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This paper presents the first graphene radiofrequency (RF) monolithic integrated balun circuit. It is composed of four integrated graphene field effect transistors (GFETs). This innovative active balun concept takes advantage of the GFET ambipolar behavior. It is realized using an advanced silicon carbide (SiC) based bilayer graphene FET technology having RF performances of about 20 GHz. Balun circuit measurement demonstrates its high frequency capability. An upper limit of 6 GHz has been achieved when considering a phase difference lower than 10° and a magnitude of amplitude imbalance less than 0.5 dB. Hence, this circuit topology shows excellent performance with large broadband performance and a functionality of up to one-third of the transit frequency of the transistor.

This paper presents the characterization of a GFET transistor using a source-pull/load-pull test set. The characterization shows that despite the good fT and fMAX, it is hard to achieve power gain using the GFET device within a circuit configuration. This is due to the very high impedance at the gate making impedance matching at the input extremely difficult. S-parameter characterization is performed and the associated small signal model is developed in order to further analyse and extrapolate the source-pull and load-pull measurement results. A good agreement is observed between small signal model simulation results and source-pull/load-pull measurements. Finally, the model is used to evaluate the optimum power gain of the transistor in a circuit configuration under matched conditions.