Graphene field-effect transistors and devices for advanced high-frequency applications

Doctoral thesis, 2019

New device technologies and materials are continuously investigated, in order to increase the bandwidth of high-speed electronics, thereby extending data rate and range of applications. The 2D-material graphene, with its intrinsically extremely high charge carrier velocity, is considered as a promising new channel material for advanced high frequency field-effect transistors. However, most fabrication processes introduce impurities and defects at the interface between graphene and adjacent materials, which degrade the device performance. In addition, at high drain fields, required for high transistor gain, the close proximity of the adjacent materials limits the saturation velocity, and there is a significant increase in the channel temperature caused by self-heating.

In this thesis, the influence of impurities and defects on charge transport, the limitations of the saturation velocity, and the effect of velocity saturation and self-heating on the transit frequency (fT) and the maximum frequency of oscillation (fmax) of graphene field effect transistor (GFETs) are analysed.

In addition, GFETs with state-of-the-art extrinsic fT =34 GHz and fmax =37 GHz, and an integrated 200-GHz GFET based receiver are presented. Also, through the development of a fabrication process of GFETs with a buried gate configuration, this work contributed to the direct nanoscopic observation of plasma waves in the GFET channel during terahertz illumination.

The study was conducted by (i) setting up a model describing the influence of impurities and defects on capacitance and transfer characteristics at low electric fields, (ii) by developing a method for studying the limiting mechanisms of the charge carrier velocity in the graphene channel at high electric fields and answering the question whether velocity saturation improves fmax, (iii) by developing a method to study the channel temperature and its effect on fT and fmax.

It was found that scattering by remote optical phonons limits the saturation velocity and charge carriers emitted from interface states at high fields are preventing the current to saturate and, hence, limiting fT and fmax. Additionally, the study shows that the channel temperature in GFETs can increase significantly causing degradation of the high frequency performance due to the decrease of charge carrier mobility and velocity.

In summary, this work shows that it is necessary to develop a GFET design and fabrication process providing clean and defect-free interfaces, to minimise parasitic effects, and to use materials with higher optical phonon energies and higher thermal conductivities than those used today. This will allow for realisation of GFETs with extrinsic fT and fmax in the sub-terahertz range.