Modelling and simulation of advanced semiconductor devices

This paper presents a modelling and simulation study of advanced semiconductor devices. Different Technology Computer Aided Design approaches and models, used in nowadays research are described here. Our discussions are based on numerous theoretical approaches starting from first principle methods and continuing with discussions based on more well stablished methods such as Drift-Diffusion, Monte Carlo and Non-Equilibrium Green’s Function formalism

Variability Study of High Current Junctionless Silicon Nanowire Transistors

Silicon nanowires have numerous potential applications, including transistors, memories, photovoltaics, biosensors and qubits [1]. Fabricating a nanowire with characteristics required for a specific application, however, poses some challenges. For example, a major challenge is that as the transistors dimensions are reduced, it is difficult to maintain a low off-current (Ioff) whilst simultaneously maintaining a high on-current (Ion). This can be the result of quantum mechanical tunnelling, short channel effects or statistical variability [2]. A variety of new architectures, including ultra-thin silicon-on-insulator (SOI), double gate, FinFETs, tri-gate, junctionless and gate all-around (GAA) nanowire transistors, have therefore been developed to improve the electrostatic control of the conducting channel. This is essential since a low Ioff implies low static power dissipation and it will therefore improve power management in the multi-billion transistor circuits employed globally in microprocessors, sensors and memories

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

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

Atomistic to circuit-level modeling of doped SWCNT for on-chip interconnects

In this article, we present a hierarchical model for doped single-wall carbon nanotube (SWCNT) for on-chip interconnect application. We study the realistic CVD grown SWCNT with defects and contacts, which induce important resistance values and worsens SWCNT on-chip interconnect performance. We investigate the fundamental physical mechanism of doping in SWCNT with the purpose of improving its electrical conductivity as well as combining mitigating the effects of defects and large contact resistance. The atomistic model provides insights on statistical variations of the number of conducting channels of doped SWCNT and SWCNT resistance variation with a various number of vacancy defects configurations. Based on atomistic simulations, we develop circuit-level models to simulate SWCNT interconnects and understand the impact of doping, defects, and contacts. Simulation results show an 80% resistance reduction by doping. Additionally, we observe that doping can mitigate the effects of defects and limited impact on contact resistance

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

Nano-electronic Simulation Software (NESS): a flexible nano-device simulation platform

The aim of this paper is to present a flexible and open-source multi-scale simulation software which has been developed by the Device Modelling Group at the University of Glasgow to study the charge transport in contemporary ultra-scaled Nano-CMOS devices. The name of this new simulation environment is Nano-electronic Simulation Software (NESS). Overall NESS is designed to be flexible, easy to use and extendable. Its main two modules are the structure generator and the numerical solvers module. The structure generator creates the geometry of the devices, defines the materials in each region of the simulation domain and includes eventually sources of statistical variability. The charge transport models and corresponding equations are implemented within the numerical solvers module and solved self-consistently with Poisson equation. Currently, NESS contains a drift–diffusion, Kubo–Greenwood, and non-equilibrium Green’s function (NEGF) solvers. The NEGF solver is the most important transport solver in the current version of NESS. Therefore, this paper is primarily focused on the description of the NEGF methodology and theory. It also provides comparison with the rest of the transport solvers implemented in NESS. The NEGF module in NESS can solve transport problems in the ballistic limit or including electron–phonon scattering. It also contains the Flietner model to compute the band-to-band tunneling current in heterostructures with a direct band gap. Both the structure generator and solvers are linked in NESS to supporting modules such as effective mass extractor and materials database. Simulation results are outputted in text or vtk format in order to be easily visualized and analyzed using 2D and 3D plots. The ultimate goal is for NESS to become open-source, flexible and easy to use TCAD simulation environment which can be used by researchers in both academia and industry and will facilitate collaborative software development

The Impact of Vacancy Defects on CNT Interconnects: From Statistical Atomistic Study to Circuit Simulations

We have performed statistical atomistic simulations with tight-binding approach to investigate the effects of randomly distributed mono-vacancy defects in metallic single-walled carbon nanotube (SWCNT) interconnects. We also extracted defective resistances from the atomistic simulations and performed circuitlevel simulations to compare the performance of interconnects with and without defects. We have found that the defects induce significant fluctuations of SWCNT resistance with a median value showing an Ohmic-like behaviour. Fortunately, the resistance depends only on the diameter of SWCNTs and not on their chirality. Moreover, our circuit simulations show that the defective resistance induces important propagation time delay ratio that should be accounted for when designing CNT interconnects

Investigation of Pt-salt-doped-standalone-multiwall carbon nanotubes for on-chip interconnect applications

In this paper, we investigate, by combining electrical measurements with an atomistic-to-circuit modeling approach, the conductance of doped standalone multiwall carbon nanotubes (CNTs) as a viable candidate for the next generation of back-end-of-line interconnects. Ab initio simulations predict a doping-related shift of the Fermi level, which reduces shell chirality variability and improves electrical resistivity up to 90% by converting semiconducting shells to metallic. Electrical measurements of Pt-salt-doped CNTs provide up to 50% of resistance reduction, which is a milestone result for future CNT interconnect technology. Moreover, we find that defects and contacts introduce additional resistance, which limits the efficiency of doping, and are the primary cause for the mismatch between theoretical predictions and experimental measurements on doped CNTs