Microwave characterisation of electrodes and field effect transistors based on graphene

Licentiate thesis, 2014

The isolation of the two-dimensional material graphene, a single hexagonal sheet of carbon atoms, is believed to trigger a revolution in electronics. Theory predicts unprecedented carrier velocities in ideal graphene, from which ultrahigh speed graphene field effect transistors (GFETs) are envisioned. In this thesis, the prospects of GFETs for microwave receivers are investigated with the emphasis on low noise amplifiers (LNAs). A microwave amplifier at 1 GHz with 10 dB small-signal gain and 6.4 dB noise figure was realised using a mechanically exfoliated graphene flake on a SiO2 substrate. Comparable GFET performance was demonstrated with large-area graphene grown by chemical vapour deposition (CVD) on copper and transferred to SiO2. From a device level noise characterisation, the CVD GFET minimum noise figure (Fmin) in the frequency range 2-8 GHz was measured to be 2.5-5 dB and estimated by de-embedding parasitics to be 1-4 dB for the intrinsic device. However, the GFET noise is sensitive to impedance mismatch as the noise resistance is high. In addition, subharmonic resistive GFET mixers utilising the symmetry of electron and hole conduction in graphene were assessed. Conversion loss (CL) and noise figure were approximately equal and ≥20 dB and the input third order intercept point (IIP3) was ≤3.9 dBm at a local oscillator power of 2 dBm, less linear then fundamental resistive mixers. Finally, the properties of graphene and metal-graphene contacts were investigated by parameter extraction based on measurements at both DC and microwave frequencies. Using a palladium based contact, a contact resistance as low as <100 Om was reached. An associated contact capacitance was extracted, for which a geometrical model was proposed. The implications of this capacitance on device performance is presumably negligible up to at least several hundred gigahertz. It is inferred, however, that the sheet resistance of graphene in this work must be reduced two orders of magnitude to improve performance of acoustic resonators when using graphene as an electrode.