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 3-86628-149-8
A three-dimensional full band Simulator for nanowire field-effect tran-sistors (FETs) is presented in this thesis. At the
nanometer scale the classical drift-diffusion 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 Non-Equilibrium Green's
Function (NEGF) formalism is reviewed, a method extensively used for
the description of
nanostructures. It is applied to the Simulation of a two-dimensional ultra-thin-body (UTB) transistor and of a
three-dimensional nanowire FET, both
treated within the effective mass approximation (EMA) and in a coupled mode-space. However, the strong
quantization effects that characterize
structures with dimensions below five nanometers oblige an accurate QT Simulator to go beyond the EMA.
The semi-empirical sp3d5s* tight-binding
(TB) method is chosen as bandstructure
model because (1) it reproduces the complete bulk (E-k) relation of a wide range of
semiconductor materials, (2) it uses an atomic
grid. and (3) its extension to nanostructures is straightfor-ward.
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 three-dimensional 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 scattering-boundary
approach is devel-oped in this work. It
significantly reduces the computational burden associated with the OBC
calculation. Furthermore, it can be formu-lated 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
self-consis-tently coupling the full-band
transport solver to the three-dimensional 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 semiconductor-oxide interface. Channel orientations along the [100], [110], [111], and [112] axis are considered.
Keywords:
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