Sylvie Galdin-Retailleau

Computationally Efficient Physics-Based Compact CNTFET Model for Circuit Design

We present a computationally efficient physics-based compact model designed for the conventional CNTFET featuring a MOSFET-like operation. A large part of its novelty lies on the implementation of a new analytical model of the channel charge. In addition, Boltzmann Monte Carlo (MC) simulation is performed with the challenge to cross-link this simulation technique to the compact modeling formulation. The comparison of the electrical characteristics obtained from the MC simulation and from the compact modeling demonstrates the compact model accuracy within its range of validity. Then, from a study of the CNT diameter dispersion for three technological processes, the compact model allows us to determine the CNTFET threshold voltage distribution and to evaluate the resulting dispersion of the propagation delay from the simulation of a ring oscillator.

Suppression of the orientation effects on bandgap in graphene nanoribbons in the presence of edge disorder

This letter shows that a moderate degree of edge disorder can explain the fact that the experimentally measured bandgaps of graphene nanoribbons (GNRs) do not depend on orientation. We argue that GNRs actually behave similarly to Anderson insulators and the measured bandgaps should thus be interpreted as quasi-mobility edges. Calculations in the tight binding approach reveal that in the presence of edge disorder, quasi-mobility edge and electronic structures become independent of orientation and that quasi-mobility edge follows a quasi-universal law similar to experimental data, although with different parameters.

On the Ability of the Particle Monte Carlo Technique to Include Quantum Effects in Nano-MOSFET Simulation

In this paper, we report on the possibility of using particle-based Monte Carlo (MC) techniques to incorporate all relevant quantum effects in the simulation of semiconductor nanotransistors. Starting from the conventional MC approach within the semiclassical Boltzmann approximation, we develop a multisubband description of transport to include quantization in ultrathin-body devices. This technique is then extended to the particle simulation of quantum transport within the Wigner formulation. This new simulator includes all expected quantum effects in nanotransistors and all relevant scattering mechanisms, which are taken into account the same way as in Boltzmann simulation. This paper is illustrated by analyzing the device operation and performance of multigate nanotransistors in a convenient range of channel lengths and thicknesses to separate the influence of all relevant effects: Significant quantization effects occur for thickness smaller than 5 nm and wave-mechanical-transport effects manifest themselves for channel length smaller than 10 nm. We also show that scattering mechanisms still have an important influence in nanoscaled double-gate transistors, both in the intrinsic part of the channel and in the resistive lateral extensions.

Analysis of CNTFET physical compact model

On the basis of acquired knowledge, the paper present a DC compact model designed for the conventional CNTFET (C-CNTFET) featuring a doping profile similar to n-MOSFET. The specific enhancement lies on the implementation of a physical based calculation of the minima of energy conduction subbands. This improvement allows a realistic analysis of the impact of CNT helicity and radius on the DC characteristics. The purpose is to enable the circuit designers to challenge CNTFET potentialities for performing logical or analogical functionalities within complex circuits

Monte Carlo study of apparent mobility reduction in nano-MOSFETs

The concept of mobility, resulting from an analysis of stationary transport where carrier velocity is limited by scattering phenomena, has been widely used till today in microelectronics as a measurable factor of merit and as a parameter of analytical models developed to predict device performance. If scatterings are still playing a major role in decananometer MOSFET and cannot be neglected, ballistic transport in the channel takes a growing importance as the gate length of MOSFETs tends to the nanometer scale. In this context, the mobility concept may appear as highly questionable.

Study of phonon modes in silicon nanocrystals using the adiabatic bond charge model

We present theoretical calculations of phonon dispersion in silicon nanocrystals using an approach based on the adiabatic bond charge model. To deal with the boundary conditions, two cases are considered: the surface atoms are either free to move or rigidly fixed. In the former case, surface modes appear at low frequencies and, in the latter case, nodes and antinodes appear near a frequency of 11 THz. By projecting the nanocrystal modes on the basis of bulk modes, one can show the increasing correlation between the nanocrystal modes and the bulk modes when increasing the dot size. Finally, the frequency shift of Raman spectra calculated as a function of the dot size is found to be in good agreement with sets of experimental data.

Electron transport in Si/SiGe modulation-doped heterostructures using Monte Carlo simulation

The electron transport in two-dimensional gas formed in tensile-strained Si1−xGex/Si/Si1−xGex heterostructures is investigated using Monte Carlo simulation. First the electron mobility is studied in ungated modulation-doped structures. Calculation matches the experimental results very well over a wide range of electron densities. The mobility typically varies between 1100 cm2/Vu200as in highly-doped structures and 2800 cm2/Vu200as at low electron density. The mobility is shown to be significantly influenced by the thickness of the spacer layer separating the strained Si channel from the pulse-doped supply layers. Then the electron transport is investigated in a gated modulation-doped structure in which the contribution of parasitic paths is negligible. The mobility is shown to be higher than in comparable ungated structures and dependent on the gate voltage as a result of the electron density dependence of remote impurity screening.

Ultra-short n-MOSFETs with strained Si : device performance and the effect of ballistic transport using Monte Carlo simulation

Semi-classical Monte Carlo simulation is used to study the electrical performance of 18-nm-long n-MOSFETs including a strained Si channel. In particular, the impact of extrinsic series resistance on the drive current Ion is quantified: we show that the large on-current improvement induced by the strain is preserved, even by including an external parasitic resistance. The importance of ballistic transport is also examined and its influence on Ion is highlighted.

Semiclassical and Quantum Transport in CNTFETs Using Monte Carlo Simulation

The effects of quasi-ballistic and quantum transport on the operation and the performance of carbon nanotube FETs are examined by means of both Boltzmann and Wigner Monte Carlo simulations including phonon scattering. The semiclassical simulation of transistors of gate length in the range 10-100 nm shows the strong ballisticity of the transport in the gated part of the channel, while the emission of high-energy phonons occurs only beyond the drain end of the channel. For a gate length of 25 nm, the fraction of ballistic electrons reaches 89%, instead of typically 31% in the silicon FET of the same gate length with the undoped channel. For a gate length higher than 20 nm, the main quantum-transport effect is the reflection of carriers due to the sharp potential drop at the drain end of the channel. In spite of strong microscopic differences observed when comparing Wigner and Boltzmann functions, this effect just makes the quantum current slightly smaller than the semiclassical one on the full range of the gate voltage. For a smaller gate length, the source-drain tunneling becomes possible in the subthreshold regime, which enhances the subthreshold slope and the off-current in the quantum simulation.

Physical Simulation of Silicon-Nanocrystal-Based Single-Electron Transistors

A 3-D simulator of semiconducting nanocrystal (NC)-based single-electron transistors (SETs) is presented. It is based on the self-consistent solution of Poisson and Schrödinger equations. The resulting wave functions are used to compute the bias-dependent tunneling rates in the weak dot-to-lead coupling limit. These rates are used as input data of a Monte Carlo code, which treats the sequential transport of electrons through the tunnel barriers. The simulator is applied to a typical silicon-NC SET. The resulting current-voltage characteristics are discussed in terms of tunneling rates, chemical potentials, and wave functions. The influence of all device parameters and of the temperature are carefully analyzed.