Weak measurement and the traversal time problem

The theory of weak measurement, proposed by Aharonov and coworkers, has been applied by Steinberg to the long-discussed traversal time problem. The uncertainty and ambiguity that characterize this concept from the perspective of von Neumann measurement theory apparently vanish, and joint probabilities and conditional averages become meaningful concepts. We express the Larmor clock and some other well-known methods in the weak measurement formalism. We also propose a method to determine higher moments of the traversal time distribution in terms of the outcome of a gedanken experiment, by introducing an appropriate operator. Since the weak measurement approach can sometimes lead to unphysical results, for example average negative reflection times and higher moments, the interpretation of the results obtained remains an open problem.Comment: Talk given at the Adriatico Research Conference on ``Tunnelling and its implications'', 30 July--2 August 1996, ICTP, Triest

NANOTCAD2D: Two-dimensional code for the simulation of nanoelectronic devices and structures

In this paper we present NANOTCAD2D, a code for the simulation of the electrical properties of semiconductor-based nanoelectronic devices and structures in two-dimensional domains. Such code is based on the solution of the Poisson/Schr\"odinger equation with density functional theory and of the continuity equation of the ballistic current. NANOTCAD2D can be applied to structures fabricated on III-IV, strained-silicon and silicon-germanium heterostructures, CMOS structures, and can easily be extended to new materials. In particular, in the case of SiGe heterostructures, it computes the effects of strain on the energy band profiles. The effects of interface states at the air/semiconductor interfaces, particularly significant in the case of devices obtained by selective etching, are also properly taken into account.Comment: 23 pages, 11 figure

On the possibility of tunable-gap bilayer graphene FET

We explore the device potential of tunable-gap bilayer graphene FET exploiting the possibility of opening a bandgap in bilayer graphene by applying a vertical electric field via independent gate operation. We evaluate device behavior using atomistic simulations based on the self-consistent solution of the Poisson and Schroedinger equations within the NEGF formalism. We show that the concept works, but bandgap opening is not strong enough to suppress band-to-band tunneling in order to obtain a sufficiently large Ion/Ioff ratio for CMOS device operation.Comment: 10 pages, 3 figures, submitted to IEEE ED

A unified approach to electron transport in double barrier structures

In this paper we show an approach to electron transport in double barrier structures which unifies the well known sequential and resonant tunneling models in the widest range of transport regimes, from completely coherent to completely incoherent. In doing so, we make a clear distinction between ``approaches'' and ``transport regimes,'' in order to clarify some ambiguities in the concept of sequential tunneling. Scattering processes in the well are accounted for by means of an effective mean free path, which plays the role of a relaxation length. Our approach is based on a recently derived formula for the density of states in a quantum well, as a function of the round trip time in the well and of trasmission and reflection probabilities for the whole structure and for each barrier.Comment: RevTeX file, 14 pages, 2 uuencoded Postscript figures, uses epsf.sty. To be published on Phys. Rev. B. Postscript files and hard copies available from the authors upon request ([email protected]

Simulation of Graphene Nanoribbon Field Effect Transistors

We present an atomistic three-dimensional simulation of graphene nanoribbon field effect transistors (GNR-FETs), based on the self-consistent solution of the 3D Poisson and Schroedinger equation with open boundary conditions within the non-equilibrium Green's Function formalism and a tight-binding hamiltonian. With respect to carbon nanotube FETs, GNR-FETs exhibit comparable performance, reduced sensitivity on the variability of channel chirality, and similar leakage problems due to band-to-band tunneling. Acceptable transistor performance requires effective nanoribbon width of 1-2 nm, that could be obtained with periodic etching patterns or stress patterns

Analytical model of 1D Carbon-based Schottky-Barrier Transistors

Nanotransistors typically operate in far-from-equilibrium (FFE) conditions, that cannot be described neither by drift-diffusion, nor by purely ballistic models. In carbonbased nanotransistors, source and drain contacts are often characterized by the formation of Schottky Barriers (SBs), with strong influence on transport. Here we present a model for onedimensional field-effect transistors (FETs), taking into account on equal footing both SB contacts and FFE transport regime. Intermediate transport is introduced within the Buttiker probe approach to dissipative transport, in which a non-ballistic transistor is seen as a suitable series of individually ballistic channels. Our model permits the study of the interplay of SBs and ambipolar FFE transport, and in particular of the transition between SB-limited and dissipation-limited transport

Signal Extraction in Continuous Time and the Generalized Hodrick- Prescott Filter

A widely used filter to extract a signal in a time series, in particular in the business cycle analysis, is the Hodrick-Prescott filter. The model that underlies the filter considers the data series as the sum of two unobserved component (signal and non signal) and a smoothing parameter which for quarterly series is set to a specified value. This paper proposes a generalization of the Hodrick-Prescott filter to a continuous time support, using the well-established relationship between cubic splines and state-space models. The spline formulation of the filter leads to a state space model with several practical advantages: first, the smoothing parameter can be either pre-specified or estimated as the other parameters in the model; second, the unobserved components can be modelled by the addition of particular ARIMA structures; lastly the model is capable of working in the presence of missing values or for irregular surveys. Monte Carlo experiments support these considerations.smoothing parameter, cubic spline, state-space model, irregular surveys.

Enhanced shot noise in carbon nanotube field-effect transistors

We predict shot noise enhancement in defect-free carbon nanotube field-effect transistors through a numerical investigation based on the self-consistent solution of the Poisson and Schrodinger equations within the non-equilibrium Green functions formalism, and on a Monte Carlo approach to reproduce injection statistics. Noise enhancement is due to the correlation between trapping of holes from the drain into quasi-bound states in the channel and thermionic injection of electrons from the source, and can lead to an appreciable Fano factor of 1.22 at room temperature.Comment: 4 pages, 4 figure

Shot noise suppression in quasi one-dimensional Field Effect Transistors

We present a novel method for the evaluation of shot noise in quasi one-dimensional field-effect transistors, such as those based on carbon nanotubes and silicon nanowires. The method is derived by using a statistical approach within the second quantization formalism and allows to include both the effects of Pauli exclusion and Coulomb repulsion among charge carriers. In this way it extends Landauer-Buttiker approach by explicitly including the effect of Coulomb repulsion on noise. We implement the method through the self-consistent solution of the 3D Poisson and transport equations within the NEGF framework and a Monte Carlo procedure for populating injected electron states. We show that the combined effect of Pauli and Coulomb interactions reduces shot noise in strong inversion down to 23 % of the full shot noise for a gate overdrive of 0.4 V, and that neglecting the effect of Coulomb repulsion would lead to an overestimation of noise up to 180 %.Comment: Changed content, 7 pages,5 figure