4,665 research outputs found
An analogue method for the analysis of current carrying semiconductor systems
In an earlier Internal Technical Memorandum (1)
and in subsequent
work(2), it has been demonstrated that a particular kind of resistance
network, in which non-linear elements are associated with each mesh
point, can be made to represent an exact analogue to a non-degenerate
semiconductor system in the equilibrium or quasi-equilibrium state.
The term !exact' in this context implies that the difference equation
which governs the potential distribution in the network becomes identical,
for the limit of vanishing mesh interval, with the differential equation
for the electrostatic potential within the semiconductor system, i.e.
the Shockley-Poisson equation. From this type of analogue network
information concerning the variation of maximum field intensity and of
junction capacitance with applied bias voltages can be obtained for one,
two and three dimensional configurations of p and n type regions of
arbitrary geometry and impurity concentration profiles.
One limitation to the applicability of the analogue technique arises
from the restriction to quasi-equilibrium conditions. This restriction
precludes the investigation of situations in which current flow contributions
to the carrier concentration pattern become significant - for example, in
the case of strongly forward biassed p-n junctions, and of p-i-n junctions
and transistors operating at high injection levels. In the present paper,
the problems involved in an extension of the basic analogue method to the
treatment of non-equilibrium situations are examined, and means for their
solution are discussed. A review of the methods previously described
and an illustration of the nature of their limitations is given in Section
2. This is followed, in Sections 3 to 7, by a detailed treatment of
the case of a current carrying semiconductor system in one dimension
which leads to a theoretically possible realization in terms of resistancenetwork/
analogue computer techniques, which is, however, too complex to. be
considered practical. Section 8 discusses means for the simplification of
the proposed schemes and leads to the description of a relatively simple
system in which a significant reduction in equipment complexity has been
made possible by the adoption of an operating mode based upon an iterative
process of successive approximations. The extension of the technique
to three dimensions is outlined in Section 9
C.V.D. annual report: January, 1967 research project ru27-1 : analogue study of semiconductor device structures
The e::tension of the resistance network analogue method to the study
of a M.O.S.T. structure is described. By means of an iterative technique,
data regarding channel current, field distribution, surface charge and
position of pinch-off point as function of gate and drain voltagen can be
obtained which do not involve the usual 'gradual' channel approximation
Results for a particular device geometry are presented.
A discussion of a digital computer approach to the solution of semiconductor
device current flow problems is included, together with preliminary
results
C.V.D. annual report: November 1965 research project RU27-1 :an analogue method for the determination of potential distributions in semiconductor systems
A general method for the solution of the nonlinear
Shockley-Poisson differential equation which
governs the potential distribution in non-degenerate
semiconductor systems is described which can be applied
to the evaluation of depletion layer widths, carrier
densities and capacitance bias relationships of p-n
junction structures.
The method is based upon the use of a particular
type of resistance network analogue and results obtained
for several one and two dimensional configurations are
discussed
The early reionization with the primordial magnetic fields
The early reionization of the intergalactic medium, which is favored from the
WMAP temperature-polarization cross-correlations, contests the validity of the
standard scenario of structure formation in the cold dark matter cosmogony. It
is difficult to achieve early enough star formation without rather extreme
assumptions such as very high escape fraction of ionizing photons from
proto-galaxies or a top-heavy initial mass function. Here we propose an
alternative scenario that is additional fluctuations on small scales induced by
primordial magnetic fields trigger the early structure formation. We found that
ionizing photons from Population III stars formed in dark haloes can easily
reionize the universe by if the strength of primordial magnetic
fields is larger than Gauss.Comment: 8 pages, 5 figures. accepted for publication in MNRA
The Mutual Interaction Between Population III Stars and Self-Annihilating Dark Matter
We use cosmological simulations of high-redshift minihalos to investigate the
effect of dark matter annihilation (DMA) on the collapse of primordial gas. We
numerically investigate the evolution of the gas as it assembles in a
Population III stellar disk. We find that when DMA effects are neglected, the
disk undergoes multiple fragmentation events beginning at ~ 500 yr after the
appearance of the first protostar. On the other hand, DMA heating and
ionization of the gas speeds the initial collapse of gas to protostellar
densities and also affects the stability of the developing disk against
fragmentation, depending on the DM distribution. We compare the evolution when
we model the DM density with an analytical DM profile which remains centrally
peaked, and when we simulate the DM profile using N-body particles (the 'live'
DM halo). When utilizing the analytical DM profile, DMA suppresses disk
fragmentation for ~ 3500 yr after the first protostar forms, in agreement with
earlier work. However, when using a 'live' DM halo, the central DM density peak
is gradually flattened due to the mutual interaction between the DM and the
rotating gaseous disk, reducing the effects of DMA on the gas, and enabling
secondary protostars of mass ~ 1 M_sol to be formed within ~ 900 yr. These
simulations demonstrate that DMA is ineffective in suppressing gas collapse and
subsequent fragmentation, rendering the formation of long-lived dark stars
unlikely. However, DMA effects may still be significant in the early collapse
and disk formation phase of primordial gas evolution.Comment: 17 pages, 11 figures, to appear in MNRA
Time domain and frequency domain measurement techniques
The principles underlying two distinct approaches to the measurement
of electrical network characteristics over a wide frequency range are
outlined, together with brief descriptions of currently available test
systems which cover the range 0.1 - 4GHZ.
The two techniques are assessed in terms of their ultimate capabilities
with regard to accuracy, speed of measurement and costs
Constraints on the Proper Motion of the Andromeda Galaxy Based on the Survival of Its Satellite M33
A major uncertainty in the dynamical history of the local group of galaxies
originates from the unknown transverse speed of the Andromeda galaxy (M31)
relative to the Milky Way. We show that the recent VLBA measurement of the
proper motion of Andromeda's satellite, M33, severely constrains the possible
values of M31's proper motion. The condition that M33's stellar disk will not
be tidally disrupted by either M31 or the Milky Way over the past 10 billion
years, favors a proper motion amplitude of 100+-20km/s for M31 with the
quadrant of a negative velocity component along Right Ascension and a positive
component along Declination strongly ruled-out. This inference can be tested by
future astrometric measurements with SIM, GAIA, or the SKA. Our results imply
that the dark halos of Andromeda and the Milky Way will pass through each other
within the next 5-10 billion years.Comment: Accepted for publication in Ap
Identifying the Environment and Redshift of GRB Afterglows from the Time-Dependence of Their Absorption Spectra
The discovery of Gamma-Ray Burst (GRB) afterglows revealed a new class of
variable sources at optical and radio wavelengths. At present, the environment
and precise redshift of the detected afterglows are still unknown. We show that
if a GRB source resides in a compact (<100pc) gas-rich environment, the
afterglow spectrum will show time-dependent absorption features due to the
gradual ionization of the surrounding medium by the afterglow radiation.
Detection of this time-dependence can be used to constrain the size and density
of the surrounding gaseous system. For example, the MgII absorption line
detected in GRB970508 should have weakened considerably during the first month
if the absorption occurred in a gas cloud of size <100pc around the source. The
time-dependent HI or metal absorption features provide a precise determination
of the GRB redshift.Comment: 13 pages, 4 figures, submitted to ApJ
Calibrating the Galaxy Halo - Black Hole Relation Based on the Clustering of Quasars
The observed number counts of quasars may be explained either by long-lived
activity within rare massive hosts, or by short-lived activity within smaller,
more common hosts. It has been argued that quasar lifetimes may therefore be
inferred from their clustering length, which determines the typical mass of the
quasar host. Here we point out that the relationship between the mass of the
black-hole and the circular velocity of its host dark-matter halo is more
fundamental to the determination of the clustering length. In particular, the
clustering length observed in the 2dF quasar redshift survey is consistent with
the galactic halo - black-hole relation observed in local galaxies, provided
that quasars shine at ~10-100% of their Eddington luminosity. The slow
evolution of the clustering length with redshift inferred in the 2dF quasar
survey favors a black-hole mass whose redshift-independent scaling is with halo
circular velocity, rather than halo mass. These results are independent from
observations of the number counts of bright quasars which may be used to
determine the quasar lifetime and its dependence on redshift. We show that if
quasar activity results from galaxy mergers, then the number counts of quasars
imply an episodic quasar lifetime that is set by the dynamical time of the host
galaxy rather than by the Salpeter time. Our results imply that as the redshift
increases, the central black-holes comprise a larger fraction of their host
galaxy mass and the quasar lifetime gets shorter.Comment: 10 pages, 5 figures. Submitted to Ap
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