727 research outputs found
Localization of non-interacting electrons in thin layered disordered systems
Localization of electronic states in disordered thin layered systems with b
layers is studied within the Anderson model of localization using the
transfer-matrix method and finite-size scaling of the inverse of the smallest
Lyapunov exponent. The results support the one-parameter scaling hypothesis for
disorder strengths W studied and b=1,...,6. The obtained results for the
localization length are in good agreement with both the analytical results of
the self-consistent theory of localization and the numerical scaling studies of
the two-dimensional Anderson model. The localization length near the band
center grows exponentially with b for fixed W but no
localization-delocalization transition takes place.Comment: 6 pages, 5 figure
Weak localisation, hole-hole interactions and the "metal"-insulator transition in two dimensions
A detailed investigation of the metallic behaviour in high quality
GaAs-AlGaAs two dimensional hole systems reveals the presence of quantum
corrections to the resistivity at low temperatures. Despite the low density
() and high quality of these systems, both weak localisation
(observed via negative magnetoresistance) and weak hole-hole interactions
(giving a correction to the Hall constant) are present in the so-called
metallic phase where the resistivity decreases with decreasing temperature. The
results suggest that even at high there is no metallic phase at T=0 in
two dimensions.Comment: 5 pages, 4 figure
A Physically Based Compact Model of Partially Depleted MOSFETs for Analog Circuit Stimulation
In this paper, the Southampton Thermal Analogue (STAG) compact model for partially depleted (PD) silicon-on-insulator (SOI) MOSFETs is presented. The model uses a single expression to model the channel current, thereby ensuring continuous transition between all operating regions. Furthermore, care has been taken to ensure that this expression is also infinitely differentiable, resulting in smooth and continuous conductances and capacitances as well as higher order derivatives. Floating-body effects, which are particular to PD SOI and which are of concern to analog circuit designers in this technology, are well modeled. Small geometry effects such as channel length modulation (CLM), drain-induced barrier lowering (DIBL), charge sharing, and high field mobility effects have also been included. Self-heating (SH) effects are much more apparent in SOI devices than in equivalent bulk devices. These have been modeled in a consistent manner, and the implementation in SPICE3f5 gives the user an additional thermal node which allows internal device temperature rises to be monitored and also accommodates the modeling of coupled heating between separate devices. The model has been successfully used to simulate a variety of circuits which commonly cause problems with convergence. Due to its inherent robustness, the model can normally achieve convergence without recourse to the setting of initial nodal voltage estimates
Pathways to economic well-being among teenage mothers in Great Britain
The present study examines pathways to independence from social welfare among 738 teenage mothers, participants of the 1970 British Cohort Study, who were followed up at age 30 years. Using a longitudinal design, a pathway model is tested, examining linkages between family social background, cognitive ability, school motivation, and individual investments in education, as well as work- and family-related roles. The most important factors associated with financial independence by age 30 are continued attachment to the labor market as well as a stable relationship with a partner (not necessarily the father of the child). Pathways to financial independence, in turn, are predicted through own cognitive resources, school motivation, and family cohesion. Implications of findings for policy making are discussed.© 2010 Hogrefe Publishing
Possible Metal/Insulator Transition at B=0 in Two Dimensions
We have studied the zero magnetic field resistivity of unique high- mobility
two-dimensional electron system in silicon. At very low electron density (but
higher than some sample-dependent critical value,
cm), CONVENTIONAL WEAK LOCALIZATION IS OVERPOWERED BY A SHARP DROP OF
RESISTIVITY BY AN ORDER OF MAGNITUDE with decreasing temperature below 1--2 K.
No further evidence for electron localization is seen down to at least 20 mK.
For , the sample is insulating. The resistivity is empirically
found to SCALE WITH TEMPERATURE BOTH BELOW AND ABOVE WITH A SINGLE
PARAMETER which approaches zero at suggesting a metal/ insulator
phase transition.Comment: 10 pages; REVTeX v3.0; 3 POSTSCRIPT figures available upon request;
to be published in PRB, Rapid Commu
The relative importance of electron-electron interactions compared to disorder in the two-dimensional "metallic" state
The effect of substrate bias and surface gate voltage on the low temperature
resistivity of a Si-MOSFET is studied for electron concentrations where the
resistivity increases with increasing temperature. This technique offers two
degrees of freedom for controlling the electron concentration and the device
mobility, thereby providing a means to evaluate the relative importance of
electron-electron interactions and disorder in this so-called ``metallic''
regime. For temperatures well below the Fermi temperature, the data obey a
scaling law where the disorder parameter (), and not the
concentration, appears explicitly. This suggests that interactions, although
present, do not alter the Fermi-liquid properties of the system fundamentally.
Furthermore, this experimental observation is reproduced in results of
calculations based on temperature-dependent screening, in the context of
Drude-Boltzmann theory.Comment: 5 pages, 6 figure
Analysis of the Metallic Phase of Two-Dimensional Holes in SiGe in Terms of Temperature Dependent Screening
We find that temperature dependent screening can quantitatively explain the
metallic behaviour of the resistivity on the metallic side of the so-called
metal-insulator transition in p-SiGe. Interference and interaction effects
exhibit the usual insulating behaviour which is expected to overpower the
metallic background at sufficiently low temperatures. We find empirically that
the concept of a Fermi-liquid describes our data in spite of the large r_s = 8.Comment: 4 pages, 3 figure
Impact of buffer charge on the reliability of carbon doped AlGaN/GaN-on-Si HEMTs
Charge trapping and transport in the carbon doped GaN buffer of an AlGaN/GaN-on-Si HEMT have been investigated. Back-gating and dynamic RON experiments show how the onset of leakage in the strain relief layer at a lower field than that through the upper part of the structure can result in serious long-term trapping leading to current collapse under standard device operating conditions. Controlling current-collapse requires control of not only the layer structures and its doping, but also the precise balance of leakage in each layer
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