The merits and limitations of local impact ionization theory

Multiplication measurements on GaAs p+-i-n+s with i-region thicknesses, w, between 1 μm and 0.025 μm and Monte Carlo (MC) calculations of the avalanche process are used to investigate the applicability of the local ionization theory. The local expressions for multiplication are able to predict the measured values surprisingly well in p+-i-n+s with i-region thicknesses, w, as thin as 0.2 μm before the effect of dead-space, where carriers have insufficient energy to ionize, causes significant errors. Moreover, only a very simple correction to the local expressions is needed to predict the multiplication accurately where the field varies rapidly in abrupt one-sided p+-n junctions doped up to 1018 cm-3. However, MC modeling also shows that complex dead-space effects cause the local ionization coefficients to be increasingly unrepresentative of the position dependent values in the device as w is reduced below 1 μm. The success of the local model in predicting multiplication is therefore attributed to the dead-space information already being contained within the experimentally determined values of local coefficients. It is suggested that these should therefore be thought of as effective coefficients which, despite the presence of dead-space effects, can be still be used with the existing local theory for efficiently quantifying multiplication and breakdown voltages

Toward Making the Constraint Hypersurface an Attractor in Free Evolution

There is an abundance of empirical evidence in the numerical relativity literature that the form in which the Einstein evolution equations are written plays a significant role in the lifetime of numerical simulations. This paper attempts to present a consistent framework for modifying any system of evolution equations by adding terms that push the evolution toward the constraint hypersurface. The method is, in principle, applicable to any system of partial differential equations which can be divided into evolution equations and constraints, although it is only demonstrated here through an application to the Maxwell equations.Comment: 6 pages, 3 figures, 1 table. Uses REVTeX

Avalanche multiplication in AlxGa1-xAs (x=0to0.60)

Electron and hole multiplication characteristics, Me and Mh, have been measured in AlxGa1-xAs (x=0-0.60) homojunction p+-i-n+ diodes with i-region thicknesses, w, from 1 μm to 0.025 μm and analyzed using a Monte Carlo model (MC). The effect of the composition on both the macroscopic multiplication characteristics and microscopic behavior is therefore shown for the first time. Increasing the alloy fraction causes the multiplication curves to be shifted to higher voltages such that the multiplication curves at any given thickness are practically parallel for different x. The Me/Mh ratio also decreases as x increases, varying from ~2 to ~1 as x increases from 0 to 0.60 in a w=1 μm p+-i-n+. The Monte-Carlo model is also used to extract ionization coefficients and dead-space distances from the measured results which cover electric field ranges from ~250 kV/cm-1200 kV/cm in each composition. These parameters can be used to calculate the nonlocal multiplication process by solving recurrence equations. Limitations to the applicability of field-dependent ionization coefficients are shown to arise however when the electric-field profile becomes highly nonunifor

The effects of nonlocal impact ionization on the speed of avalanche photodiodes

The nonlocal enhancement in the velocities of charge carriers to ionization is shown to outweigh the opposing effects of dead space, increasing the avalanche speed of short avalanche photodiodes (APDs) over the predictions of a conventional local model which ignores both of these effects. The trends in the measured gain-bandwidth product of two short InAlAs APDs reported in the literature support this result. Relatively large speed benefits are predicted to result from further small reductions in the lengths of short multiplication regions

Low multiplication noise thin Al0.6Ga0.4As avalanche photodiodes

Avalanche multiplication and excess noise were measured on a series of Al0.6Ga0.4As p+in+ and n+ip+ diodes, with avalanche region thickness, w ranging from 0.026 μm to 0.85 μm. The results show that the ionization coefficient for electrons is slightly higher than for holes in thick, bulk material. At fixed multiplication values the excess noise factor was found to decrease with decreasing w, irrespective of injected carrier type. Owing to the wide Al0.6Ga0.4As bandgap extremely thin devices can sustain very high electric fields, giving rise to very low excess noise factors, of around F~3.3 at a multiplication factor of M~15.5 in the structure with w=0.026 μm. This is the lowest reported excess noise at this value of multiplication for devices grown on GaAs substrates. Recursion equation modeling, using both a hard threshold dead space model and one which incorporates the detailed history of the ionizing carriers, is used to model the nonlocal nature of impact ionization giving rise to the reduction in excess noise with decreasing w. Although the hard threshold dead space model could reproduce qualitatively the experimental results, better agreement was obtained from the history-dependent mode

Major growth in some business related uses of climate information

Uses of climate information have grown considerably in the past 15 years as a wide variety of weathersensitive businesses sought to deal effectively with their financial losses and manage risks associated with various weather and climate conditions. Availability of both long-term quality climate data and new technologies has facilitated development of climate-related products by private-sector atmospheric scientists and decision makers. Weather derivatives, now widely used in the energy sector, allow companies to select a financially critical seasonal weather threshold, and, for a price paid to a provider, to obtain financial reparation if this threshold is exceeded. Another new product primarily used by the insurance industry is weather-risk models, which define the potential risks of severe-weather losses across a region where few historical insured loss data exist. Firms develop weather-risk models based on historical storm information combined with a target region’s societal, economic, and physical conditions. Examples of the derivatives and weather-risk models and their uses are presented. Atmospheric scientists who want to participate in the development and use of these new risk-management products will need to broaden their educational experience and develop knowledge and skills in fields such as finance, geography, economics, statistics, and information technology

A critical exploration of ‘access’ in qualitative International Business field research: towards a concept of socio-cultural and multidimensional research practice

This is the author accepted manuscript. It is currently under an indefinite embargo pending publication by EmeraldThis is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.Purpose: Researchers often face challenges in locating and obtaining relevant and meaningful information during qualitative International Business (IB) field research in other countries. This process constitutes an immensely critical phase, which determines the success or failure of the research endeavour. This article discusses ‘access’ as a multidimensional and contestable concept, that poses particular challenges in international and multicultural research contexts. Design/methodology/approach This article builds on our experience as field researchers in China/Hong Kong (120 in-depth interviews) and the need to disseminate acquired field experiences, in particular concerning ‘access’. The multifaceted issue of ‘access’ is rarely featured on the IB methodological agenda, and has become a silent feature of qualitative IB research. Findings This article is devoted to this nexus: the lack of focus on ‘access’ issues, and the rich sources of acquired, but mostly veiled, field experiences that feature in both international business and management research programmes. A plausible explanation for this circumstance relates to the influence of mainstream positivist and objectivist paradigms in which researchers are not recognised as having an impact on research processes, hence taking this silent feature for granted. Originality/value By viewing the multiple dimensions of ‘access’, we move beyond the mainstream understanding that merely relates it to the question of gaining access to a physical site and/or the time of an individual, and in which ‘access’ is only an enterprise of securing pre-existing, tangible information. Drawing upon specific international field-research experiences, this article contributes to the methodological debate concerning ‘access’ – beyond ‘technicality’ and towards a concept of socio-cultural and multidimensional research practice.The authors would like to thank the two anonymous reviewers and the Guest Editor, Prof Karen Locke, for the very helpful and insightful recommendations. The authors are also very grateful to the David C. Lam Institute for East-West Studies (Hong Kong Baptist University) for bestowing upon both researchers a Resident Graduate Scholarship towards conducting field research in China/Hong Kong.The authors would like to thank the two anonymous reviewers and the Guest Editor, Professor Karen Locke, for the very helpful and insightful recommendations. The authors are also very grateful to the David C. Lam Institute for East-West Studies (Hong Kong Baptist University) for bestowing upon both researchers a Resident Graduate Scholarship towards conducting field research in China/Hong Kong

Avalanche noise characteristics of thin GaAs structures with distributed carrier generation

It is known that both pure electron and pure hole injection into thin GaAs multiplication regions gives rise to avalanche multiplication with noise lower than predicted by the local noise model. In this paper, it is shown that the noise from multiplication initiated by carriers generated throughout a 0.1 μm avalanche region is also lower than predicted by the local model but higher than that obtained with pure injection of either carrier type. This behavior is due to the effects of nonlocal ionization brought about by the dead space; the minimum distance a carrier has to travel in the electric field to initiate an ionization even

Spontaneous R-Symmetry Breaking in O'Raifeartaigh Models

We study the question of whether spontaneous U(1)_R breaking can occur in O'Raifeartaigh-type models of spontaneous supersymmetry breaking. We show that in order for it to occur, there must be a field in the theory with R-charge different from 0 or 2. We construct the simplest O'Raifeartaigh model with this property, and we find that for a wide range of parameters, it has a meta-stable vacuum where U(1)_R is spontaneously broken. This suggests that spontaneous U(1)_R breaking actually occurs in generic O'Raifeartaigh models.Comment: 19 pages; v2: reference added, minor changes; v3: important typo fixe

Visibility and activity: foreign affairs think tanks in the United Kingdom

publication-status: Publishedtypes: Articlepeer-reviewed electronic journal publishing postgraduate research in the field of politicsIf politics is about transforming ‘reality’, then think tanks are in the business of interpreting politics. However, there is a lack of research dealing with the way think tanks disseminate ideas. Although think tanks are publicly recognised, researchers face a number of difficulties in determining their exact impact on the policy process. As think tanks are mostly concerned with the climate of opinion, we aim to explore the ‘visibility’ and ‘activity’ of a comparable sample of three United Kingdom (UK) foreign policy think tanks, namely Chatham House, the International Institute for Strategic Studies, and the Royal United Services Institute for Defence and Security Studies. These are ranked amongst the most influential ‘foreign affairs’ think tanks in the UK. ‘Visibility’ signals the presence of think tanks on the Internet and in the media. ‘Activity’ reflects the understanding of ‘the political’ as outcomes generated by their publications, and networking activities of their members and staff. For this purpose, we combine the usage of digital methods for ‘visibility aspects’, and elite methods for ‘activity aspects’ as a means to explore a possible reconceptualisation of ‘influence’ by encouraging the academic debate to approach this concept beyond the conventional quantitative and/or self-referential inquiry