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 …

I and i

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

Principles of Neural Science

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.

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Phonons and related crystal properties from density-functional perturbation theory

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.

Band parameters for III–V compound semiconductors and their alloys

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.

Conjugated polymer-based organic solar cells

In this work we present a novel concept of piezoelectrically actuated MEMS resonators based on AlGaN/GaN-heterostructures. The 2DEG, which is confined at the AlGaN/GaN interface, serves as back electrode for piezoelectric actuation and read-out. Using the inverse piezoelectric effect longitudinal in-plane mechanical oscillations could be exited by applying a RF signal to the drive contacts. Through the charges, which were generated by the direct piezoelectric effect a sensor signal could be detected at the sense contacts of the integrated resonator structure. This enables the measurement of longitudinal resonant frequencies up to the 4th mode with values from 3 to 63 MHz. The investigations were carried out in various ambient pressure conditions, which exemplarily demonstrate the sensitivity of the quality factor to environmental parameters. Thereby the AlGaN/GaN-resonators showed the highest sensitivity in the region of viscous damping, which leads to the possibility of sensor applications concerning the property determination of gaseous or even liquid surrounding conditions. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Piezoelectric actuated epitaxially grown AlGaN/GaN‐resonators

GaN based pH-sensors were used to measure the reaction and the kinetics of a lipase assay producing caprylic acid. Sensor structures consist of an AlGaN/GaN-heterostructure grown by plasma-induced molecular beam epitaxy with a thin GaN cap layer. The carrier concentration of the two-dimensional electron gas is manipulated by changing the surface potential due to changes in pH value.

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AlGaN/GaN-sensors for Monitoring of Enzyme Activity by pH-Measurements

This work shows a GaN-based current sense amplifier as a readout circuit for a power GaN HEMT with integrated current shunt. In the GaN power transistor, the voltage drop of the outermost metal finger of a large-area comb structure effectively realizes a shunt acting as a current sensor. This small voltage drop of the integrated shunt (approx. 4 mV/A at $100 \mathrm{m}\Omega$ on-resistance) is amplified by a two-stage GaN-based amplifier with a gain of $\gt 40$ dB at room temperature. The amplifier consisting of two series-connected differential amplifiers with source follower intermediate stages was verified in a double pulse test setup up to a peak current of 11.5 A. Consequently, the proposed current sense amplifier allows a simple and potentially integratable further processing for the monitoring or control of the current signal.

A GaN-based Current Sense Amplifier for GaN HEMTs with Integrated Current Shunts

In this paper a low-power wireless data transmitter MMIC at 113 GHz with data rates up to 25 Gbit/s is presented. The power consumption of the transmitter MMIC is 9.48mW and the output power is about 4.3 dBm. The measured bit error rate is less than 1 × 10
 -12
 for at least up to 13 Gbit/s. The transmitter is a fully integrated MMIC based on a 50nm InGaAs metamorphic HEMT technology. The circuit consists of a low-power voltage controlled oscillator as signal generator and a low-loss high-speed switch as amplitude modulator, only needing a single-ended data input signal. The whole transmitter MMIC has a size of only 1.25×1mm
 2
.

Low-power wireless data transmitter MMIC with data rates up to 25 Gbit/s and 9.5mW power consumption using a 113 GHz carrier

The degradation of packaged GaN HEMTs for high power applications has been studied under long term reverse bias step stress tests. Increases of leakage current and dynamic R on resistance have been found. This degradation is possibly caused by the formation of localized defects which have been observed by backside electroluminescence imaging. In addition the effect of device layout and substrate material on the dynamic R on as well as its temperature, recovery behavior, and drain voltage dependence have been investigated on wafer-level. The recovery behavior and the temperature dependence indicate that the dynamic R on resistance increase is caused by surface or buffer carrier trapping. By reducing the buffer trap density the dynamic R on resistance was reduced. A slightly higher dynamic R on of GaN HEMTs on silicon compared to transistors on SiC substrate has been observed.

Oliver Ambacher

Papers

Piezoelectric actuated epitaxially grown AlGaN/GaN‐resonators

AlGaN/GaN-sensors for Monitoring of Enzyme Activity by pH-Measurements

A GaN-based Current Sense Amplifier for GaN HEMTs with Integrated Current Shunts

Low-power wireless data transmitter MMIC with data rates up to 25 Gbit/s and 9.5mW power consumption using a 113 GHz carrier

Reverse bias stress test of GaN HEMTs for high-voltage switching applications