389 research outputs found
A unified approach to electron transport in double barrier structures
In this paper we show an approach to electron transport in double barrier
structures which unifies the well known sequential and resonant tunneling
models in the widest range of transport regimes, from completely coherent to
completely incoherent. In doing so, we make a clear distinction between
``approaches'' and ``transport regimes,'' in order to clarify some ambiguities
in the concept of sequential tunneling. Scattering processes in the well are
accounted for by means of an effective mean free path, which plays the role of
a relaxation length. Our approach is based on a recently derived formula for
the density of states in a quantum well, as a function of the round trip time
in the well and of trasmission and reflection probabilities for the whole
structure and for each barrier.Comment: RevTeX file, 14 pages, 2 uuencoded Postscript figures, uses epsf.sty.
To be published on Phys. Rev. B. Postscript files and hard copies available
from the authors upon request ([email protected]
Analytical model of 1D Carbon-based Schottky-Barrier Transistors
Nanotransistors typically operate in far-from-equilibrium (FFE) conditions,
that cannot be described neither by drift-diffusion, nor by purely ballistic
models. In carbonbased nanotransistors, source and drain contacts are often
characterized by the formation of Schottky Barriers (SBs), with strong
influence on transport. Here we present a model for onedimensional field-effect
transistors (FETs), taking into account on equal footing both SB contacts and
FFE transport regime. Intermediate transport is introduced within the Buttiker
probe approach to dissipative transport, in which a non-ballistic transistor is
seen as a suitable series of individually ballistic channels. Our model permits
the study of the interplay of SBs and ambipolar FFE transport, and in
particular of the transition between SB-limited and dissipation-limited
transport
Enhanced shot noise in carbon nanotube field-effect transistors
We predict shot noise enhancement in defect-free carbon nanotube field-effect
transistors through a numerical investigation based on the self-consistent
solution of the Poisson and Schrodinger equations within the non-equilibrium
Green functions formalism, and on a Monte Carlo approach to reproduce injection
statistics. Noise enhancement is due to the correlation between trapping of
holes from the drain into quasi-bound states in the channel and thermionic
injection of electrons from the source, and can lead to an appreciable Fano
factor of 1.22 at room temperature.Comment: 4 pages, 4 figure
Shot noise suppression in quasi one-dimensional Field Effect Transistors
We present a novel method for the evaluation of shot noise in quasi
one-dimensional field-effect transistors, such as those based on carbon
nanotubes and silicon nanowires. The method is derived by using a statistical
approach within the second quantization formalism and allows to include both
the effects of Pauli exclusion and Coulomb repulsion among charge carriers. In
this way it extends Landauer-Buttiker approach by explicitly including the
effect of Coulomb repulsion on noise. We implement the method through the
self-consistent solution of the 3D Poisson and transport equations within the
NEGF framework and a Monte Carlo procedure for populating injected electron
states. We show that the combined effect of Pauli and Coulomb interactions
reduces shot noise in strong inversion down to 23 % of the full shot noise for
a gate overdrive of 0.4 V, and that neglecting the effect of Coulomb repulsion
would lead to an overestimation of noise up to 180 %.Comment: Changed content, 7 pages,5 figure
Electric-Field-control of spin rotation in bilayer graphene
The manipulation of the electron spin degree of freedom is at the core of the
spintronics paradigm, which offers the perspective of reduced power
consumption, enabled by the decoupling of information processing from net
charge transfer. Spintronics also offers the possibility of devising hybrid
devices able to perform logic, communication, and storage operations. Graphene,
with its potentially long spin-coherence length, is a promising material for
spin-encoded information transport. However, the small spin-orbit interaction
is also a limitation for the design of conventional devices based on the
canonical Datta-Das spin-FET. An alternative solution can be found in magnetic
doping of graphene, or, as discussed in the present work, in exploiting the
proximity effect between graphene and Ferromagnetic Oxides (FOs). Graphene in
proximity to FO experiences an exchange proximity interaction (EPI), that acts
as an effective Zeeman field for electrons in graphene, inducing a spin
precession around the magnetization axis of the FO. Here we show that in an
appropriately designed double-gate field-effect transistor, with a bilayer
graphene channel and FO used as a gate dielectric, spin-precession of carriers
can be turned ON and OFF with the application of a differential voltage to the
gates. This feature is directly probed in the spin-resolved conductance of the
bilayer
Model and performance evaluation of field-effect transistors based on epitaxial graphene on SiC
In view of the appreciable semiconducting gap of 0.26 eV observed in recent
experiments, epitaxial graphene on a SiC substrate seems a promising channel
material for FETs. Indeed, it is two-dimensional - and therefore does not
require prohibitive lithography - and exhibits a wider gap than other
alternative options, such as bilayer graphene. Here we propose a model and
assess the achievable performance of a nanoscale FET based on epitaxial
graphene on SiC, conducting an exploration of the design parameter space. We
show that the current can be modulated by 4 orders of magnitude; for digital
applications an Ion /Ioff ratio of 50 and a subthreshold slope of 145 mV/decade
can be obtained with a supply voltage of 0.25 V. This represents a significant
progress towards solid-state integration of graphene electronics, but not yet
sufficient for digital applications
Analytical model of nanowire FETs in a partially ballistic or dissipative transport regime
The intermediate transport regime in nanoscale transistors between the fully
ballistic case and the quasi equilibrium case described by the drift-diffusion
model is still an open modeling issue. Analytical approaches to the problem
have been proposed, based on the introduction of a backscattering coefficient,
or numerical approaches consisting in the MonteCarlo solution of the Boltzmann
transport equation or in the introduction of dissipation in quantum transport
descriptions. In this paper we propose a very simple analytical model to
seamlessly cover the whole range of transport regimes in generic quasi-one
dimensional field-effect transistors, and apply it to silicon nanowire
transistors. The model is based on describing a generic transistor as a chain
of ballistic nanowire transistors in series, or as the series of a ballistic
transistor and a drift-diffusion transistor operating in the triode region. As
an additional result, we find a relation between the mobility and the mean free
path, that has deep consequences on the understanding of transport in nanoscale
devices
Model of tunneling transistors based on graphene on SiC
Recent experiments shown that graphene epitaxially grown on Silicon Carbide
(SiC) can exhibit a energy gap of 0.26 eV, making it a promising material for
electronics. With an accurate model, we explore the design parameter space for
a fully ballistic graphene-on-SiC Tunnel Field-Effect Transistors (TFETs), and
assess the DC and high frequency figures of merit. The steep subthreshold
behavior can enable I_{ON}/I_{OFF} ratios exceeding 10^4 even with a low supply
voltage of 0.15 V, for devices with gatelength down to 30 nm. Intrinsic
transistor delays smaller than 1 ps are obtained. These factors make the device
an interesting candidate for low-power nanoelectronics beyond CMOS
A Backscattering Model Incorporating the Effective Carrier Temperature in Nano MOSFET
In this work we propose a channel backscattering model in which increased
carrier temperature at the top of the potential energy barrier in the channel
is taken into account. This model represents an extension of a previous model
by the same authors which highlighted the importance of considering the
partially ballistic transport between the source contact and the top of the
potential energy barrier in the channel. The increase of carrier temperature is
precisely due to energy dissipation between the source contact and the top of
the barrier caused by the high saturation current. To support our discussion,
accurate 2D full band Monte Carlo device simulations with quantum correction
have been performed in double gate nMOSFETs for different geometries (gate
length down to 10 nm), biases and lattice temperatures. Including the effective
carrier temperature is especially important to properly treat the high
inversion regime, where previous backscattering models usually fail
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