Transistor-based measurements of electron injection currents in p-type GaAs doped 1018–1020 cm-3

Measurements of electron currents injected into p+‐GaAs are presented for molecular beam epitaxially grown material doped from 2×1018 to 8×1019 cm−3 with Be. The collector current versus base‐emitter voltage characteristics of n‐p+‐n GaAs homojunction bipolar transistors are analyzed, and the results are interpreted in terms of the quantity (n0Dn), where n0 is the equilibrium minority‐carrier concentration and Dn is the minority‐carrier diffusion coefficient. The results are consistent with earlier measurements of (n0Dn) made using metalorganic chemical vapor deposited p+‐n GaAs solar cells, Zn doped as heavily as 1×1019 cm−3. The large electron injection currents observed are interpreted as evidence for significant effective band‐gap shrinkage. These effects must be accounted for in the modeling and design of GaAs‐based heterojunction bipolar transistors and solar cells

Recent advancements in monolithic AlGaAs/GaAs solar cells for space applications

High efficiency, two terminal, multijunction AlGaAs/GaAs solar cells were reproducibly made with areas of 0.5 sq cm. The multiple layers in the cells were grown by Organo Metallic Vapor Phase Epitaxy (OMVPE) on GaAs substrates in the n-p configuration. The upper AlGaAs cell has a bandgap of 1.93 eV and is connected in series to the lower GaAs cell (1.4 eV) via a metal interconnect deposited during post-growth processing. A prismatic coverglass is installed on top of the cell to reduce obscuration caused by the gridlines. The best 0.5 sq cm cell has a two terminal efficiency of 23.0 pct. at 1 sun, air mass zero (AM0) and 25 C. To date, over 300 of these cells were grown and processed for a manufacturing demonstration. Yield and efficiency data for this demonstration are presented. As a first step toward the goal of a 30 pct. efficient cell, a mechanical stack of the 0.5 sq cm cells described above, and InGaAsP (0.95 eV) solar cells was made. The best two terminal measurement to date yields an efficiency of 25.2 pct. AM0. This is the highest reported efficiency of any two terminal, 1 sun space solar cell

Effective minority‐carrier hole confinement of Si‐doped, n+‐n GaAs homojunction barriers

he electrical performance of Si‐doped n+‐n GaAs homojunction barriers grown by molecular‐beam epitaxy (MBE) is characterized and analyzed. We employed a successive etch technique to study hole injection currents in GaAs n+‐n‐p+ solar cells. The results of the analysis show that minority‐carrier holes in our MBE‐grown material have a mobility of 293 cm2/V s for an n‐type Si‐doping level of 1.5×1016 cm−3 at 300 K. The interface recombination velocity for these homojunction barriers is estimated to be less than 1×103 cm/s, and it appears to be comparable to that recently observed for Si‐doped n+‐n GaAs homojunction barriers grown by metalorganic chemical vapor deposition. We present evidence that these n+‐n GaAs homojunctions, unlike p+‐p GaAs homojunctions, are almost as effective as AlGaAs heterojunctions in minority‐carrier 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+‐n diodes are presented for a range of p‐type dopant concentrations. A successive etch technique was used to characterize the electron injection current in terms of the product (noDn). Measurements are presented for Zn‐doped GaAs solar cells with p‐layer hole concentrations in the range 6.3×1017−1.3×1019 cm−3. The results demonstrate that so‐called band‐gap narrowing effects substantially increase the injected electron current in heavily doped p‐type 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 =~1.4×1018 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

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 molecular‐beam 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

Evidence for band-gap narrowing effects in Be-doped, p-p+ GaAs homojunction barriers

The electrical performance of Be‐doped, p‐p+ GaAs homojunction barriers is characterized and analyzed. The results of the analysis show that minority‐carrier electrons, at 300 K, have a mobility of 4760 cm2/V s at a hole concentration of 2.3×1016 cm−3, and that the effective recombination velocity for these homojunction barriers is about 6×104 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 band‐gap shrinkage in this Be‐doped material grown by molecular‐beam epitaxy appears to be comparable to that already observed for Zn‐doped GaAs grown by metalorganic chemical vapor deposition. This work demonstrates that so‐called band‐gap narrowing effects significantly influence the electrical performance of GaAs devices

Extinction Debt in Source-Sink Metacommunities

In an increasingly modified world, understanding and predicting the consequences of landscape alteration on biodiversity is a challenge for ecologists. To this end, metacommunity theory has developed to better understand the complexity of local and regional interactions that occur across larger landscapes. While metacommunity ecology has now provided several alternative models of species coexistence at different spatial scales, predictions regarding the consequences of landscape alteration have been done exclusively for the competition-colonization trade off model (CC). In this paper we investigate the effects of landscape perturbation on source-sink metacommunities. We show that habitat destruction perturbs the equilibria among species competitive effects within the metacommunity, driving both direct extinctions and an indirect extinction debt. As in CC models, we found a time lag for extinction following habitat destruction that varied in length depending upon the relative importance of direct and indirect effects. However, in contrast to CC models, we found that the less competitive species are more affected by habitat destruction. The best competitors can sometimes even be positively affected by habitat destruction, which corresponds well with the results of field studies. Our results are complementary to those results found in CC models of metacommunity dynamics. From a conservation perspective, our results illustrate that landscape alteration jeopardizes species coexistence in patchy landscapes through complex indirect effects and delayed extinctions patterns