Random dopant-induced variability in Si-InAs nanowire tunnel FETs: a quantum transport simulation study

In this letter, we report a quantum transport simu- lation study of the impact of Random Discrete Dopants (RDD)s on Si-InAs nanowire p-type Tunnel FETs. The band-to-band tunneling is simulated using the non-equilibrium Green’s func- tion formalism in effective mass approximation, implementing a two-band model of the imaginary dispersion. We have found that RDDs induce strong variability not only in the OFF-state but also in the ON-state current of the TFETs. Contrary to the nearly normal distribution of the RDD induced ON-current variations in conventional CMOS transistors, the TFET’s ON- currents variations are described by a logarithmic distribution. The distributions of other Figures of Merit (FoM) such as threshold voltage and subthreshold swing are also reported. The variability in the FoM is analysed by studying the correlation between the number and the position of the dopants

Comprehensive study of cross-section dependent effective masses for silicon based gate-all-around transistors

The use of bulk effective masses in simulations of the modern-day ultra-scaled transistor is erroneous due to the strong dependence of the band structure on the cross-section dimensions and shape. This has to be accounted for in transport simulations due to the significant impact of the effective masses on quantum confinement effects and mobility. In this article, we present a methodology for the extraction of the electron effective masses, in both confinement and the transport directions, from the simulated electronic band structure of the nanowire channel. This methodology has been implemented in our in-house three-dimensional (3D) simulation engine, NESS (Nano-Electronic Simulation Software). We provide comprehensive data for the effective masses of the silicon-based nanowire transistors (NWTs) with technologically relevant cross-sectional area and transport orientations. We demonstrate the importance of the correct effective masses by showing its impact on mobility and transfer characteristics

Density Gradient Based Quantum-Corrected 3D Drift-Diffusion Simulator for Nanoscale MOSFETs

In this work, we have developed a solver for the three-dimensional density gradient (DG) equation which is used to apply quantum corrections (QC) to the classical drift-diffusion (DD) simulator in a self-consistent manner. This module has been implemented in C++ using the finite volume method and has been incorporated into NESS (Nano-Electronic Simulation Software) which is being developed in the Device Modelling Group, University of Glasgow. Here, we summarise the implementation details and particularly highlight the impact of the three anisotropic DG masses, which are used as fitting parameters, on the charge profiles and current-voltage (I-V) characteristics in nano-transistors

Quantum Transport Investigation of Threshold Voltage Variability in Sub-10 nm JunctionlessSi Nanowire FETs

In this paper, we use the Non-Equilibrium Green's Function formalism to study the dependence of the threshold voltage variability on the cross-section shape and the gate length in Junction Less Field Effect Transistors. Each configuration, i.e. gate length and cross-section, was investigated using a statistical ensemble of 100 samples. We found that the variability in threshold voltage is increased independently of the cross-section shape when the gate length isshrunk down to 5 nm. We attribute this results to the higher wave function “randomization” in longer gate lengths

Self-Consistent Enhanced S/D Tunneling Implementation in a 2D MS-EMC Nanodevice Simulator

The implementation of a source to drain tunneling in ultrascaled devices using MS-EMC has traditionally led to overestimated current levels in the subthreshold regime. In order to correct this issue and enhance the capabilities of this type of simulator, we discuss in this paper two alternative and self-consistent solutions focusing on different parts of the simulation flow. The first solution reformulates the tunneling probability computation by modulating the WKB approximation in a suitable way. The second corresponds to a change in the current calculation technique based on the utilization of the Landauer formalism. The results from both solutions are compared and contrasted to NEGF results from NESS. We conclude that the current computation modification constitutes the most suitable and advisable strategy to improve the MS-EMC tool.Spanish Ministry of Economy, Industry and Competitivity under grant TEC2017-89800-REPSRC UKRI Innovation Fellowship scheme under grant agreement No. EP/S001131/1 (QSEE)Juan de la Cierva Incorporación Fellowship scheme under grant agreement No. IJC2019-040003-I (MICINN/AEI

A Multi-Scale Simulation Study of the Strained Si Nanowire FETs

In this work, we study 2.1nm-diameter uniaxial strained Si gate-all-around nanowire field-effect transistors, focusing on the electron mobility and the variability due to random discrete dopants (RDDs). Firstly, we extract the electron effective masses under various strains from Density Functional Theory (DFT) simulations. Secondly, we present the impact of the strain on the electron mobility in the Si nanowire using the Kubo-Greenwood formalism with a set of multi-subband phonon, surface roughness, and ionized impurity scattering mechanisms. Finally, we perform quantum transport simulations to investigate the effect of RDD on the threshold voltage and ON-state current variation

Impact of Strain on S/D tunneling in FinFETs: a MS-EMC study

As device dimensions are scaled down, the use of strained channels as performance booster becomes of special relevance. Moreover, the inclusion of quantum effects in the transport direction is imperative to predict the performance of future transistors. In particular, Source-to-Drain tunneling (S/D tunneling) is presented as a scaling limit in sub-10nm nodes. In this work, a Multi-Subband Ensemble Monte Carlo (MS-EMC) study of the impact of S/D tunneling in relaxed and biaxially strained channel FinFETs is presented

Mobility of circular and elliptical si nanowire transistors using a multi-subband 1d formalism

We have studied the impact of the cross-sectional shape on the electron mobility of n-type silicon nanowire transistors (NWTs). We have considered circular and elliptical cross-section NWTs including the most relevant multisubband scattering processes involving phonon, surface roughness, and impurity scattering. For this purpose, we use a flexible simulation framework, coupling 3D Poisson and 2D Schrödinger solvers with the semi-classical Kubo-Greenwood formalism. Moreover, we consider cross-section dependent effective masses calculated from tight binding simulations. Our results show significant mobility improvement in the elliptic NWTs in comparison to the circular one for both 100 and 110 transport directions

Enhanced Capabilities of the Nano-Electronic Simulation Software (NESS)

The aim of this paper is to present a flexible TCAD platform called Nano-Electronic Simulation Software (NESS) which enables the modelling of contemporary future electronic devices combining different simulation paradigms (with different degrees of complexity) in a unified simulation domain. NESS considers confinement-aware band structures, generates the main sources of variability, and can study their impact using different transport models. In particular, this work focuses on the new modules implemented: Kubo-Greenwood solver, Kinetic Monte Carlo solver, Gate Leakage calculation, and a full-band quantum transport solver in the presence of hole-phonon interactions using a mode-space k⋅p approach in combination with the existing NEGF module

Multisubband ensemble Monte Carlo analysis of tunneling leakage mechanisms in ultrascaled FDSOI, DGSOI, and FinFET devices

Leakage phenomena are increasingly affecting the performance of nanoelectronic devices, and therefore, advanced device simulators need to include them in an appropriate way. This paper presents the modeling and implementation of direct source-to-drain tunneling (S/D tunneling), gate leakage mechanisms (GLMs) accounting for both direct tunneling and trap-assisted tunneling, and nonlocal band-to-band tunneling (BTBT) phenomena in a multissubband ensemble Monte Carlo (MS-EMC) simulator along with their simultaneous application for the study of ultrascaled fully depleted silicon-on-insulator, double-gate silicon-on-insulator, and FinFET devices. We find that S/D tunneling is the prevalent phenomena for the three devices, and it is increasingly relevant for short-channel lengths