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This paper provides an overview of typical non-destructive test (NDT) procedures currently used for pavement structural evaluation applications. In particular, surface deflection measurements, spectral analysis of surface waves (SASW), and ground penetrating radar (GPR) approaches are described and discussed. Data interpretation and application, including problems encountered with each type of measurement, are also covered. Potential future developments in this field are discussed.

The coronavirus disease 2019 crisis has caused a sharp rise in demand for ventilators and respiratory therapists (RTs) who are trained to operate them. Shortages of these resources in hospitals have put patients who need them at increased risk. In this Perspective, researchers describe a model they developed to help assess needs and to allocate ventilators and RTs efficiently among hospitals in order to minimize patient wait time.

Flip-chip heterogeneously integrated n-p-n InGaP/GaAs heterojunction bipolar transistors (HBTs) with integrated thermal management on wide-bandgap AlN substrates followed by GaAs substrate removal are demonstrated. Without thermal management, substrate removal after integration significantly aggravates self-heating effects, causing poor $I$ – $V$ characteristics due to excessive device self-heating. An electrothermal codesign scheme is demonstrated that involves simulation (design), thermal characterization, fabrication, and evaluation. Thermoreflectance thermal imaging, electrical-temperature sensitive parameter-based thermometry, and infrared thermography were utilized to assess the junction temperature rise in HBTs under diverse configurations. In order to reduce the thermal resistance of integrated devices, passive cooling schemes assisted by structural modification, i.e., positioning indium bump heat sinks between the devices and the carrier, were employed. By implementing thermal heat sinks in close proximity to the active region of flip-chip integrated HBTs, the junction-to-baseplate thermal resistance was reduced over a factor of two, as revealed by junction temperature measurements and improvement of electrical performance. The suggested heterogeneous integration method accounts for not only electrical but also thermal requirements providing insight into realization of advanced and robust III-V/Si heterogeneously integrated electronics.

California Senate Bill 1953 requires structural and nonstructural upgrades to hospital buildings to enhance resilience to seismic events. This report estimates the costs to hospitals of compliance, focusing on the 2030 deadline. The potential costs have raised significant concerns regarding the financial burden on hospital systems and the opportunity costs associated with hospitals investing large pools of capital to implement seismic upgrades.

Our teacher leadership story comes from two schools collaborating on a New Zealand Teacher Led Innovation Fund (TLIF) project exploring the effect of student-led learning practices on literacy achievement within modern learning environments (MLEs). Our rationale is that learning which is individualised for all learners leads to more equitable outcomes for all. It also enables student ownership of learning, which in turn increases success for all learners, measured through improved student engagement, positive shifts in attitude, and improved progress and achievement. We undertook two cycles of participatory action-based inquiry to find out how successful collaboration and student ownership within the MLEs could impact on literacy engagement and achievement. We noted considerable progress in the development of key competencies, influenced by the transfer of ownership from teacher to student through choice, sharing of the curriculum, and engagement with the wider range of resources readily available in an MLE. Giving students a say in their topic and context increased their engagement and led to improved outcomes in literacy achievement. The support provided by our school management for teacher-leadership of the innovations has enabled research-informed student-led pedagogy to be developed at both schools.

Step-stress experiments on high-voltage Npn InGaP/GaAs HBTs are shown to reveal a number of degradation mechanisms, singly or in combinations: defect buildup in the emitter depletion region, defect buildup in the neutral base region, possible degradation of Ohmic contacts or increase in epitaxial layer resistances. Defect buildup in the emitter depletion region often precedes other types of degradation. Two less commonly reported degradation mechanisms are also suggested: base Ohmic metal punch-through to the collector and deterioration of the die attach material. It was found that the vast majority of devices failed within 5% of a maximum attainable power level, but when stressed just shy of this catastrophic level, electrical characteristics typically degraded gradually.

This article explores challenges in ensuring effective student transition from school or college to university. It examines the complex liaison needed for students to progress to appropriate courses, settle into university life and succeed as higher education learners. Secondary data (international literature on transition and the formation of learner identity) are analysed to identify underpinning concepts. Primary data are taken from two studies of student transition in England using student and staff surveys, student focus groups, staff interviews and staff–student conferences that discussed selected project data sets. The article goes on to offer a model of the process of transition and the formation of learner identity. It proposes that the development of higher education learner identity is essential to student achievement and is initially encouraged where schools, colleges and universities adopt integrated systems of transition. This has clear implications for practice for higher education administ...

Professional identities in further education (FE) colleges and schools in England have been substantially affected by recent educational reform; additionally, recent reform of provision for the 14–19 age group requires close collaboration between the two sectors. Drawing on case-study data from four English FE colleges, this paper takes as its empirical base an analysis of professional identities of middle leaders in FE colleges in order to explore implications for professional practice in both colleges and schools. Three elements of professional identity are proposed and examined: professional values (What I profess), professional location (The profession to which I belong) and professional role (My role within the institution). These are used to explore commonality, difference and differential status across middle managers holding a range of roles in FE, and a model for analysing professional practice is proposed. The study indicates problems of professional identity that need to be resolved if the two ...

Over the past two years noteworthy experimental and theoretical progress has been made towards the top-level scientific question for the US programme on heavy-ion-fusion-science and high energy density physics: ‘How can heavy-ion beams be compressed to the high intensity required to create high energy density matter and fusion conditions?’ New results in transverse and longitudinal beam compression, high-brightness transport and beam acceleration will be reported. Central to this campaign is final beam compression. With a neutralizing plasma, we demonstrated transverse beam compression by an areal factor of over 100 and longitudinal compression by a factor of >50. We also report on the first demonstration of simultaneous transverse and longitudinal beam compression in plasma. High beam brightness is key to high intensity on target, and detailed experimental and theoretical studies on the effect of secondary electrons on beam brightness degradation are reported. A new accelerator concept for nearterm low-cost target heating experiments was invented, and the predicted beam dynamics validated experimentally. We show how these scientific campaigns have created new opportunities for interesting target experiments in the warm dense matter regime. Finally, we summarize progress towards heavy-ion fusion, including the demonstration of a compact driver-size high-brightness ion injector. For all components of our high intensity campaign, the new results have been obtained via tightly coupled efforts in experiments, simulations and theory.

Beta titanium alloys have been available since the 1950s (Ti-13V-11Cr-3Mo or B120VCA), but significant applications of these alloys, beyond the SR-71 Blackbird, have been slow in coming. The next significant usage of a β alloy did not occur until the mid-1980s on the B-1B bomber. This aircraft used Ti-15V-3Cr-3Al-3Sn sheet due to its capability for strip rolling, improved formability, and higher strength than Ti-6Al-4V. The next major usage was on a commercial aircraft, the Boeing 777, which made extensive use of Ti-10V-2Fe-3Al high-strength forgings. Ti-15V-3Cr-3Al-3Sn environmental control system ducting, castings, and springs were also used, along with Ti-3Al-8V-6Cr-4Mo-4Zr (β-C) springs. Beta-21S was also introduced for high-temperature usage. More recent work at Boeing has focused on the development of Ti-5Al-5Mo-5V-3Cr, a high-strength alloy that can be used at higher strength than Ti-10V-2Fe-3Al and is much more robust; it has a much wider, or friendlier, processing window. This, along with additional studies at Boeing, and from within the aerospace industry in general will be discussed in detail, summarizing applications and the rationale for the selection of this alloy system for aerospace applications.

A contacted electroreflectance technique was used to investigate AlGaN/GaN heterostructures and their intrinsic electric field-induced properties. By studying variations in the electroreflectance with applied field, spectral features associated with the AlGaN barrier, the two-dimensional electron gas at the interface, and bulk GaN were identified. Barrier-layer composition and electric field were determined from the AlGaN Franz–Keldysh oscillations. For a high mobility heterostructure grown on SiC, measured AlGaN polarization electric field and two-dimensional electron gas density approached values predicted by a standard bandstructure model. The two-dimensional electron gas produced a broad, field-tunable first derivative electroreflectance feature. With a dielectric function calculation, we describe the line shape and relative amplitude of the two-dimensional electron gas electroreflectance feature for a wide range of electron density and applied field values.

Al0.2Ga0.8N/GaN HEMTs have been fabricated from MOCVD-grown epilayers on (0 0 0 1) sapphire substrates. DC current densities from 0.5 to ∼1 A/mm were achieved for gate lengths ranging from 0.75 to 0.1 μ m. A maximum transconductance of 207 mS/mm was measured for short gate length devices. From s-parameter measurements, ft of 59 GHz and fmax of 90 GHz were extracted. Load pull results from 0.25×150 μm 2 gate dimension devices indicated an output power density of 2.75 W/mm at 3 GHz and 1.7 W/mm at 10 GHz. Small signal modeling of the measured s-parameters is also presented.

Emitter orientation effects in AlGaAs/GaAs heterojunction bipolar transistors (HBTs) in which DC current gain was greater for the [0 1 1] emitter orientation compared to the [0 1 1 ] orientation were previously attributed to the piezoelectric effect, however no effects of dielectric overlayers were examined. In this work, we establish that for InGaP/GaAs HBTs, dielectric passivation effects can be as important as the piezoelectric effect. Non-self-aligned InGaP/GaAs HBTs with ECR SiON dielectric deposited at 25 °C exhibited greater stress and showed less current gain difference and lower base current ideality factors for these orientations than identical HBTs that were passivated with plasma enhanced chemical vapor deposition (PECVD) SiN dielectric. Non-self-aligned HBTs with ECR SiON passivation also showed slightly better rf performance for the [0 1 1] orientation with ft of 48 GHz compared to ft of 44 GHz for the [0 1 1 ] orientation. These differences in dielectrics are attributed to higher dielectric-induced damage in emitter–base space-charge region for the PECVD SiN film.

The temperature and gate length effects on dc performance of AlGaN/GaN high-electron mobility transistors (HEMTs) grown on AlN/SiC templates or sapphire substrates are reported. The defect density in the structures grown on the AlN/SiC template is significantly lower than those grown on sapphire, as measured by transmission electron microscopy. Reverse breakdown voltages above 40 V were obtained for 0.25 μm gate length devices on both types of substrate. Extrinsic transconductances of ∼200 mS/mm for HEMTs on sapphire and ∼125 mS/mm for devices on AlN/SiC were achieved, with the latter devices showing significantly lower self-heating effects. Both types of HEMTs showed similar trends of drain current and transconductance with increasing temperature. There was a clear signature of optical phonon scattering as the dominant scattering mechanism from room temperature to 300 °C.

The DC performance of AlGaN/GaN high electron mobility transistors grown by plasma-assisted molecular beam epitaxy was investigated for gate lengths in the range 0.1–1.2 μm. On 0.25 μm gate length devices we obtained 40 V DS operation with >50 mA peak I D . The peak drain current density was 0.44 A/mm for 100 μm gate width devices with 1.2 μm gate lengths. The extrinsic transconductance ( g m ) decreased with both gate length and gate width and was ⩾75 mS/mm for all gate widths for 0.25 μm devices. E-beam written gates typically produced a slightly lower Schottky barrier height than optically patterned gates.

The aim of this study was to retrospectively review the clinical presentation, diagnostic features, in particular cervical vestibular evoked myogenic potentials (cVEMPs), and the outcomes of surgical repair for superior semicircular canal dehiscence syndrome (SSCDS). SSCDS is a well-described syndrome of auditory and vestibular symptoms due to a bony dehiscence of the superior semicircular canal in the middle cranial fossa. A series of six procedures on five patients with SSCDS who underwent surgical repair via a middle fossa craniotomy were retrospectively reviewed. Preoperative and postoperative audiometric and vestibular symptoms as well as investigation findings were reviewed. Auditory and vestibular symptoms improved and hearing was preserved in all patients. The low frequency pseudo-conductive loss was corrected in four out of five patients, and the lowered preoperative cVEMP thresholds normalised following successful middle cranial fossa repair. In this series, middle fossa repair of SSCD was safe and effective with excellent sensorineural hearing preservation.

Learning Resource Centres (LRCs) are an increasingly common provision in Further Education Colleges. As they have derived from other types of open‐access learning facilities — libraries, Open Learning centres, subject‐based workshops — there is no common pattern of management. Rugby College set up a small facility in 1996, with a view to further expansion the following year. The study was used to support the developments, and as a management tool for the expanded Learning Resource Centre. Students attending the College in 1997 were surveyed to produce profiles of users and non‐users of the existing facility. Attractions and constraints upon use were examined in more detail through interviews with key members of the College staff. From the profiles generated by both surveys, a succession of models were built, showing the interaction of factors which influence utilisation. The key elements were: the College infrastructure; methods of curriculum delivery; the physical and social provision of the fac...

A survey of ohmic contact materials and properties to GaAs, InP, GaN will be presented along with critical issues pertaining to each semiconductor material. Au-based alloys (e.g., GeAuNi for n-type GaAs) are the most commonly used contacts for GaAs and InP materials for both n- and p-type contacts due to the excellent contact resistivity, reliability, and usefulness over a wide range of doping levels. Research into new contacting schemes for these materials has focused on addressing limitations of the conventional Au-alloys in thermal stability, propensity for spiking, poor edge definition, and new approaches for a non-alloyed contact. The alternative contacts to GaAs and InP include alloys with higher temperature stability, contacts based on solid phase regrowth, and contacts that react with the substrate to form lower bandgap semiconductors alloys at the interface. A new area of contact studies is for the wide bandgap group III-Nitride materials. At present, low resistivity ohmic contact to p-type GaN has not been obtained primarily due to the large acceptor ionization energy and the resultant difficulty in achieving high free hole concentrations at room temperature. For n-type GaN, however, significant progress has been reported with reactive Ti-based metalization schemes or the use of graded InGaN layers. The present status of these approaches will be reviewed.

SiC Schottky rectifiers with moderate breakdown voltages of ∼450 V and with either WSiX or Ni rectifying contacts were irradiated with Co-60 γ-rays to doses up to ∼315 Mrad. The Ni/SiC rectifiers show severe reaction of the contact after irradiation at the highest dose, badly degrading the forward current characteristics and increasing the on-state resistance by up to a factor of 6 after irradiation. By sharp contrast, the WSiX/SiC devices show little deterioration of the contact with the same conditions and changes in on-state resistance of

The H-Tx rat has inherited hydrocephalus with an onset in late gestation. Ventriculosubcutaneous shunts were placed in a group of hydrocephalic pups at 3-6 days after birth and in another group at 8-12 days after birth. Multislice proton magnetic resonance (MR) images were taken of shunt-treated pups at 7, 14, or 21 days and of age-matched control and untreated hydrocephalic rats and were subjected to quantitative analysis. Some rats were also imaged before surgery. The volume of the ventricles of untreated hydrocephalic pups increased linearly with age at a rate of 52 μl/day. The ventricles of shunted pups were reduced from the preshunt condition and were around 20% of age-matched hydrocephalic rats. There was no significant difference in the post-shunt volume between early and late shunt groups. The cortical mantle thickness in both groups of shunt-treated rats was significantly thicker than in untreated pups and close to that of control rats. It is concluded that shunt treatment at both ages reversed the hydrocephalus as measured from MR images, although other evidence from fixed brains suggests that normal morphology may not be achieved at the cellular level even for early shunts.

A comparison was made of specific contact resistivity and morphology of Ti/Al/Pt/WSi/Ti/Au and Ti/Al/Pt/W/Ti/Au ohmic contacts to AlGaN/GaN heterostructures relative to the standard Ti/Al/Pt/Au metallization. The W- and WSi-based contacts show comparable specific resistivities to that of the standard contact on similar layer structures, reaching minimum values of ∼10−5 Ω cm2 after annealing in the range 850–900 °C. However, the W- and WSi-based contacts exhibit much smoother surface morphologies, even after 950 °C annealing. For example, the root-mean-square roughness of the Ti/Al/Pt/WSi/Ti/Au contact annealed at 950 °C was unchanged from the as-deposited values whereas the Ti/Al/Pt/Au contact shows significant deterioration of the morphology under these conditions. The improved thermal stability of the W- and WSix-based contacts is important for maintaining edge acuity during high-temperature operation.

Unpassivated AlGaN/GaN high-electron-mobility transistors show significant gate lag effects due to the presence of surface states in the region between the gate and drain contact. Low-temperature (100 °C) layers of MgO or Sc2O3 deposited by plasma-assisted molecular-beam epitaxy are shown to effectively mitigate the collapse in drain current through passivation of the surface traps. These dielectrics may have advantages over the more conventional SiNX passivation in terms of long-term device stability.

We present the design, fabrication, and characterization of a pixelated, hyperspectral arrayed component for Focal Plane Array (FPA) integration in the Long-Wave IR. This device contains tens of pixels within a single super-pixel which is tiled across the extent of the FPA. Each spectral pixel maps to a single FPA pixel with a spectral FWHM of 200nm. With this arrayed approach, remote sensing data may be accumulated with a non-scanning, "snapshot" imaging system. This technology is flexible with respect to individual pixel center wavelength and to pixel position within the array. Moreover, the entire pixel area has a single wavelength response, not the integrated linear response of a graded cavity thickness design. These requirements bar tilted, linear array technologies where the cavity length monotonically increases across the device.

In this work, we describe the most recent progress towards the device modeling, fabrication, testing and system integration of active resonant subwavelength grating (RSG) devices. Passive RSG devices have been a subject of interest in subwavelength-structured surfaces (SWS) in recent years due to their narrow spectral response and high quality filtering performance. Modulating the bias voltage of interdigitated metal electrodes over an electrooptic thin film material enables the RSG components to act as actively tunable high-speed optical filters. The filter characteristics of the device can be engineered using the geometry of the device grating and underlying materials. Using electron beam lithography and specialized etch techniques, we have fabricated interdigitated metal electrodes on an insulating layer and BaTiO3 thin film on sapphire substrate. With bias voltages of up to 100V, spectral red shifts of several nanometers are measured, as well as significant changes in the reflected and transmitted signal intensities around the 1.55um wavelength. Due to their small size and lack of moving parts, these devices are attractive for high speed spectral sensing applications. We will discuss the most recent device testing results as well as comment on the system integration aspects of this project.

We have successfully demonstrated an integrated optical system for collecting the fluorescence from a trapped ion. The system, consisting of an array of transmissive, dielectric micro-optics and an optical fiber array, has been intimately incorporated into the ion-trapping chip without negatively impacting trapping performance. Epoxies, vacuum feedthrough, and optical component materials were carefully chosen so that they did not degrade the vacuum environment, and we have demonstrated light detection as well as ion trapping and shuttling behavior comparable to trapping chips without integrated optics, with no modification to the control voltages of the trapping chip., Comment: 14 pages, 12 figures

In this paper, we describe progress towards a multi-color spectrometer and radiometer based upon an active resonant subwavelength grating (RSG). This active RSG component acts as a tunable high-speed optical filter that allows device miniaturization and ruggedization not realizable using current sensors with conventional bulk optics. Furthermore, the geometrical characteristics of the device allow for inherently high speed operation. Because of the small critical dimensions of the RSG devices, the fabrication of these sensors can prove challenging. However, we utilize the state-ofthe-art capabilities at Sandia National Laboratories to realize these subwavelength grating devices. This work also leverages previous work on passive RSG devices with greater than 98% efficiency and ~1nm FWHM. Rigorous coupled wave analysis has been utilized to design RSG devices with PLZT, PMN-PT and BaTiO3 electrooptic thin films on sapphire substrates. The simulated interdigitated electrode configuration achieves field strengths around 3×10 7 V/m. This translates to an increase in the refractive index of 0.05 with a 40V bias potential resulting in a 90% contrast of the modulated optical signal. We have fabricated several active RSG devices on selected electro-optic materials and we discuss the latest experimental results on these devices with variable electrostatic bias and a tunable wavelength source around 1.5µm. Finally, we present the proposed data acquisition hardware and system integration plans.

We design and fabricate guided-mode resonant subwavelength gratings using an active layer of barium titanate. Loss mechanisms in the metal and in the guiding layer are investigated. Modeling and experimental results are shown.

We will discuss a passive thermal emission management surface that can manipulate the direction and wavelength bands of emission. We are designing and fabricating diffractive optics in materials that support surface-polariton plasmons. We use a grating in this material to couple the thermally-generated plasmons to photons. Grating parameters, such as grating depth and duty cycle, are varied to optimize the plasmon/photon coupling efficiency. The grating configuration ensures a phased, radiative response if the plasmon decay length along the surface traverses many grating periods. All of these parameters, material indices and dimensions, determine the specular and angular "shape" of emission.

GaN-based microwave power amplifiers have been identified as critical components in Sandia's next generation micro-Synthetic-Aperture-Radar (SAR) operating at X-band and Ku-band (10-18 GHz). To miniaturize SAR, GaN-based amplifiers are necessary to replace bulky traveling wave tubes. Specifically, for micro-SAR development, highly reliable GaN high electron mobility transistors (HEMTs), which have delivered a factor of 10 times improvement in power performance compared to GaAs, need to be developed. Despite the great promise of GaN HEMTs, problems associated with nitride materials growth currently limit gain, linearity, power-added-efficiency, reproducibility, and reliability. These material quality issues are primarily due to heteroepitaxial growth of GaN on lattice mismatched substrates. Because SiC provides the best lattice match and thermal conductivity, SiC is currently the substrate of choice for GaN-based microwave amplifiers. Obviously for GaN-based HEMTs to fully realize their tremendous promise, several challenges related to GaN heteroepitaxy on SiC must be solved. For this LDRD, we conducted a concerted effort to resolve materials issues through in-depth research on GaN/AlGaN growth on SiC. Repeatable growth processes were developed which enabled basic studies of these device layers as well as full fabrication of microwave amplifiers. Detailed studies of the GaN and AlGaN growth of SiC were conducted and techniques to measure the structural and electrical properties of the layers were developed. Problems that limit device performance were investigated, including electron traps, dislocations, the quality of semi-insulating GaN, the GaN/AlGaN interface roughness, and surface pinning of the AlGaN gate. Surface charge was reduced by developing silicon nitride passivation. Constant feedback between material properties, physical understanding, and device performance enabled rapid progress which eventually led to the successful fabrication of state of the art HEMT transistors and amplifiers.

Background: The severity of allergic reactions to food appears to be affected by many interacting factors. It is uncertain whether challenge-based reactions reflect the severity of past reactions or can predict future risk. Objective: To explore the relationship of a subject's clinical history of past reactions to the severity of reaction elicited by a low-dose, double-blind, placebo-controlled food challenge (DBPCFC) with peanut. Method: Cross-sectional questionnaire assessment of community-based allergic reactions and low-dose DBPCFC in self-selected peanut-allergic subjects. Reaction severity was assessed using a novel scoring system, taking account of the dose of allergen ingested. Results: Forty subjects (15 males, 23 children, 23 asthmatics by history) were studied. Only the most recent community reaction predicted the severity of reaction in the DBPCFC, but even this association was weak (r=0.37, P=0.03). Peanut-specific IgE (PsIgE) and skin prick test (SPT) weal size were not associated with community score but PsIgE level correlated well with the challenge score (r=0.6, P=0.001). Asthma did not affect the eliciting dose or challenge score directly but the association of PsIgE and challenge score was stronger in those without asthma (r=0.72, P=0.001) than in those with asthma (r=0.48, P=0.02). Conclusions: The scoring system developed appears to improve the sensitivity of assessment of reactions induced by DBPCFC. This is the first prospective study showing an association between PsIgE levels and clinical reactivity in DBPCFC, an effect that is more pronounced in non-asthmatics. This finding has important implications for the clinical care of subjects with food allergy. There is a poor correlation between the severity of reported reactions in the community and the severity of reaction elicited during low-dose DBPCFC with peanut.

In this paper, high temperature hydrogen gas sensors with GaN and SiC based Schottky diodes were studied using Pd and Pt as Schottky contacts. Here we investigated thermal stability of W and WSi based Schottky metals for high temperature gas sensing. Schottky characteristics were observed for Au/Ti/W/SiC up to 900/spl deg/C annealing.

This paper presents the design for a HACT-based tapped delay line for adaptive equalization of frequency selective multipath faded channels for a wireless terrestrial digital microwave communication system. This approach allows monolithic integration of the electronics and improvements in insertion loss and noise figure over the SAW approach currently used in this system. >

A contacted electroreflectance technique was used to characterize the electronic properties of AlGaN/GaN heterostructure field-effect transistors (HFETs). By studying variations in the electroreflectance with applied electric field, spectral features associated with the AlGaN barrier, the 2-dimensional electron gas at the interface, and bulk GaN were observed. Barrier-layer composition and electric field were determined from the AlGaN Franz-Keldysh oscillations. Comparing HFETs grown on SiC and sapphire substrates, the measured AlGaN polarization electric field (0.25±0.05 MV/cm) approached that predicted by a standard model (0.33 MV/cm) for the higher mobility HFET grown on SiC.

Since InGaP/GaAs heterojunction bipolar transistors (HBTs) are utilized in a wide variety of RF and other applications, a great deal has been learned about their reliability. Nevertheless, this knowledge is limited by the fact that most reliability studies heavily emphasize the stress-related evolution of a single parameter, the DC current gain, beta. We have found that interrupted stressing experiments, with complete characterization of HBTs during the interruption, can give a more complete picture of the degradation that occurs during bias stressing of HBTs. We have previously correlated electrical signatures with degradation in step-stressing of packaged InGaP/GaAs HBTs [1]. In this work we will present some results of step-stress experiments which have been conducted at the wafer level for InGaP/GaAs HBTs. Wafer-level evaluation is advantageous due to the potential for greater automation and flexibility in the testing as well as the ability to identify problems at an earlier stage in the process. The HBTs are stressed at room temperature at a constant collector voltage. Stressing is initiated at a given base current, held for a fixed time, which is typically 10 s. The applied stress is stepped up to a higher level of base current and the process is iterated until a failure is recorded. Full DC parametric electrical characterization of the HBT gets carried out initially and after certain predefined stress steps. Although the stress is typically carried near room temperature, it is understood that the junction of the HBT is effectively self-heated and can reach extremely high temperatures. A certain set of degradation processes are responsible for stress-induced failures and these occur singly or in combinations. In addition, many HBTs within a wafer and among duplicate wafers have not only a number of common failure modes, but a tendency towards a consistently realized destructive power limit. The consistency of these results and the utility of electrical signature analysis lead us to conclude that wafer-level step-stressing is an excellent tool for quickly monitoring the HBT quality. We have also found many commonalities in the wafer-level and package level results for the ceramic packages used in a previous study [1]. The degradation of HBTs by these stressing methods leads eventually to a large drop in current gain. Thus, the interrupted step stressing method arrives at the same end state as continuous, single-parameter monitoring, but with much more clarity about the degradation processes that occur on the way to the end state. The interrupted step-stress process in combination with electrical signature analysis is very likely to find the weak link in an HBT process. We will discuss the utility of the technique in terms of the opportunities to ruggedize the process. It is also possible that the identified weak link is fundamental to the technology or inconsequential in terms of meeting reliability requirements. In that case, wafer-level step-stressing is still an excellent tool for early identification of quality issues that may affect reliability through monitoring the consistency (or lack thereof) in degradation observed across different wafers over different periods of time. A.G. Baca, A.J. Scruggs, A. Gorenz, T.R. Fortune, J.F. Klem, R.D. Briggs, J.B. Clevenger, G.A. Patrizi, and C.T. Sullivan, “A Survey of Electrical Signatures Characteristic of Step-Stressed InGaP/GaAs HBTS,” ECS Transactions vol. 50 (6), pp. 273-282 (2012). Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000.

Great strides have been made in the development of ultraviolet LED materials and devices. Power levels in the near UV (below 390 nm) have been improved from the 10 W to the 1 mW level through improvements in the growth and design of AlInGaN alloys. High frequency AlGaN/GaN HEMTs have been developed with ft of 65 GHz and fmax of 85 GHz, all while attaining breakdown voltage greater than 100 V. A new breakthrough in the lateral overgrowth of GaN materials promises to further improve these devices.

Massively parallel optical interconnects are appropriate to ease the data bandwidth bottleneck that will occur in future computing applications. Vertical cavity surface emitting lasers (VCSELs) are promising sources for emerging 2D optical systems such as free space and guided wave optical interconnects. We discuss the development of high performance VCSEL arrays, including individually addressable and matrix addressable arrays. We also show the characteristics of GaAs microelectronic driver and photoreceiver chips that have been designed to interface with Si-based CMOS circuitry. Finally, the potential of these source and receiver modules for use in free space or guided wave parallel channel optical interconnect architectures will be described.

Vertical cavity surface emitting lasers (VCSELs) which operate in multiple transverse optical modes have been rapidly adopted into present data communication applications which rely on multi-mode optical fiber. However, operation only in the fundamental mode is required for free space interconnects and numerous other emerging VCSEL applications. Two device design strategies for obtaining single mode lasing in VCSELs based on mode selective loss or mode selective gain are reviewed and compared. Mode discrimination is attained with the use of a thick tapered oxide aperture positioned at a longitudinal field null. Mode selective gain is achieved by defining a gain aperture within the VCSEL active region to preferentially support the fundamental mode. VCSELs which exhibit greater than 3 mW of single mode output power at 850 nm with mode suppression ratio greater than 30 dB are reported.

Ultra high-speed digital photography has been used to record the onset and build-up of reaction in nitromethane charges that have been impacted by steel fragments. The nitromethane charges were housed in PMMA cylinders and back-lit using conventional flash bulbs. Flat plates of aluminum were glued to one end of the cylinder and PMMA plates to the other. The completed charge was positioned to allow normal impact of the projectiles through the aluminum barrier plate. The events were filmed using an Imacon 468, ultra high-speed digital image system, capable of framing at up to 100 million pictures per second. Using this system it was possible to record detailed photographic information concerning the onset and growth of reaction due to shock initiation of the nitromethane charges. The results obtained to date are consistent with the established concepts for initiation of homogeneous and heterogeneous materials.

Optocouplers are used for a variety of applications aboard spacecraft including electrical isolation, switching and power transfer. Commercially available light emitting diode (LED)-based optocouplers have experienced severe degradation of light output due to extensive displacement damage occurring in the semiconductor lattice caused by energetic proton bombardment. A new optocoupler has been designed and fabricated which utilizes vertical cavity surface emitting laser (VCSEL) and resonant cavity photodetector (RCPD) technologies for the optocoupler emitter and detector, respectively. Linear arrays of selectively oxidized GaAs/AlGaAs VCSELS and RCPDS, each designed to operate at a wavelength of 850nm, were fabricated using an airbridge contacting scheme. The airbridged contacts were designed to improve packaging yields and device reliability by eliminating the use of a polyimide planarizing layer which provided poor adhesion to the bond pad metallization. Details of the airbridged optocoupler fabrication process are reported. Discrete VCSEL and RCPD devices were characterized at temperatures between {minus}100 to 100 C. Devices were packaged in a face-to-face configuration to form a single channel optocoupler and its performance was evaluated under conditions of high-energy proton bombardment.

Fabrication of group-Ill nitride electronic and photonic devices relies heavily on the ability to pattern features with anisotropie profiles, smooth surface morphologies, etch rates often exceeding 0.5 μm/min, and a low degree of plasma-induced damage. Patterning these materials has been especially difficult due to their high bond energies and their relatively inert chemical nature as compared to other compound semiconductors. However, high-density plasma etching has been an effective patterning technique due to ion fluxes which are 2 to 4 orders of magnitude higher than conventional RIE systems. GaN etch rates as high as -1.3 μm/min have been reported in ECR generated ICI plasmas at -150 V dc-bias. In this study, we report high-density GaN etch results for ECR- and ICP-generated plasmas as a function of Cl2- and BCl3-based plasma chemistries.

There is a paucity of data on which to base estimates of the energy requirements of the elderly. In general, ageing appears to be associated with a reduction in energy requirement arising from a reduction in physical activity and loss of fat-free mass. The aim of the present study was to measure the total energy expenditure (TEE), basal metabolic rate (BMR), and energy expended on physical activity (calculated as TEE–BMR) in a group of healthy elderly women living in the community in Southampton. Mean rates of TEE (9.21 (SD 1.48) MJd) and energy expended on physical activity (4.12 (SD 1.19) MJ/d) were higher than those observed in some studies of younger adults in the UK, and higher than the factors used to estimate the average energy requirement for the elderly. The results suggest that an age-related reduction in energy requirement is not inevitable and support the hypothesis that the effects of ageing on physical activity, body composition, and hence energy requirements, are variable.

A 300 ft (91.4 m) cable stayed bridge near Skagway, Alaska carries in addition to normal traffic 160,000 lb (71.576 kg) ore trucks. It has a laminated timber deck supported by transverse floor beams spanning two stiffened ASTM A588 steel box girders. Two inclined cables stay the girders near midlength to a single tower. The abutments and tower were rock anchored to the canyon walls.

Rationale The delayed introduction of solids, particularly of allergenic foods, into an infants diet is advocated by the American Academy of Pediatrics as an allergy prevention measure. Such advice is also incorporated into many allergy prevention studies. Methods We compared retrospectively sensitization rates of control groups of two intervention allergy prevention studies. Recruitment to the studies ran consecutively, with the same criteria and procedures being applied to each. Mothers from the first intervention study (S1) received no weaning advice, whereas mothers from the later study (S2) were advised to delay the introduction of solids until 4 months of age and not to introduce allergenic foods until after 6 months of age. Skin prick test results and presence of eczema were compared at 1 year (n=185) and 3 years of age (n=79). Results At one year, sensitization rates in S2 mothers was 21% to food allergens and 36% to food and aeroallergens combined. In S1 mothers these values were 8% and 20% respectively. These differences are significant (p=0.023 and p=0.015). At three years of age there was no difference in sensitization rates (p=1.0 and p=0.79). There was no significant difference in eczema rates at either time point (p=0.3 and p=0.3). Conclusions Delayed weaning strategies may not be as an effective allergy prevention measure as is currently thought and should be further investigated.

MgO or Sc 2 O 3 passivated AlGaN/GaN high electron mobility transistors (HEMTs) were irradiated with 40 MeV protons at a dose equivalent to ∼10 years in low-earth orbit. Very little change in drain-source current, transconductance, or diode ideality factor was observed under these conditions, but the reverse breakdown increased due to a decrease in channel electron density. In addition, no significant change was observed in the drain-source current under pulsed conditions, indicating that the proton irradiation did not alter the effectiveness of the MgO and Sc 2 O 3 in passivating surface states.

The authors describe an (InGaAl)As/InGaAs vertical-cavity optical transmission modulator operating at 1.32 /spl mu/m, and having a single pass contrast /spl Delta/T/T of 24% at