Fabrication, Characterisation and Modelling of Subharmonic Graphene FET Mixers

Licentiate thesis, 2011

Graphene has exceptional carrier transport properties which makes it a promising material for future nanoelectronics. The high carrier mobility along with the ability to switch between n- and p-channel in a graphene field effect transistor (G-FET) truly distinguishes it from other types of FET technologies and enables completely new high frequency devices. In this thesis, a novel subharmonic resistive G-FET mixer is presented. The mixer operation is based on the G-FET’s symmetrical transfer characteristic. Due to this property, 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. This makes the mixer circuit more compact. The mixer conversion loss (CL) is measured with fRF=2 GHz, fLO=1.01 GHz and fIF=20 MHz in a 50 Ω impedance system, and for a G-FET with an on-off ratio of 3, a CL of 24 dB is obtained. In addition, the mixer performance is analysed based on the GFET parameters, the LO power and the embedding impedances. It is predicted that by having a G-FET with an on-off ratio of 10 and selecting proper embedding impedances, a CL of 17 dB is attainable. Also, by further improvement of the G-FET on-off ratio, the CL is optimised to about 14.2 dB. Moreover, a process technology for 1 μm gate-length G-FETs based on exfoliated graphene has been developed. A contact resistance as low as 500-600 Ω.μm is obtained, which is close to the lowest reported value. In addition, different gate dielectric materials have been investigated. A plasma enhanced chemical vapour deposition (PECVD) process for deposition of a silicon nitride film as a gate dielectric is developed. The process maintains the carrier mobility of the graphene film largely intact after deposition. Also, to form Al2O3 gate dielectric films, a protective layer of naturally oxidised Al is used prior to e-gun evaporation of Al2O3. This layer prevents further degradation of the carrier mobility. Finally, a novel closed-form large-signal model for G-FETs is developed. The model is semiempirical and can be utilised in standard Electronic Design Automation (EDA) tools for designing and analysing G-FET circuits. The model is implemented in Agilent’s Advanced Design System (ADS) software and experimentally verified for a G-FET under both DC and RF operation. The DC results agree with the model. The RF verification includes S-parameters and power spectrum measurements. The S-parameters measurements essentially coincide with the model and the power spectrum analysis shows good agreement up to the 4th order. Moreover, the model is used to simulate the G-FET mixer CL and the results follow the measurements.