This thesis is largely based on my papers on numerical and analytical mod-
eling of graphene-based devices, to consider possible approaches to engineer a
bandgap in graphene and to evaluate the perspectives of different technological
options toward graphene nanoelectronics.
The thesis is organized as follows:
In Chapter 1 some basic concepts are presented. In the first section the
basic operation of a field effect transistor (FET) and of a tunnel field effect
transistor (TFET) are described. In the second section the graphene lattice
geometry, the primitive lattice vectors of both the real and reciprocal lattice
space are presented. The third section is dedicated to the description of the
Tight Binding (TB) method applied to graphene.
In Chapter 2 contains an analytical model for a bilayer-graphene field-
effect transistor. This kind of approach is suitable for exploring the design
parameter space for a device structure with promising performance in terms
of transistor operation. The model, based on the effective mass approximation
and ballistic transport assumptions, takes into account bilayer-graphene tun-
able gap and self-polarization, and includes all band-to-band tunneling current
components, which are shown to represent the major limitation to transistor
operation, because the achievable energy gap is not sufficient to obtain a large
Ion /Ioff ratio.
Chapter 3 is presented an analytical model of a nanoscale FET based on
epitaxial graphene on SiC, and assess the achievable performance in the case
of fully ballistic transport.
Chapter 4 presents an accurate model and a exploration of the design
parameter space for a fully ballistic graphene-on-SiC Tunnel Field-Effect Tran-
sistor (TFET). The DC and high frequency figures of merit are shown. The
steep subthreshold behavior can enable Ion /Ioff ratios exceeding 104 even with
a low supply voltage of 0.15 V, for devices with gate length down to 30 nm. In-
trinsic transistor delays smaller than 1 ps are obtained. These factors make the
device an interesting candidate for low-power nanoelectronics beyond CMOS.
In Chapter 5 shows an accurate estimation of intrinsic mobility in the low-
field limit, analyzing the effects of temperature and of charge concentration
on phonon-limited mobility. Full-band calculations are performed, using the
tight binding approximation, both for electrons and longitudinal and transverse
phonons (acoustic and optical). For the electron-phonon scattering we use the
Su-Schrieffer-Hegger (SSH) model and according to that approach, longitudinal
and transverse modes have different angle dependencies for the electron-phonon
coupling. This model allows us to consider the effect on mobility of all phonons
modes. Finally, the low-field mobility has been calculated by the quantum
mechanical Kubo-Greenwood formula adapted for bidimensional systems.