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.

Carrier Recombination at Screw Dislocations in n‐Type AlGaN Layers

Abstract: We analyse the recombination and scattering at dislocations in Si doped AlGaN layers by transmission electron microscopy (TEM), electron beam induced current (EBIC) and cathodoluminescence. We show that c-type screw dislocations are nonradiative recombination centres. From temperature dependent measurements of the EBIC contrast we find a shallow acceptor state 20 meV above the valence band. The level can be understood iri terms of one-dimensional dislocation bands split off by the dislocation deformation field. In addition piezoelectric potentials play a role.

A 40 dBm AlGaN/GaN HEMT power amplifier MMIC for SatCom applications at K-band

Abstract: The design, realization, and characterization of a K-band high power amplifier with a saturated output power of 40dBm is described in this paper. The amplifier is realized using a 250nm gate length AlGaN/GaN HEMT MMIC technology on semi-insulating SiC substrates. The two-stage amplifier is designed with two 6×90 µm HEMT cells in the driver and four 8×100 µm HEMT cells in the final stage and thus exhibits a relatively aggressive staging ratio of 1∶3. When measured with a supply voltage of 32V, the amplifier delivers a saturated output power of 40dBm at 18 GHz. The peak PAE at this frequency is 30%, and the linear gain exceeds 20 dB. These results are state-of-the-art performance with regard to power/efficiency at K-band.

A Study of Hydrogen Sensing Performance of Pt–GaN Schottky Diodes

Abstract: The performance of hydrogen-gas detectors based on Pt-GaN Schottky diodes with 24-nm-thick Pt contact was investigated. Current-voltage (I-V) Characteristics were measured in two ambients (e.g., synthetic air (20% O2 in N2) and 1-vol.% H2 in synthetic air) at different temperatures. The forward current of the diodes is found to increase significantly upon introduction of H2 into the synthetic air ambient. Analysis of the I-V characteristics as a function of temperature demonstrated that the observed current increase is due to a decrease in the effective barrier height (BH) through a decrease in the Pt work function upon absorption of hydrogen. The decrease in the BH was measured as high as 30 and 152 meV at 25 degC and at 280 degC, respectively, upon introduction of H 2 into the ambient. The changes in the BH were completely reversible upon restoration of the synthetic air ambient. The sensitivity to the hydrogen gas was investigated in dependence on the operating temperature for 1-vol.% hydrogen in synthetic air. The changes in the forward bias at a constant current density of 3.2 A/cm2 was 90 and 330 mV at 25 degC and at 310 degC, respectively, upon introduction of 1-vol.% H2 into the ambient. Additionally, a significant increase in the sensitivity and a decrease in the response and recovery times have been observed after increasing the operating temperature up to ~310degC

High Efficiency Digital GaN MMIC Power Amplifiers for Future Switch-Mode Based Mobile Communication Systems

Abstract: A high efficiency digital MMIC amplifier for mobile communication switch-mode concepts was designed by utilizing a 0.25 μm GaN HEMT technology with fT of 32 GHz. A comparative investigation of two different driver concepts for a 1.2 mm GaN HEMT PA is shown. The MMICs were on-wafer evaluated for class-D and class-S operation. A drain efficiency of 70% for an output power of 4.4 W for a band pass delta-sigma (BPDS) class-S input signal at a bit rate of 3.6 Gbps equivalent to a 0.9 GHz fundamental was obtained. For the first time the operating mode up to 8 Gbps (2 GHz) is shown with an efficiency of 62%, demonstrating the prospect of future use of GaN HEMTs for switch mode amplifier concepts.

Effect of nanoscale surface morphology on the phase stability of 3C-AlN films on Si(111)

Abstract: In this work, we report on the stabilization of 3C-AlN polytype by molecular-beam epitaxy (MBE) on 3C-SiC∕Si(111) pseudosubstrates. The main purpose of the present studies is to analyze the mechanisms forcing the epitaxy of the 3C-AlN at typical MBE conditions. The forces driving the cubic polytype formation have been considered including supersaturation, macroscopic stress, interfacial energy, and interface morphology. We conclude that the growth of 3C-AlN phase can be stabilized on the “rough” 3C-SiC(111) surface by the polytype replication due to the lateral character of the nucleation and the reduction in the total energy of the film provided by periodical undulations of the template surface.

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.