No abstract

In this paper we report on the possibility to use particle-based Monte Carlo techniques to incorporate all relevant quantum effects in the simulation of semiconductor nanotransistors. Starting from the conventional Monte Carlo approach within the semi-classical Boltzmann approximation, we develop a multi-subband description of transport to include quantization in ultra-thin 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 nano-transistors and all relevant scattering mechanisms which are taken into account the same way as in Boltzmann simulation. This work is illustrated by analyzing the device operation and performance of multi-gate nano-transistors in a convenient range of channel lengths and thicknesses to separate the influence of all relevant effects: significant quantization effects occurs 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.

Based on Monte Carlo simulation, we report the study of the inversion layer mobility in n-channel strained Si/ Si 1-x Ge x MOS structures. The influence of the strain in the Si layer and of the doping level is studied. Universal mobility curves µ eff as a function of the effective vertical field E eff are obtained for various state of strain, as well as a fall-off of the mobility in weak inversion regime, which reproduces correctly the experimental trends. We also observe a mobility enhancement up to 120 % for strained Si/ Si 0.70 Ge 0.30 , in accordance with best experimental data. The effect of the strained Si channel thickness is also investigated: when decreasing the thickness, a mobility degradation is observed under low effective field only.

The band offsets for strained Si 1−x−y Ge x C y layers grown on Si(001) substrate and for strained Si 1−x Ge x layers grown on fully relaxed Si 1−z Ge z virtual substrates are estimated. The hydrostatic strain, the uniaxial strain and the intrinsic chemical effect of Ge and C are considered separately. Unknown material parameters relative to the latter effect are chosen to give the best agreement with the available experimental results for Si 1−x Ge x and Si 1−y C y layers on Si. As a general trend concerning carrier confinement opportunities, it is found that a compressive strain is required to obtain a sizeable valence band offset, while a tensile strain is needed to obtain a conduction band discontinuity. In most cases the strain is responsible for a bandgap narrowing with respect to that of the substrate. The obtained results are in very good agreement with available experimental determinations of band offsets and bandgap changes for ternary alloys on Si(001).

Electron transport in Ge at various temperatures down to 20 mK has been investigated using particle Monte Carlo simulation taking into account ionized impurity and inelastic phonon scattering. The simulations account for the essential features of electron transport at cryogenic temperature: Ohmic regime, anisotropy of the drift velocity relative to the direction of the electric field, as well as a negative differential mobility phenomenon along the <111> field orientation. Experimental data for the electron velocities are reproduced with a satisfactory accuracy. Examples of electron position in the real space during the simulations are given and evidence separated clouds of electrons propagating along different directions depending on the valley they belong.

http://www.gianlucafiori.org/articles/SSE09.pd

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 I on 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 I on is highlighted.

No abstract