Graphene field-effect transistors for high frequency applications

Licentiatavhandling, 2019

Rapid development of wireless and internet communications requires development of new generation high frequency electronics based on new device concepts and new materials. The very high intrinsic velocity of charge carriers in graphene makes it promising new channel material for high frequency electronics.

In this thesis, the graphene field-effect transistors (GFETs) are fabricated using chemical vapor deposition (CVD) graphene and investigated for high frequency electronics applications. The characterization and simulation of high frequency performance of the state-of-the-art GFETs devices are given. A modified fabrication process is used. This allows for preserving intrinsic graphene properties in the GFET channel and, simultaneously, achieving extremely low graphene/metal contact resistance. As a result, GFETs with state-of-the-art high frequency performance were fabricated and used in further analysis for development of GFETs with continuously improved performance.

In particular, the dependencies between the material quality and the high-field high-frequency performance of GFETs fabricated on Si chip have been studied. It was shown, that the low-field carrier mobility can be selected as the material quality parameter. The high-frequency performance of GFETs is characterized by fT and fmax. The surface distribution of the graphene/dielectric material quality across the chip has been exploited as a tool to study the dependencies of GFET high-frequency performance on the material quality. The fT and fmax increase in the range of 20-40 GHz with low-field mobility in the range of 600-2000 cm2/V s. The dependencies are analyzed by combining the models of the drain resistance, carrier velocity, saturation velocity and small-signal equivalent circuit. Additionally, this allows for clarifying the effects of the equivalent-circuit parameters, such as contact resistance (Rc), transconductance (gm) and differential drain conductance (gds), on the fT and fmax. The observed variations of fT and fmax are mainly governed by corresponding variations of gm and gds. Analysis allows for identifying a most promising approach for improving the GFET high-frequency performance, which is selection of adjacent dielectric materials with optical phonon energy higher than that of SiO2, resulting in higher saturation velocity and, hence, higher fT and fmax.