molecular beam epitaxy on Si substrates is demonstrated. Atomic force microscopy was applied showing the possibility to control the lateral ordering in linear chains and in dense dot arrays. The large lattice mismatch between Si and SiC of 20% stimulates a three-dimensional nucleation on the substrate. The formation of well ordered monoatomic, and biatomic steps as well as step bands on (100) and (111) Si was used, offering the advantage to not need additional processing steps to define the alignment. However, during the SiC nucleation on Si the substrate participates in the reaction to SiC by lateral Si diffusion resulting in an unstable surface during the growth. Thus, the reproducible control of the nucleation sites independent on the movement of the steps during the growth is a critical issue to achieve the lateral alignment. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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.