Electron transport was studied in an open square quantum dot with a dimension typical
for current experiments. A numerical analysis of the probability density distribution
inside the dot was performed which enabled us to unambiguously map the resonant states
which dominate the conductance of the structure. It was shown that, despite of the
presence of dot openings, transport through the dot is effectively mediated by just a few
(or even a single) eigenstates of the corresponding closed structure. In a single-mode
regime in the leads, the broadening of the resonant levels is typically smaller than the
mean energy level spacing, Δ. On the contrary, in the many-mode regime this broadening
typically exceeds Δ and has an irregular, essentially non-Lorentzian, character. It was
demonstrated that in the latter case eigenlevel spacing statistics of the corresponding
closed system are not relevant to the averaged transport properties of the dot. This
conclusion seems to have a number of experimental as well as numerical verifications.

I. V. Zozoulenko

K.-F. Berggren

Quantum Transport in Open Nanostructures

Nonparabolicity effects in two-dimensional electron systems are quantitativelyanalyzed. A formalism has been developed which allows to incorporate a nonparabolicbulk dispersion relation into the Schr&#246;dinger equation. As a consequence ofnonparabolicity the wave functions depend on the in-plane momentum. Each subbandis parametrized by its energy, effective mass and a subband nonparabolicity coefficient.The formalism is implemented in a one-dimensional Schr&#246;dinger-Poisson solver whichis applicable both to silicon inversion layers and heterostructures.

H.  Kosina

C.  Troger

SPIN &#8211; A Schr&#246;dinger-Poisson Solver IncludingNonparabolic Bands

We present the effects of resonant tunneling through doubly-coupled attractors in seriesin a quantum nanosystem. It is found that multiple split-tunneling peaks arise fromresonant tunneling through multiple nondegenerate quasi-bound-states of the coupledattractors. We show that the tunability of coupling is achieved by modulating thestrength of the attractor for a fixed separation length between the attractors, and byadjusting the geometrical separation length for a fixed intervening barrier strength.

Yong S. Joe

Ronald M. Cosby

Resonances in Conductance Through Tunable Attractors

One of the most promising applications of single-electronics is a single-electron memorychip. Such a chip would have orders of magnitude lower power consumption comparedto state-of-the-art dynamic memories, and would allow integration densities beyond thetera bit chip.We studied various single-electron memory designs. Additionally we areproposing a new memory cell which we call the T-memory cell. This cell can bemanufactured with state-of-the-art lithography, it operates at room temperature andshows a strong resistance against random background charge.

Christoph  Wasshuber

Hans  Kosina

Siegfried  Selberherr

Single-Electron Memories

In a companion presentation, we have discussed the theory of a mesoscopic/
macroscopic model, which can be viewed as an augmented drift-diffusion model. Here,
we describe how that model is used. The device we consider for this presentation is the
one dimensional GaAs <mml:math id="E1" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:msup><mml:mi>n</mml:mi><mml:mo>+</mml:mo></mml:msup><mml:mo>−</mml:mo><mml:mi>n</mml:mi><mml:mo>−</mml:mo><mml:msup><mml:mi>n</mml:mi><mml:mo>+</mml:mo></mml:msup></mml:math> structure of length 0.8μm. First, a full Hydro-
Dynamic (HD) model, proven reliable when compared with Monte Carlo simulations, is
used to simulate the device via the ENO finite difference method. As applied to the full
device, the new model is not necessarily superior to traditional Drift-Diffusion (DD).
Indeed, when we plot the quantity <mml:math id="E2" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>η</mml:mi><mml:mo>=</mml:mo><mml:mtext> </mml:mtext><mml:msub><mml:mi>μ</mml:mi><mml:mn>0</mml:mn></mml:msub><mml:mi>E</mml:mi><mml:mo>/</mml:mo><mml:msqrt><mml:mrow><mml:mi>k</mml:mi><mml:msub><mml:mi>T</mml:mi><mml:mn>0</mml:mn></mml:msub><mml:mo>/</mml:mo><mml:mi>m</mml:mi></mml:mrow></mml:msqrt></mml:mrow></mml:math>, where μ0 is the mobility constant
and <mml:math id="E3" xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>E</mml:mi><mml:mo>=</mml:mo><mml:mo>−</mml:mo><mml:msup><mml:mi>ϕ</mml:mi><mml:mo>′</mml:mo></mml:msup></mml:mrow></mml:math> is the electric field, we verify that the high field assumption η › 1, required
for the high field model, is satisfied only in an interval given approximately by [0.2, 0.5].
When we run both the DD model and the new high field model in this restricted interval,
with boundary conditions of concentration n and potential ϕ provided by the HD
results, we demonstrate that the new model outperforms the DD model. This indicates
that the high field and DD models should be used only in parts of the device, connected
by a transition kinetic regime. This will be a domain decomposition issue involving
interface conditions and adequate numerical methods.

Carlo Cercignani

Irene M. Gamba

Joseph W. Jerome

Chi-Wang Shu

Applicability of the High Field Model: A Preliminary
Numerical Study

The nanoelectronic modeling (NEMO) program is the result of a three-yeardevelopment effort involving four universities and the former Corporate Researchand Development Laboratory of Texas Instruments, now Applied Research Laboratory,Raytheon TI Systems, to create a comprehensive quantum device modeling toolfor layered semiconductor structures. Based on the non-equilibrium Green functionformalism, it includes the effects of quantum charging, bandstructure and incoherentscattering from alloy disorder, interface roughness, acoustic phonons, and polar opticalphonons. NEMO addresses the diverse needs of two different types of users: (i) theengineer/experimentalist who desires a black-box design tool and (ii) the theorist who isinterested in a detailed investigation of the physics. A collection of models trade offphysical content with speed and memory requirements. Access to this comprehensivetheoretical framework is accommodated by a Graphical User Interface (GUI) thatfacilitates device prototyping and in situ data analysis. We describe a hierarchicalsoftware design that allows rapid incorporation of theory enhancements whilemaintaining a user-friendly GUI, thus satisfying the conflicting criteria of ease of useand ease of development. The theory and GUI modules share data structures that definethe device structure, material parameters, and simulation parameters. These datastructures may contain general data such as integer and real numbers, option lists,vectors, matrices and the labels for both batch and GUI operation. NEMO generatesthe corresponding GUI elements at run-time for display and entry of these datastructures.

Gerhard  Klimeck

Dan  Blanks

Roger  Lake

R. Chris Bowen

Chenjing L. Fernando

Manhua  Leng

 William R. Frensley

Dejan  Jovanovic

Paul  Sotirelis

Writing Research Software in a Large Group for the NEMO Project

In this paper, we present a mesoscopic-macroscopic model of self-consistent charge
transport. It is based upon an asymptotic expansion of solutions of the Boltzmann
Transport Equation (BTE). We identify three dimensionless parameters from the BTE.
These parameters are, respectively, the quotient of reference scales for drift and thermal
velocities, the scaled mean free path, and the scaled Debye length. Such parameters
induce domain dependent macroscopic approximations. Particular focus is placed upon
the so-called high field model, defined by the regime where drift velocity dominates
thermal velocity. This model incorporates kinetic transition layers, linking mesoscopic
to macroscopic states. Reference scalings are defined by the background doping levels
and distinct, experimentally measured mobility expressions, as well as locally
determined ranges for the electric fields. The mobilities reflect a coarse substitute for
reference scales of scattering mechanisms. See [9] for elaboration.The high field approximation is a formally derived modification of the augmented
drift-diffusion model originally introduced by Thornber some fifteen years ago [25]. We
are able to compare our approach with the earlier kinetic approach of Baranger and
Wilkins [5] and the macroscopic approach of Kan, Ravaioli and Kerkhoven [20].

Applicability of the High Field Model:
An Analytical Study Via Asymptotic Parameters
Defining Domain Decomposition

Physical simulation of semiconductor devices at high frequencies involves not onlysemiconductor transport issues but also electromagnetic wave propagation issues. Inorder to obtain the nonlinear and the large-signal characteristics of the semiconductordevices, an electromagnetic model should replace the traditional quasi-static model inthe device simulator. In this paper, the advantages of a semiconductor device simulatorcombining an electromagnetic and an electron transport models are presented. Thisstudy is based on a semiconductor device simulator that couples a semiconductor modelto the 3D time-domain solution of Maxwell&#39;s equations. The electromagnetic wavepropagation effects on the millimeter-wave FETs are thoroughly analyzed. The use ofthe electromagnetic model over the conventional quasi-static model provides the actualdevice response at high frequencies. It also shows the nonlinear energy build-up alongthe device width whereas the quasi-static model provides a linear increase of energy. Thecombined model is capable of predicting the device nonlinearity and harmonicdistortion of amplifier circuits at large signal.

S. M. Sohel Imtiaz

Samir M. El-Ghazaly

Robert O. Grondin

Advantages of Semiconductor Device SimulatorCombining Electromagnetic and ElectronTransport Models

Intersubband relaxation due to electron interactions with the localized phonon modesplays an important role for population inversion in quantum well laser structuresdesigned for intersubband lasers operating at mid-infrared to submillimeter wavelengths.In this work, intersubband relaxation rates between subbands in step quantumwell structures are evaluated numerically using Fermi&#39;s golden rule, in which thelocalized phonon modes including the asymmetric interface modes, symmetric interfacemodes, and confined phonon modes and the electron &#8211; phonon interaction Hamiltoniansare derived based on the macroscopic dielectric continuum model, whereas theelectron wave functions are obtained by solving the Schr&#246;dinger equation for theheterostructures under investigation. The sum rule for the relationship between the formfactors of the various localized phonon modes and the bulk phonon modes is examinedand verified for these structures. The intersubband relaxation rates due to electronscattering by the asymmetric interface phonons, symmetric interface phonons, andconfined phonons are calculated and compared with the relaxation rates calculatedusing the bulk phonon modes and the Fr&#246;hlich interaction Hamiltonian for stepquantum well structures with subband separations of 36 meV and 50meV, correspondingto the bulk longitudinal optical phonon energy and interface phonon energy,respectively. Our results show that for preferential electron relaxation in intersubbandlaser structures, the effects of the localized phonon modes, especially the interfacephonon modes, must be included for optimal design of these structures.

J. P. Sun

H. B. Teng

G. I. Haddad

M. A. Stroscio

G. J. Iafrate

lntersubband Relaxation in Step QuantumWell Structures

Tunneling between vertically stacked quantum wires has been investigated. The wires
are assumed to have the dimension perpendicular to the tunneling barrier much smaller
than the other transverse dimension, so that only the lowest mode in such direction is to
be taken into account, while many modes in the other direction are filled. A model with
hard-wall confinement has been used for the investigation of the transport problem, and
the tunneling conductance has been computed, via a recursive Green's functions
procedure.

M. Macucci

A. T. Galick

U. Ravaioli

Tunneling Between Multimode Stacked Quantum Wires

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