31 research outputs found
Transistor-based measurements of electron injection currents in p-type GaAs doped 1018â1020 cm-3
Measurements of electron currents injected into p+âGaAs are presented for molecular beam epitaxially grown material doped from 2Ă1018 to 8Ă1019 cmâ3 with Be. The collector current versus baseâemitter voltage characteristics of nâp+ân GaAs homojunction bipolar transistors are analyzed, and the results are interpreted in terms of the quantity (n0Dn), where n0 is the equilibrium minorityâcarrier concentration and Dn is the minorityâcarrier diffusion coefficient. The results are consistent with earlier measurements of (n0Dn) made using metalorganic chemical vapor deposited p+ân GaAs solar cells, Zn doped as heavily as 1Ă1019 cmâ3. The large electron injection currents observed are interpreted as evidence for significant effective bandâgap shrinkage. These effects must be accounted for in the modeling and design of GaAsâbased heterojunction bipolar transistors and solar cells
Effective minorityâcarrier hole confinement of Siâdoped, n+ân GaAs homojunction barriers
he electrical performance of Siâdoped n+ân GaAs homojunction barriers grown by molecularâbeam epitaxy (MBE) is characterized and analyzed. We employed a successive etch technique to study hole injection currents in GaAs n+ânâp+ solar cells. The results of the analysis show that minorityâcarrier holes in our MBEâgrown material have a mobility of 293 cm2/Vâs for an nâtype Siâdoping level of 1.5Ă1016 cmâ3 at 300 K. The interface recombination velocity for these homojunction barriers is estimated to be less than 1Ă103 cm/s, and it appears to be comparable to that recently observed for Siâdoped n+ân GaAs homojunction barriers grown by metalorganic chemical vapor deposition. We present evidence that these n+ân GaAs homojunctions, unlike p+âp GaAs homojunctions, are almost as effective as AlGaAs heterojunctions in minorityâcarrier confinement, and that their electrical performance is not degraded by heavy doping effects
Effects of Heavy Impurity Doping on Electron Injection in p+-n GaAs Diodes
Measurements of electron injection currents in p+ân diodes are presented for a range of pâtype dopant concentrations. A successive etch technique was used to characterize the electron injection current in terms of the product (noDn). Measurements are presented for Znâdoped GaAs solar cells with pâlayer hole concentrations in the range 6.3Ă1017â1.3Ă1019 cmâ3. The results demonstrate that soâcalled bandâgap narrowing effects substantially increase the injected electron current in heavily doped pâtype GaAs. These heavy doping effects must be accounted for in the modeling and design of GaAs solar cells and heterostructure bipolar transistors
Zero-Field Time-of-Flight Measurements of Electron Diffusion in P+-GaAs
Minority electron diffusivities in p+-GaAs-doped NA =~1.4Ă1018 and ~1019 cm-3 have been measured in zero-field conditions with an extension of the zero-field time-of-flight technique. Extension of the technique to make it applicable to heavily doped p+-GaAs is described and zero-field data are discussed. Unexpectedly, majority carrier drag effects are not evident in a comparison of this data with recently reported high-field data. Low zero-field mobility of electrons in p+-GaAs has important implications for high-speed devices such as heterojunction bipolar transistors
Evidence for band-gap narrowing effects in Be-doped, p-p+ GaAs homojunction barriers
The electrical performance of Beâdoped, pâp+ GaAs homojunction barriers is characterized and analyzed. The results of the analysis show that minorityâcarrier electrons, at 300 K, have a mobility of 4760 cm2/V s at a hole concentration of 2.3Ă1016 cmâ3, and that the effective recombination velocity for these homojunction barriers is about 6Ă104 cm/s. We present evidence that this unexpectedly high recombination velocity is a consequence of an effective reduction in band gap due to the heavy impurity doping. The effective bandâgap shrinkage in this Beâdoped material grown by molecularâbeam epitaxy appears to be comparable to that already observed for Znâdoped GaAs grown by metalorganic chemical vapor deposition. This work demonstrates that soâcalled bandâgap narrowing effects significantly influence the electrical performance of GaAs devices
Recombination-current suppression in GaAs p-n junctions grown on AlGaAs buffer layers by molecular-beam epitaxy
n+pp+GaAs and n+pP+ GaAs/GaAs/Al0.3Ga0.7As mesa diodes have been fabricated from films grown by molecularâbeam epitaxy. The diodes made from films employing an AlGaAs buffer layer show marked improvements (a factor of 5 reduction) in recombination current densities. Deep level transient spectroscopy measurements moreover indicate that deep level concentrations are reduced by the AlGaAs buffer
Multifractal Spatial Patterns and Diversity in an Ecological Succession
We analyzed the relationship between biodiversity and spatial biomass heterogeneity along an ecological succession developed in the laboratory. Periphyton (attached microalgae) biomass spatial patterns at several successional stages were obtained using digital image analysis and at the same time we estimated the species composition and abundance. We show that the spatial pattern was self-similar and as the community developed in an homogeneous environment the pattern is self-organized. To characterize it we estimated the multifractal spectrum of generalized dimensions Dq. Using Dq we analyze the existence of cycles of heterogeneity during succession and the use of the information dimension D1 as an index of successional stage. We did not find cycles but the values of D1 showed an increasing trend as the succession developed and the biomass was higher. D1 was also negatively correlated with Shannon's diversity. Several studies have found this relationship in different ecosystems but here we prove that the community self-organizes and generates its own spatial heterogeneity influencing diversity. If this is confirmed with more experimental and theoretical evidence D1 could be used as an index, easily calculated from remote sensing data, to detect high or low diversity areas
Best practices in heterotrophic high-cell-density microalgal processes: achievements, potential and possible limitations
Microalgae of numerous heterotrophic genera (obligate or facultative) exhibit considerable metabolic versatility and flexibility but are currently underexploited in the biotechnological manufacturing of known plant-derived compounds, novel high-value biomolecules or enriched biomass. Highly efficient production of microalgal biomass without the need for light is now feasible in inexpensive, well-defined mineral medium, typically supplemented with glucose. Cell densities of more than 100Â gâlâ1 cell dry weight have been achieved with Chlorella, Crypthecodinium and Galdieria species while controlling the addition of organic sources of carbon and energy in fedbatch mode. The ability of microalgae to adapt their metabolism to varying culture conditions provides opportunities to modify, control and thereby maximise the formation of targeted compounds with non-recombinant microalgae. This review outlines the critical aspects of cultivation technology and current best practices in the heterotrophic high-cell-density cultivation of microalgae. The primary topics include (1) the characteristics of microalgae that make them suitable for heterotrophic cultivation, (2) the appropriate chemical composition of mineral growth media, (3) the different strategies for fedbatch cultivations and (4) the principles behind the customisation of biomass composition. The review confirms that, although fundamental knowledge is now available, the development of efficient, economically feasible large-scale bioprocesses remains an obstacle to the commercialisation of this promising technology
Modeling the interactions between river morphodynamics and riparian vegetation
The study of river-riparian vegetation interactions is an important and intriguing research field in geophysics. Vegetation is an active element of the ecological dynamics of a floodplain which interacts with the fluvial processes and affects the flow field, sediment transport, and the morphology of the river. In turn, the river provides water, sediments, nutrients, and seeds to the nearby riparian vegetation, depending on the hydrological, hydraulic, and geomorphological characteristic of the stream. In the past, the study of this complex theme was approached in two different ways. On the one hand, the subject was faced from a mainly qualitative point of view by ecologists and biogeographers. Riparian vegetation dynamics and its spatial patterns have been described and demonstrated in detail, and the key role of several fluvial processes has been shown, but no mathematical models have been proposed. On the other hand, the quantitative approach to fluvial processes, which is typical of engineers, has led to the development of several morphodynamic models. However, the biological aspect has usually been neglected, and vegetation has only been considered as a static element. In recent years, different scientific communities (ranging from ecologists to biogeographers and from geomorphologists to hydrologists and fluvial engineers) have begun to collaborate and have proposed both semiquantitative and quantitative models of river-vegetation interconnections. These models demonstrate the importance of linking fluvial morphodynamics and riparian vegetation dynamics to understand the key processes that regulate a riparian environment in order to foresee the impact of anthropogenic actions and to carefully manage and rehabilitate riparian areas. In the first part of this work, we review the main interactions between rivers and riparian vegetation, and their possible modeling. In the second part, we discuss the semiquantitative and quantitative models which have been proposed to date, considering both multi- and single-thread river