4,954 research outputs found
Introducing the hybrid unipolar bipolar field effect transistor : the HUBFET
Modern commercial aircraft are becoming increasingly dependent on electrical
power. More and more of the systems traditionally powered by hydraulics or
pneumatics are being migrated to run on electricity. One consequence of the
move towards electrical power is the increase in the storage capacity of the bat-
teries used to supplement the power generation. The increase in battery size
increases the maximum stress that a short circuit failure can put on the power
distribution system. Although such failures are extremely rare, the fail safe
switches in the distribution system must be capable of handling extremely high
energy short circuits and turning off the power to protect the electrical systems
from damage. Traditionally aircraft have used electromechanical relays in this
role. However, they are large, heavy and slow to switch. As the potential power
level is increased, the slow switching becomes more of a problem. The solution is
a semiconductor switch. An IGBT can handle the high short circuit currents and
switches fast enough to prevent short circuits damaging key systems. However,
the inherent voltage drop in the forward current path significantly reduces its
efficiency during nominal operation. A power MOSFET would be considerably
more efficient than an IGBT during nominal operation. However, during high
current surges, the ohmic behaviour of the switch leads to extremely high power
loss and thermal failure. In this thesis a solution to this problem is presented.
A new class of semiconductor device is proposed that has the highly efficient
low current performance of the power MOSFET and the high current handling
capability of the IGBT. The device has been named the Hybrid Unipolar Bipolar
Field Effect Transistor or HUBFET. The HUBFET operates in unipolar mode,
like a MOSFET, at low currents and in bipolar mode, like an IGBT, at high
currents. The structure of the HUBFET is a merging of the MOSFET and
IGBT. It is a vertical device with a traditional MOS gate structure, however
the backside consists of alternating regions of both N-type and P-type doping.
Through simulation the key on-state characteristics of the HUBFET have been
shown. Fabricated test modules have been tested to validate the simulations and
to show how the HUBFET can dynamically transistion from unipolar to bipolar
mode during a short circuit event. Following the proof of concept the pattern of
implants on the backside of the device that give the HUBFET its characteristic
were investigated and potential improvements to the design were identified
Structural behavior of uranium dioxide under pressure by LSDA+U calculations
The structural behavior of UO2 under high pressure up to 300GPa has been
studied by first-principles calculations with LSDA+U approximation. The results
show that a pressure-induced structural transition to the cotunnite-type
(orthorhombic Pnma) phase occurs at 38GPa. It agrees well with the
experimentally observed ~42 GPa. An isostructural transition following that is
also predicted to take place from 80 to 130GPa, which has not yet been observed
in experiments. Further high compression beyond 226GPa will result in a
metallic and paramagnetic transition. It corresponds to a volume of 90A^3 per
cell, in good agreement with a previous theoretical analysis in the reduction
of volume required to delocalize 5f states.Comment: 10 pages, 8 figure
Dynamical excitations in the collision of 2D Bose-Einstein condensates
We carry out simulations of the collision of two components of an
adiabatically divided, quasi-2D BEC. We identify under, over and critically
damped regimes in the dipole oscillations of the components according to the
balance of internal and centre-of-mass (c.m.) energies of the components and
investigate the creation of internal excitations. We distinguish the behaviour
of this system from previous studies of quasi-1D BEC's. In particular we note
that the nature of the internal excitations is only essentially sensitive to an
initial phase difference between the components in the overdamped regime.Comment: 17 pages, 9 figure
Fuel cycles in nuclear reactors
Series numbering from publisher's list"61"--stamped on cover"Unclassified. NYO-2131."Originally issued by the first author as an Sc. D. thesis, Massachusetts Institute of Technology, Dept. of Nuclear Engineering, 1959Contract AT(30-1)-207
Localized states in strong magnetic field: resonant scattering and the Dicke effect
We study the energy spectrum of a system of localized states coupled to a 2D
electron gas in strong magnetic field. If the energy levels of localized states
are close to the electron energy in the plane, the system exhibits a kind of
collective behavior analogous to the Dicke effect in optics. The latter
manifests itself in ``trapping'' of electronic states by localized states. At
the same time, the electronic density of states develops a gap near the
resonance. The gap and the trapping of states appear to be complementary and
reflect an intimate relation between the resonant scattering and the Dicke
effect. We reveal this relation by presenting the exact solution of the problem
for the lowest Landau level. In particular, we show that in the absence of
disorder the system undergoes a phase transition at some critical concentration
of localized states.Comment: 28 pages + 9 fig
THE CONTRIBUTION OF ENVIRONMENTAL AMENITIES TO AGRICULTURAL LAND VALUES: HEDONIC MODELLING USING GEOGRAPHIC INFORMATION SYSTEMS DATA
Geographic Information Systems (GIS) data are used in a hedonic model to measure the impact of recreational and scenic amenities on agricultural land values. Results indicate agricultural land values are determined by environmental amenities as well as production attributes. Significant amenity variables included scenic view, elk habitat and fishery productivity.Environmental Economics and Policy, Land Economics/Use,
- …