367 research outputs found
Analytical Modelling Of Breakdown Effect In Graphene Nanoribbon Field Effect Transistor
Since 2004, graphene as transistor channel has drawn huge amount of attention due to its extraordinary scalability and high carrier mobility. In order to open required bandgap, its nanoribbon form is used in transistors. Breakdown effect modelling of the graphene nanoribbon field effect transistors (GNRFET) is needed to investigate the limits on operating voltage of the transistor. However, until now there is no study in analytical approach and modelling of the breakdown voltage (BV) effects on the graphene-based transistors. Thus, in this project, semi-analytical models for lateral electric field, length of velocity saturation region (LVSR), ionization coefficient (α), and breakdown voltage (BV) of single- and double-gate graphene nanoribbon field effect transistors (GNRFET) are proposed. As the methodology, the application of Gauss’s law at drain and source regions is employed in order to derive surface potential and lateral electric field equations. Then, LVSR is calculated as a solution of surface potential at saturation condition. The ionization coefficient is modelled and calculated by deriving equations for probability of collisions in ballistic and drift modes based on lucky drift theory of ionization. Then the threshold energy of ionization is computed using simulation and an empirical equation is derived semi-analytically. Finally avalanche breakdown condition is employed to calculate the lateral BV. As a result of this research, simple analytical and semi-analytical models are proposed for the LVSR,α, and BV, which could be used in design and optimization of semiconductor devices and sensors
Effect of Device Variables on Surface Potential and Threshold Voltage in DG-GNRFET
In this paper we present four simple analytical threshold voltage model for short- channel and length of saturation velocity region (LVSR) effect that takes into account the built – in potential of the source and drain channel junction, the surface potential and the surface electric field effect on double – gate graphene nanoribbon transistors. Four established models for surface potential, lateral electric field, LVSR and threshold voltage are presented. These models are based on the easy analytical solution of the two dimensional potential distribution in the graphene and Poisson equation which can be used to obtain surface potential, lateral electric field, LVSR and threshold voltage. These models give a closed form solution of the surface potential and electrical field distribution as a function of structural parameters and drain bias. Most of analytical outcomes are shown to correlate with outcomes acquired by Matlab simulation and the end model applicability to the published silicon base devices is demonstrated
Monolithically Patterned Wide-Narrow-Wide All-Graphene Devices
We investigate theoretically the performance advantages of all-graphene
nanoribbon field-effect transistors (GNRFETs) whose channel and source/drain
(contact) regions are patterned monolithically from a two-dimensional single
sheet of graphene. In our simulated devices, the source/drain and interconnect
regions are composed of wide graphene nanoribbon (GNR) sections that are
semimetallic, while the channel regions consist of narrow GNR sections that
open semiconducting bandgaps. Our simulation employs a fully atomistic model of
the device, contact and interfacial regions using tight-binding theory. The
electronic structures are coupled with a self-consistent three-dimensional
Poisson's equation to capture the nontrivial contact electrostatics, along with
a quantum kinetic formulation of transport based on non-equilibrium Green's
functions (NEGF). Although we only consider a specific device geometry, our
results establish several general performance advantages of such monolithic
devices (besides those related to fabrication and patterning), namely the
improved electrostatics, suppressed short-channel effects, and Ohmic contacts
at the narrow-to-wide interfaces.Comment: 9 pages, 11 figures, 2 table
Introduction to Graphene Electronics -- A New Era of Digital Transistors and Devices
The speed of silicon-based transistors has reached an impasse in the recent
decade, primarily due to scaling techniques and the short-channel effect.
Conversely, graphene (a revolutionary new material possessing an atomic
thickness) has been shown to exhibit a promising value for electrical
conductivity. Graphene would thus appear to alleviate some of the drawbacks
associated with silicon-based transistors. It is for this reason why such a
material is considered one of the most prominent candidates to replace silicon
within nano-scale transistors. The major crux here, is that graphene is
intrinsically gapless, and yet, transistors require a band-gap pertaining to a
well-defined ON/OFF logical state. Therefore, exactly as to how one would
create this band-gap in graphene allotropes is an intensive area of growing
research. Existing methods include nano-ribbons, bilayer and multi-layer
structures, carbon nanotubes, as well as the usage of the graphene substrates.
Graphene transistors can generally be classified according to two working
principles. The first is that a single graphene layer, nanoribbon or carbon
nanotube can act as a transistor channel, with current being transported along
the horizontal axis. The second mechanism is regarded as tunneling, whether
this be band-to-band on a single graphene layer, or vertically between adjacent
graphene layers. The high-frequency graphene amplifier is another talking point
in recent research, since it does not require a clear ON/OFF state, as with
logical electronics. This paper reviews both the physical properties and
manufacturing methodologies of graphene, as well as graphene-based electronic
devices, transistors, and high-frequency amplifiers from past to present
studies. Finally, we provide possible perspectives with regards to future
developments.Comment: This is an updated version of our review article, due to be published
in Contemporary Physics (Sept 2013). Included are updated references, along
with a few minor corrections. (45 pages, 19 figures
Physical Modeling of Graphene Nanoribbon Field Effect Transistor Using Non-Equilibrium Green Function Approach for Integrated Circuit Design
The driving engine for the exponential growth of digital information processing systems is scaling down the transistor dimensions. For decades, this has enhanced the device performance and density. However, the International Technology Roadmap for Semiconductors (ITRS) states the end of Moore’s law in the next decade due to the scaling challenges of silicon-based CMOS electronics, e.g. extremely high power density. The forward-looking solutions are the utilization of emerging materials and devices for integrated circuits. The Ph.D. dissertation focuses on graphene, one atomic layer of carbon sheet, experimentally discovered in 2004. Since fabrication technology of emerging materials is still in early stages, transistor modeling has been playing an important role for evaluating futuristic graphene-based devices and circuits. The GNR FET has been simulated by solving a numerical quantum transport model based on self-consistent solution of the 3D Poisson equation and 1D Schrödinger equations within the non-equilibrium Green’s function (NEGF) formalism. The quantum transport model fully treats short channel-length electrostatic effects and the quantum tunneling effects, leading to the technology exploration of graphene nanoribbon field effect transistors (GNRFETs) for the future. A comprehensive study of static metrics and switching attributes of GNRFET has been presented including the performance dependence of device characteristics to the GNR width and the scaling of its channel length down to 2.5 nanometer. It has been found that increasing the GNR width deteriorate the off-state performance of the GNRFET, such that, narrower armchair GNRs improved the device robustness to short channel effects, leading to better off-state performance considering smaller off-current, larger ION/IOFF ratio, smaller subthreshold swing and smaller drain-induced barrier-lowering. The wider armchair GNRs allow the scaling of channel length and supply voltage resulting in better on-state performance such as higher drive current, smaller intrinsic gate-delay time and smaller power-delay product. In addition, the width-dependent characteristics of GNR FETs is investigated for two GNR semiconducting families (3p,0) and (3p+1,0). It has been found that the GNRs(3p+1,0) demonstrates superior off-state performance, while, on the other hand, GNRs(3p,0) shows superior on-state performance. Thus, GNRs(3p+1,0) are promising for low-power design, while GNRs(3p,0) indicate a more preferable attribute for high frequency applications
Analytical model of 1D Carbon-based Schottky-Barrier Transistors
Nanotransistors typically operate in far-from-equilibrium (FFE) conditions,
that cannot be described neither by drift-diffusion, nor by purely ballistic
models. In carbonbased nanotransistors, source and drain contacts are often
characterized by the formation of Schottky Barriers (SBs), with strong
influence on transport. Here we present a model for onedimensional field-effect
transistors (FETs), taking into account on equal footing both SB contacts and
FFE transport regime. Intermediate transport is introduced within the Buttiker
probe approach to dissipative transport, in which a non-ballistic transistor is
seen as a suitable series of individually ballistic channels. Our model permits
the study of the interplay of SBs and ambipolar FFE transport, and in
particular of the transition between SB-limited and dissipation-limited
transport
Model and performance evaluation of field-effect transistors based on epitaxial graphene on SiC
In view of the appreciable semiconducting gap of 0.26 eV observed in recent
experiments, epitaxial graphene on a SiC substrate seems a promising channel
material for FETs. Indeed, it is two-dimensional - and therefore does not
require prohibitive lithography - and exhibits a wider gap than other
alternative options, such as bilayer graphene. Here we propose a model and
assess the achievable performance of a nanoscale FET based on epitaxial
graphene on SiC, conducting an exploration of the design parameter space. We
show that the current can be modulated by 4 orders of magnitude; for digital
applications an Ion /Ioff ratio of 50 and a subthreshold slope of 145 mV/decade
can be obtained with a supply voltage of 0.25 V. This represents a significant
progress towards solid-state integration of graphene electronics, but not yet
sufficient for digital applications
Application of Graphene within Optoelectronic Devices and Transistors
Scientists are always yearning for new and exciting ways to unlock graphene's
true potential. However, recent reports suggest this two-dimensional material
may harbor some unique properties, making it a viable candidate for use in
optoelectronic and semiconducting devices. Whereas on one hand, graphene is
highly transparent due to its atomic thickness, the material does exhibit a
strong interaction with photons. This has clear advantages over existing
materials used in photonic devices such as Indium-based compounds. Moreover,
the material can be used to 'trap' light and alter the incident wavelength,
forming the basis of the plasmonic devices. We also highlight upon graphene's
nonlinear optical response to an applied electric field, and the phenomenon of
saturable absorption. Within the context of logical devices, graphene has no
discernible band-gap. Therefore, generating one will be of utmost importance.
Amongst many others, some existing methods to open this band-gap include
chemical doping, deformation of the honeycomb structure, or the use of carbon
nanotubes (CNTs). We shall also discuss various designs of transistors,
including those which incorporate CNTs, and others which exploit the idea of
quantum tunneling. A key advantage of the CNT transistor is that ballistic
transport occurs throughout the CNT channel, with short channel effects being
minimized. We shall also discuss recent developments of the graphene tunneling
transistor, with emphasis being placed upon its operational mechanism. Finally,
we provide perspective for incorporating graphene within high frequency
devices, which do not require a pre-defined band-gap.Comment: Due to be published in "Current Topics in Applied Spectroscopy and
the Science of Nanomaterials" - Springer (Fall 2014). (17 pages, 19 figures
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