The two-dimensional multi-layered HEMT is modelled isothermally by solving thePoisson and current continuity equations consistently with the Schr&#246;dinger equation. Amultigrid method is used on the Poisson and current continuity equations while theelectron density is calculated at each level by solving the Schr&#246;dinger equation in onedimensionalslices perpendicular to the layer structure. A correction factor is introducedwhich enables relatively accurate solutions to be obtained using a low number ofeigensolutions. A novel method for discretising the current density which can begeneralised to the non-isothermal case is described. Results are illustrated using a twolayer AlGaAs-GaAs HEMT.

Eric A. B. Cole

Tobias  Boettcher

Christopher M. Snowden

Two-dimensional Modelling of HEMTs Using Multigrids with Quantum Correction

A theoretical and computational analysis of the quantum dynamics of charge carriers in
presence of electron-phonon interaction based on the Wigner function is here applied to
the study of transport in mesoscopic systems. Numerical applications are shown for a) a
wave packet scattering with phonons while crossing a potential profile and b) electrons
scattering with phonons in a finite device with open boundary conditions.

C. Jacoboni

A. Abramo

P. Bordone

R. Brunetti

M. Pascoli

Application of the Wigner-Function Formulation to
Mesoscopic Systems in Presence of Electron-Phonon
Interaction

The use of a wavelet basis can lead to efficient methods for performing ab initio
electronic structure calculations of inherently localized structures. In this work wavelets
are used to construct an adaptive basis which is optimized dynamically throughout the
calculation. The computational effort of such a method should scale linearly with the
number of basis functions. The adaptive basis is tested for the case of bulk Si using only
a local s-pseudopotential.

D. A. Richie

P. von Allmen

K. Hess

Richard M. Martin

Electronic Structure Calculations Using
An Adaptive Wavelet Basis

The importance of 2-dimensional (2D) features of carriers in Si MOSFETs on the deviceperformance is re-examined experimentally and theoretically from the viewpoint of low-fieldmobility, velocity in high tangential fields and the inversion-layer capacitance. It isconfirmed that low-field mobility and inversion-layer capacitance can be understoodwell in terms of the 2D subbands and the 2D carrier transport. In order to obtain fully-quantitativeunderstanding of low-field mobility, however, it is still necessary to moreaccurately determine the amount of the scattering parameters in the inversion layer. Onthe other hand, saturation velocity is considered to be less influenced by the 2Dquantization, while it is found experimentally that saturation velocity is slightlydependent on surface carrier concentration.According to the knowledge of 2-dimensional carrier transport in Si inversion layer,an effective way to have higher current drive is to increase the occupancy of the 2-foldvalleys, which have lower conductivity mass, on a (100) surface. From this viewpoint,two device structures, strained Si MOSFETs and SOI MOSFETs with ultra-thin SOIfilms, are introduced and the behavior of low-field mobility is analyzed through thecalculations of the subband structures and phonon-limited mobility.

Shin-Ichi  Takagi

Two-dimensional Carrier Transport in Si MOSFETs

A tight-binding models which account for band mixing, strain and external appliedpotentials in a self-consistent fashion has been developed. This allows us to describeelectronic and optical properties of nanostructured devices beyond the usual envelopefunction approximation. This model can be applied to direct and indirect gapsemiconductors thus allowing for instance the self-consistent calculation of band profileand carrier control in pseudomorphic InGaAs/GaAs HEMTs and SiGe/Si MODFETs.

Andrea  Reale

Aldo Di Carlo

Sara  Pescetelli

Marco  Paciotti

Paolo  Lugli

Optical and Electronic Properties of Semiconductor2D Nanosystems: Self-consistentTight-binding Calculations

We examine an alternative layout geometry for the quantum cellular automata (QCA)architecture. In the traditional QCA geometry, all of the cells are placed in a singleplane, so that each cell interacts with a particular neighbor only along one of its edges.By rotating the cells out of the plane, we make it possible for neighbors to interact alongall four edges at once. This increased interaction leads to a more bistable cell-cellresponse function and a 50&#37; higher excitation energy. We also present a majority logicgate designed using three-dimensional cells.

Paul G. Krause

Rachel M. Mueller

P. Douglas Tougaw

Janelle M. Weidner

An Alternative Geometry for Quantum Cellular Automata

In recent years, artificially layered microstructure have been considered as candidates
for better thermoelectrics. In this work we examine transport properties of the type-II
broken-gap InAs/GaSb superlattice. We use the effective bond orbital model for an
accurate description of the band structures. Theoretical results of thermoelectric
transport coefficients and the dimensionless figure of merit for an (8, 8)-InAs/GaSb
type-II superlattice are presented.

J.-F. Lin

D. Z.-Y. Ting

Quantum Transport and Thermoelectric Properties
of InAs/GaSb Superlattices

The Mutation Operator Monte Carlo method (MOMC) is a new type of Monte Carlo
technique for the study of hot electron related effects in semiconductor devices. The
MOMC calculates energy distributions of electrons by a physical mutation of the
distribution towards a stationary condition. In this work we compare results of an one
dimensional simulation of an 800nm Si-MOSFET with full band Monte Carlo
calculations and measurement results. Starting from the potential distribution resulting
from a drift diffusion simulation, the MOMC calculates electron distributions which are
comparable to FBMC-results within minutes on a modern workstation. From these
distributions, substrate and gate currents close to experimental results can be calculated.
These results show that the MOMC is useful as a post-processor for the investigation of
hot electron related problems in Si-MOSFETs. Beside the computational efficiency, a
further advantage of the MOMC compared to standard MC techniques is the good
resolution of the high energy tail of the distribution without the necessity of any
statistical enhancement.

Jürgen Jakumeit

Amanda Duncan

Umberto Ravaioli

Karl Hess

Simulation of Si-MOSFETs with the Mutation Operator
Monte Carlo Method

A boundary condition specifically designed to model open-circuited devices in amacroscopic device simulator is introduced. Other simulation techniques have relied onan external circuit model to regulate the current flow out of a contact thus allowing thepotential to remain the controlled variable at the boundary. The limitations of thesemethods become apparent when modeling open-circuited devices with an exceptionallysmall or zero output current. In this case, using a standard ohmic-type Dirichletboundary condition would not yield satisfactory results and attaching the device to anarbitrarily large load resistance is physically and numerically unacceptable. Thisproposed condition is a true current controlled boundary where the external current isthe specified parameter rather than the potential. Using this model, the external currentis disseminated into electron and hole components relative to their respectiveconcentration densities at the contact. This model also allows for the inclusion oftrapped interface charge and a finite surface recombination velocity at the contact.An example of the use of this boundary condition is performed by modeling a siliconavalanche photodiode operating in the flux integrating mode for use in an imagingsystem. In this example, the device is biased in steady-state to just below the breakdownvoltage and then open-circuited. The recovery of the isolated photodiode back to itsequilibrium condition is then determined by the generation lifetime of the material, thequantity of signal and background radiation incident upon the device, and the impactionization rates.

Joseph W. Parks

Kevin F. Brennan

Boundary Condition for the Modeling of Open-circuitedDevices in Non-equilibrium

We introduced a novel high-frequency source based on interband tunneling. A
polarization-induced oscillation of trapped-hole-charge occurs in an AlGaSb/InAs/
AlGaSb resonant tunneling device. Rate equations for Zener tunneling, polarization,
and electron-hole recombination is used to analyze the nonlinear dynamics of this device
structure. The nonoscillatory state is unstable against the limit-cycle operation. The
amplitude of trapped hole oscillation increases with bias, but the time-averaged values
can be approximated by a step function. These lead to the hysteresis of the averaged
trapped hole charge in AlGaSb barrier, and to the experimental intrinsic bistability in
AlGaSb/InAs/AlGaSb resonant tunneling device. Large-scale time-dependent simulation
of quantum transport with interband-tunneling dynamics is needed for the design
optimization of this novel class of oscillator useful for high-bandwidth applications.

F. A. Buot

An lnterband Tunnel Oscillator: Intrinsic Bistability
and Hysteresis of Trapped Hole Charge
in a Double-Barrier Structure

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