Modeling of graphene electron devices

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.