Arnaud Bournel

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.

Effect of discrete impurities on electron transport in ultrashort MOSFET using 3D MC simulation

This paper discusses the influence of the channel impurity distribution on the transport and the drive current in short-gate MOSFETs. A careful description of electron-ion interaction suitable for the case of discrete impurities has been implemented in a three-dimensional particle Monte Carlo simulator. This transport model is applied to the investigation of 50-nm MOSFET operation. The results show that a small change in the number of doping impurities or in the position of a single discrete impurity in the inversion layer may significantly influence the drain current. This effect is not only related to threshold voltage fluctuations but also to variations in transport properties in the inversion layer, especially at high drain voltage. The results are analyzed in terms of local fluctuations of electron velocity and current density. In a set of fifteen simulated devices the drive current I/sub on/, determined at V/sub GS/=V/sub DS/=0.6 V, is found to vary in a range of 23% according to the position of channel impurities.

Electronic transport and spin-polarization effects of relativisticlike particles in mesoscopic graphene structures

Motivated by recent studies on the use of graphene for new concepts of electronic/spintronic devices, the authors develop an efficient calculation method based on the nonequilibrium Green’s function to solve the quantum relativisticlike Dirac’s equation that governs the low-energy excited states in graphene. The approach is then applied to investigate the electronic transport and the spin polarization in a single-graphene barrier structure. The obtained results are presented and analyzed in detail aiming to highlight typical properties of the considered graphene structure as well as the efficiency of the developed approach that both may be helpful for further development in electronic devices and in spintronics.

Impact of the Radial Ionization Profile on SEE Prediction for SOI Transistors and SRAMs Beyond the 32-nm Technological Node

The relative contribution of the radial ionization profile on SEE prediction is investigated using MUSCA-SEP3 , in comparison with the classical approach considering the ion track as a series of punctual charges. The new approach is validated against experimental results, for three technology generations of PDSOI transistors and for two generations of SOI SRAM cells, showing better agreement than the punctual approach. The impact of the radial approach on the evaluation of SEU cross section as compared to the punctual approach is then investigated for nanometric SOI SRAM cells, beyond the 32-nm technological node. The influence of the radial dimension of the ion track is shown to increase with technology generation. The impact of the ion mass and energy on the ratio between radial and punctual SEU cross section is also investigated.

Controllable spin-dependent transport in armchair graphene nanoribbon structures

Using the nonequilibrium Green’s functions formalism in a tight binding model, the spin-dependent transport in armchair graphene nanoribbons controlled by a ferromagnetic gate is investigated. Beyond the oscillatory behavior of conductance and spin polarization with respect to the barrier height, which can be tuned by the gate voltage, we especially analyze the effects of width-dependent band gap and of the nature of contacts. The oscillation of spin polarization in graphene nanoribbons with a large band gap is strong in comparison with that in infinite graphene sheets. Very high spin polarization (close to 100%) is observed in normal-conductor/graphene/normal-conductor junctions. Moreover, we find that the difference in electronic structure between normal conductor and graphene generates confined states which have a strong influence on the transport properties of the device. This study suggests that the device should be carefully designed to obtain a high controllability of spin-polarized current.

A Study of Quantum Transport in End-of-Roadmap DG-MOSFETs Using a Fully Self-Consistent Wigner Monte Carlo Approach

We present results of ultrascaled double-gate MOSFET operation and performance obtained from a new self-consistent particle-based quantum Monte Carlo (MC) approach. The simulation of quantum transport along the source-drain direction is based on the Wigner transport equation and the mode-space approximation of multi subband description. An improved method for correctly reproducing the Wigner function in the phase space by means of pseudo-particles is proposed. Our approach includes scattering effects for a two-dimensional (2-D) electron gas via standard MC algorithm. Detailed comparisons with both ballistic nonequilibrium Greens function and semiclassical multi subband Monte Carlo approaches show the ability of this Wigner transport model to incorporate correctly quantum effect into particle ensemble Monte Carlo simulation together with accurate description of scattering. This study of 6-nm-long MOSFET emphasizes the prevalent contribution of source-drain tunneling in subthreshold regime and the significant effect of quantum reflections in on-state. The influence of scattering in both the source access region and the gated part of the channel appears to be of prime importance for the correct evaluation of the on-state current, even for such small device in which the fraction of ballistic electrons is high

Resonant tunnelling diodes based on graphene/h-BN heterostructure

In this work, we propose a resonant tunnelling diode (RTD) based on a double-barrier graphene/boron nitride (BN) heterostructure as a device suitable to take advantage of the elaboration of atomic sheets containing different domains of BN and C phases within a hexagonal lattice. The device operation and performance are investigated by means of a self-consistent solution within the non-equilibrium Greens function formalism on a tight-binding Hamiltonian. This RTD exhibits a negative differential conductance effect, which involves the resonant tunnelling through both the electron and hole bound states in the graphene quantum well. It is shown that the peak-to-valley ratio reaches a value of ?4 at room temperature even for zero bandgap and can be higher than 10 when a finite gap opens in the graphene channel.

Graphene nanomesh-based devices exhibiting a strong negative differential conductance effect

Using atomistic quantum simulation based on a tight binding model, we have investigated the transport characteristics of graphene nanomesh-based devices and evaluated the possibilities of observing negative differential conductance. It is shown that by taking advantage of bandgap opening in the graphene nanomesh lattice, a strong negative differential conductance effect can be achieved at room temperature in pn junctions and n-doped structures. Remarkably, the effect is improved very significantly (with a peak-to-valley current ratio of a few hundred) and appears to be weakly sensitive to the transition length in graphene nanomesh pn hetero-junctions when inserting a pristine (gapless) graphene section in the transition region between n and p zones. The study therefore suggests new design strategies for graphene electronic devices which may offer strong advantages in terms of performance and processing over the devices studied previously.

Effect of the Ion Mass and Energy on the Response of 70-nm SOI Transistors to the Ion Deposited Charge by Direct Ionization

The response of SOI transistors under heavy ion irradiation is analyzed using Geant4 and Synopsys Sentaurus device simulations. The ion mass and energy have a significant impact on the radial ionization profile of the ion deposited charge. For example, for an identical LET, the higher the ion energy per nucleon, the wider the radial ionization track. For a 70-nm SOI technology, the track radius of high energy ions (> 10 MeV/a) is larger than the transistor sensitive volume; part of the ion charge recombines in the highly doped source or drain regions and does not participate to the transistor electric response. At lower energy (<; 10 MeV/a), as often used for ground testing, the track radius is smaller than the transistor sensitive volume, and the entire charge is used for the transistor response. The collected charge is then higher, corresponding to a worst-case response of the transistor. Implications for the hardness assurance of highly-scaled generations are discussed.