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S

Series in Microelectronics
edited by Wolfgang Fichtner
Qiuting Huang
Heinz Jäckel
Gerhard Tröster
Bernd Witzigmann
Mathieu Luisier,
Quantum Transport Beyond the
Effective Mass Approximation.
2007, xiv, 130 pages. € 64,00. ISBN 3866281498
A threedimensional full band Simulator for nanowire
fieldeffect transistors (FETs) is
presented in this thesis. At the nanometer scale the
classical driftdiffusion transport theory reaches its limits; quantum transport (QT) phenomena govern the motion of
electrons and holes. The development of a QT Simulator
requires the assembly of several physical models
and the choice of appropriate simplifications.
In the first part, the NonEquilibrium Green's
Function (NEGF) formalism is reviewed, a method
extensively used for the description
of nanostructures. It is applied to the Simulation of a twodimensional
ultrathinbody (UTB) transistor and of a
threedimensional nanowire FET, both
treated within the effective mass approximation (EMA) and in a coupled modespace. However, the strong
quantization effects that characterize
structures with dimensions below five nanometers oblige an accurate QT Simulator to go beyond the
EMA.
The semiempirical sp^{3}d^{5}s*
tightbinding (TB) method is chosen as bandstructure model
because (1) it reproduces the complete bulk (Ek) relation of a wide range of semiconductor materials, (2) it uses an atomic grid. and (3)
its extension to nanostructures is straightforward.
The Integration of the TB
method into a transport code is only possible,
if open boundary conditions (OBC) are introduced. The available procedures to
apply OBC in a threedimensional multiband QT Simulator are computationally too intensive since they represent a generalized
eigenvalue problem or require iterative solvers. Therefore, a new method based on the scatteringboundary
approach is developed in this work. It
significantly reduces the computational burden associated with the OBC
calculation. Furthermore, it can be formulated either in the NEGF or in the Wave Function formalism, and it works for any channel orientation, material
composition, and cross section shape.
Finally, simulations of nanowire FETs are
achieved by selfconsistently coupling the
fullband transport solver to the threedimensional computation of the electrostatic potential in the
device (Poisson's equation). Two different wire types are studied,
one with a perfect stoichiometric structure (atoms occupy all the lattice positions) and another with
atomic roughness at the semiconductoroxide interface. Channel orientations along the [100], [110], [111],
and [112] axis are considered.
Keywords:
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