Umberto Ravaioli

Theory for a quantum modulated transistor

We present a theoretical study of semiconductor T‐structures which may exhibit transistor action based on quantum interference. The electron transmission through a semiconductor quantum wire can be controlled by an external gate voltage that modifies the penetration of the electron wavefunction in a lateral stub, affecting in this way its interference pattern. The structures are modeled as ideal two‐dimensional electron waveguides and a tight‐binding Green’s function technique is used to compute the electron transmission and reflection coefficients. The calculations show that relatively small changes in the stub length can induce strong variations in the electron transmission across the structure. Operation in the fundamental transverse mode appears to be important for applications. We also show that a bound state of purely geometrical origin nucleates at the intersection between waveguide and stub. The performance of the device can be improved by inserting additional stubs of slightly different lengths. ...

Impact of phonon-surface roughness scattering on thermal conductivity of thin si nanowires.

We present a novel approach for computing the surface roughness-limited thermal conductivity of silicon nanowires with diameter D<100 nm. A frequency-dependent phonon scattering rate is computed from perturbation theory and related to a description of the surface through the root-mean-square roughness height Delta and autocovariance length L. Using a full phonon dispersion relation, we find a quadratic dependence of thermal conductivity on diameter and roughness as (D/Delta)(2). Computed results show excellent agreement with experimental data for a wide diameter and temperature range (25-350 K), and successfully predict the extraordinarily low thermal conductivity of 2 W m(-1) K-1 at room temperature in rough-etched 50 nm silicon nanowires.

On the possibility of transistor action based on quantum interference phenomena

A theoretical study of quantum interference phenomena in a T‐shaped semiconductor structure is presented. Transmission and reflection coefficients are computed by use of a tight‐binding Green function technique. As expected, the results resemble the well‐known solutions for the electromagnetic field in waveguides with the main difference that the penetration of the wave function of the electrons can be controlled by external voltages. We conclude that transistor action based on quantum interference should be observable in such structures, and we present general results for the functional dependences of the transmission coefficient which corresponds to a transconductance.

Iteration scheme for the solution of the two-dimensional Schrödinger-Poisson equations in quantum structures

A fast and robust iterative method for obtaining self-consistent solutions to the coupled system of Schrodinger’s and Poisson’s equations is presented. Using quantum mechanical perturbation theory, a simple expression describing the dependence of the quantum electron density on the electrostatic potential is derived. This expression is then used to implement an iteration scheme, based on a predictor-corrector type approach, for the solution of the coupled system of differential equations. We find that this iteration approach simplifies the software implementation of the nonlinear problem, and provides excellent convergence speed and stability. We demonstrate the approach by presenting an example for the calculation of the two-dimensional bound electron states within the cross section of a GaAs-AlGaAs based quantum wire. For this example, the convergence is six times faster by applying our predictor-corrector approach compared to a corresponding underrelaxation algorithm.

An improved energy transport model including nonparabolicity and non-Maxwellian distribution effects

An improved energy transport model for device simulation is derived from the zeroth and second moments of the Boltzmann transport equation (BTE) and from the presumed functional form of the even part of the carrier distribution in momentum space. Energy-band nonparabolicity and non-Maxwellian distribution effects are included to first order. The model is amenable to an efficient self-consistent discretization taking advantage of the similarity between current and energy flow equations. Numerical results for ballistic diodes and MOSFETs are presented. Typical spurious velocity overshoot spikes, obtained in conventional hydrodynamic (HD) simulations of ballistic diodes, are virtually eliminated.<<ETX>>

STUDY OF INTERFACE ROUGHNESS DEPENDENCE OF ELECTRON MOBILITY IN SI INVERSION LAYERS USING THE MONTE CARLO METHOD

The electron mobility in the inversion layer of a metal–oxide semiconductor field effect transistor formed on the (100) silicon surface is calculated by using a Monte Carlo approach which takes into account size quantization, acoustic phonon scattering, intervalley phonon scattering and surface roughness scattering. Degeneracy is also considered because it is important at higher normal effective fields (high gate voltages). The main emphasis is placed on the influence of the specific autocovariance function, used to describe the surface roughness, on the electron mobility. Here we compare the mobilities obtained using exponential and Gaussian autocovariance functions. It is found that the electron mobility calculated with roughness scattering rates based on the exponential function shows good agreement with experiments. The effect of the degeneracy and screening on the roughness scattering is also discussed.

Simulation of Schottky barrier MOSFETs with a coupled quantum injection/Monte Carlo technique

A full-band Monte Carlo device simulator has been used to analyze the performance of sub-0.1 /spl mu/m Schottky barrier MOSFETs. In these devices, the source and drain contacts are realized with metal silicide, and the injection of carriers is controlled by gate voltage modulation of tunneling through the source barrier. A simple model treating the silicide regions as metals, coupled with an Airy function approach for tunneling through the barrier, provides injecting boundary conditions for the Monte Carlo procedure. Simulations were carried out considering a p-channel device with 270 /spl Aring/ gate length for which measurements are available. Our results show that in these structures there is not a strong interaction with the oxide interface as in conventional MOS devices and carriers are injected at fairly wide angles from the source into the bulk of the device. The Monte Carlo simulations not only give good agreement with current-voltage (I-V) curves, but also easily reproduce the subthreshold behavior since all the computational power is devoted to simulation of channel particles. The simulations also clarify why these structures exhibit a large amount of leakage in subthreshold regime, due to both thermionic and tunneling emission. Computational experiments suggest ways to modify the doping profile to reduce to some extent the leakage.

Electronic Response and Bandstructure Modulation of Carbon Nanotubes in a Transverse Electrical Field

The electronic properties of carbon nanotubes in a uniform transverse field are investigated within a single orbital tight-binding model. For doped nanotubes, the dielectric function is found to depend not only on symmetry of the tube, but also on radius and Fermi level position. Bandgap opening/closing is predicted for zigzag tubes, while it is found that armchair tubes always remain metallic, which is explained by the symmetry in their configuration. The bandstructures for both types are considerably modified when the field strength is large enough to mix neighboring subbands.

Reduced thermal conductivity in nanoengineered rough Ge and GaAs nanowires.

We model and compare the thermal conductivity of rough semiconductor nanowires (NWs) of Si, Ge, and GaAs for thermoelectric devices. On the basis of full phonon dispersion relations, the effect of NW surface roughness on thermal conductivity is derived from perturbation theory and appears as an efficient way to scatter phonons in Si, Ge, and GaAs NWs with diameter D < 200 nm. For small diameters and large root-mean-square roughness Delta, thermal conductivity is limited by surface asperities and varies quadratically as (D/Delta)(2). At room temperature, our model previously agreed with experimental observations of thermal conductivity down to 2 W m(-1) K(-1) in rough 56 nm Si NWs with Delta = 3 nm. In comparison to Si, we predict here remarkably low thermal conductivity in Ge and GaAs NWs of 0.1 and 0.4 W m(-1) K(-1), respectively, at similar roughness and diameter.

Integration of a particle-particle-particle-mesh algorithm with the ensemble Monte Carlo method for the simulation of ultra-small semiconductor devices

A particle-particle-particle-mesh (P/sup 3/M) algorithm is integrated with the ensemble Monte Carlo (EMC) method for the treatment of carrier-impurity (c-i) and carrier-carrier (c-c) effects in semiconductor device simulation. Ionized impurities and charge carriers are treated granularly as opposed to the normal continuum methods and c-i and c-c interactions are calculated in three dimensions. The combined P/sup 3/M-EMC method follows the approach of Hockney (1981), but is modified to treat nonuniform rectilinear meshes with arbitrary boundary conditions. Bulk mobility results are obtained for a three-dimensional (3-D) resistor and are compared with previously reported experimental and numerical results.