81 research outputs found
Diffusive Transport in Quasi-2D and Quasi-1D Electron Systems
Quantum-confined semiconductor structures are the cornerstone of modern-day
electronics. Spatial confinement in these structures leads to formation of
discrete low-dimensional subbands. At room temperature, carriers transfer among
different states due to efficient scattering with phonons, charged impurities,
surface roughness and other electrons, so transport is scattering-limited
(diffusive) and well described by the Boltzmann transport equation. In this
review, we present the theoretical framework used for the description and
simulation of diffusive electron transport in quasi-two-dimensional and
quasi-one-dimensional semiconductor structures. Transport in silicon MOSFETs
and nanowires is presented in detail.Comment: Review article, to appear in Journal of Computational and Theoretical
Nanoscienc
Two Dimensional Quantum Mechanical Modeling of Nanotransistors
Quantization in the inversion layer and phase coherent transport are
anticipated to have significant impact on device performance in 'ballistic'
nanoscale transistors. While the role of some quantum effects have been
analyzed qualitatively using simple one dimensional ballistic models, two
dimensional (2D) quantum mechanical simulation is important for quantitative
results. In this paper, we present a framework for 2D quantum mechanical
simulation of a nanotransistor / Metal Oxide Field Effect Transistor (MOSFET).
This framework consists of the non equilibrium Green's function equations
solved self-consistently with Poisson's equation. Solution of this set of
equations is computationally intensive. An efficient algorithm to calculate the
quantum mechanical 2D electron density has been developed. The method presented
is comprehensive in that treatment includes the three open boundary conditions,
where the narrow channel region opens into physically broad source, drain and
gate regions. Results are presented for (i) drain current versus drain and gate
voltages, (ii) comparison to results from Medici, and (iii) gate tunneling
current, using 2D potential profiles. Methods to reduce the gate leakage
current are also discussed based on simulation results.Comment: 12 figures. Journal of Applied Physics (to appear
Collision duration time for optical phonon emission in semiconductors
The time required to emit an optical (polar and intervalley) phonon by a nearly-free electron in a semiconductor is evaluated using a nonequilibrium Green's-function formalism. The leading idea of the work is that the so-called "collision duration" is related to the time required to build up correlation between the initial and the final state, and then to destroy this correlation as the collision is completed. The use of the nonequilibrium Green's-function formalism gives us the possibility to evaluate explicitly the effects of the correlations in time. Our approach is based on two crucial assumptions: we build the self-energy from only the polarization field of the polar-optical phonon; that is, the self-energy is a function of a single time and position, and we introduce the electron correlation function between the initial and the final states, written in terms of a generalized less-than Green's function in the momentum variables. We derive an analytical expression for the probability for a carrier to end up in a final state k as a consequence of the emission of a phonon as a function of time. We find that the probability rises to the "Fermi golden rule" result within a few femtoseconds. If the total lifetime broadening of the initial state is comparable to the scattering time, the probability oscillates as it approaches the asymptotic value. For larger initial-state broadening (due to more scattering processes), these oscillations disappear
Electrical plasmon detection in graphene waveguides
We present a simple device architecture that allows all-electrical detection
of plasmons in a graphene waveguide. The key principle of our electrical
plasmon detection scheme is the non-linear nature of the hydrodynamic equations
of motion that describe transport in graphene at room temperature and in a wide
range of carrier densities. These non-linearities yield a dc voltage in
response to the oscillating field of a propagating plasmon. For illustrative
purposes, we calculate the dc voltage arising from the propagation of the
lowest-energy modes in a fully analytical fashion. Our device architecture for
all-electrical plasmon detection paves the way for the integration of graphene
plasmonic waveguides in electronic circuits.Comment: 9 pages, 3 figure
Electron mobility in silicon nanowires
The low-field electron mobility in rectangular silicon nanowire (SiNW)
transistors was computed using a self-consistent Poisson-Schr\"{o}dinger-Monte
Carlo solver. The behavior of the phonon-limited and surface-roughness-limited
components of the mobility was investigated by decreasing the wire width from
30 nm to 8 nm, the width range capturing a crossover between two-dimensional
(2D) and one-dimensional (1D) electron transport. The phonon-limited mobility,
which characterizes transport at low and moderate transverse fields, is found
to decrease with decreasing wire width due to an increase in the
electron-phonon wavefunction overlap. In contrast, the mobility at very high
transverse fields, which is limited by surface roughness scattering, increases
with decreasing wire width due to volume inversion. The importance of acoustic
phonon confinement is also discussed briefly
Collision duration for polar optical and intervalley phonon scattering
The use of fs laser pulses to excite plasmas in semiconductors has become a major method for studying fast processes. The transition times from the Gamma valley to the satellite X and L valleys are comparable to the reciprocal of the frequency of the phonons involved, bringing into question the use of the standard perturbation-theory approaches. Our aim is to evaluate the time required to emit a phonon, either the intravalley LO or the intervalley, by a nearly-free electron in semiconductors. The leading idea of our work is that the so-called ''collision duration'' is related to the time required to build up correlation between the initial and final state, and then to destroy this correlation as the collision is completed. The calculations are developed using e nonequilibrium Green's function formalism, which allows us to evaluate explicitly the effects of the correlations in time
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