Graphene FETs in Microwave Applications

Doctoral thesis, 2012

Graphene is a one-atom-thick sheet of carbon with numerous impressive properties. It is a promising material for future high-speed nanoelectronics due to its intrinsic superior carrier mobility and very high saturation velocity. These exceptional carrier transport properties suggest that graphene field effect transistors (G-FETs) can potentially outperform other FET technologies. This doctoral thesis presents the realisation of G-FET circuits at microwave frequencies (0.3-30 GHz) with emphasis on a novel subharmonic resistive mixer. The work covers device manufacturing, modelling, circuit design, and characterisation. The developed mixer exploits the G-FETs ability to conduct current in both n-channel and p-channel modes for subharmonic (×2) mixing. Consequently, the mixer operates with a single transistor and unlike the conventional subharmonic resistive FET mixers, it does not need any balun at the local oscillator (LO) port. In addition, the mixer has potential to operate unbiased. These aspects enable us to utilise G-FET subharmonic mixers in compact high frequency heterodyne detectors. A 30 GHz mixer is realised in microstrip technology on a 250 μm high resistivity silicon substrate. A conversion loss (CL) of 19 ± 1 dB in the frequency range of 24 to 31 GHz is obtained with an LO to RF isolation better than 20 dB. For designing and analysing G-FET circuits a closed-form semiempirical largesignal model is proposed and experimentally verified under both DC and RF operation. The model is implemented in a standard Electronic Design Automation (EDA) software for device-circuit co-design. By using the model, the first G-FET microwave amplifier is realised. The amplifier exhibits a small-signal power gain of 10 dB at 1 GHz.