# A Broadband Active Microwave Monolithically Integrated Circuit Balun in Graphene Technology  
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1 University of Lille--IEMN CNRS50, Avenue Poincare, CS 60069, 59652 Villeneuve d'Ascq, France; [PERSON]DRESS] (D.F.); [EMAIL_ADDRESS] (W.W.); [PERSON]S.B.S.); [EMAIL_ADDRESS] (D.Z.); [EMAIL_ADDRESS] (E.P.); [EMAIL_ADDRESS] (H.H.) 1  
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1 University of Lille--IEMN CNRS50, Avenue Poincare, CS 60069, 59652 Villeneuve d'Ascq, France; [PERSON]DRESS] (D.F.); [EMAIL_ADDRESS] (W.W.); [PERSON]S.B.S.); [EMAIL_ADDRESS] (D.Z.); [EMAIL_ADDRESS] (E.P.); [EMAIL_ADDRESS] (H.H.) 1  
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1 University of Lille--IEMN CNRS50, Avenue Poincare, CS 60069, 59652 Villeneuve d'Ascq, France; [PERSON]DRESS] (D.F.); [EMAIL_ADDRESS] (W.W.); [PERSON]S.B.S.); [EMAIL_ADDRESS] (D.Z.); [EMAIL_ADDRESS] (E.P.); [EMAIL_ADDRESS] (H.H.) 1  
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1 University of Lille--IEMN CNRS50, Avenue Poincare, CS 60069, 59652 Villeneuve d'Ascq, France; [PERSON]DRESS] (D.F.); [EMAIL_ADDRESS] (W.W.); [PERSON]S.B.S.); [EMAIL_ADDRESS] (D.Z.); [EMAIL_ADDRESS] (E.P.); [EMAIL_ADDRESS] (H.H.) 1  
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1 University of Lille--IEMN CNRS50, Avenue Poincare, CS 60069, 59652 Villeneuve d'Ascq, France; [PERSON]DRESS] (D.F.); [EMAIL_ADDRESS] (W.W.); [PERSON]S.B.S.); [EMAIL_ADDRESS] (D.Z.); [EMAIL_ADDRESS] (E.P.); [EMAIL_ADDRESS] (H.H.) 1  
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1 University of Lille--IEMN CNRS50, Avenue Poincare, CS 60069, 59652 Villeneuve d'Ascq, France; [PERSON]DRESS] (D.F.); [EMAIL_ADDRESS] (W.W.); [PERSON]S.B.S.); [EMAIL_ADDRESS] (D.Z.); [EMAIL_ADDRESS] (E.P.); [EMAIL_ADDRESS] (H.H.) 1  
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(balanced) and vice versa. Thus, the balun is a device that consists of an unbalanced single-ended input port and two balanced output ports. The main figures of merit (FOMs) of the balun are the phase error and the amplitude difference between the output balanced ports. Finally, specifically for the active balun, the gain is also an important FOM. In [15], we proposed two balun architectures based on graphene FET specificities. The basic working principle of these circuits was demonstrated but this demonstration was limited to very low frequencies (<10 MHz). The circuits were tested by adding external elements such as lumped resistors, basic coaxial cables, and DC probes; thus preventing the evaluation of the high frequency figures of merits of the circuits.  
In this paper and for the first time, we have designed and fabricated a RF monolithic integrated balun circuit using a SiC bilayer graphene FET technology. Our monolithically integrated balun consists of four transistors, two of them split the single ended signal in two differential signals, the other two act as active loads, their role is to ensure optimal biasing of the first two transistors.  
The SiC substrate is well suited for high-frequency applications: it is highly insulating ensuring low losses at high-frequency and has high thermal conductivity, that is advantageous for thermal management. Therefore, graphene on SiC does not require a transfer step that may contaminate the graphene and reduces the device performances. Here we choose graphene bilayer grown by CVD on silicon carbide since this technique produces wafer scale graphene films that cover the whole substrate, and, in our experience, and are more homogenous than the monolayer graphene. Moreover, comparison of monolayer and bilayer GFETs shows that bilayer have better DC and RF characteristics [16]. The first part of the paper describes the technology used and presents the transistor performances. Second, the balun topology and layout are presented together with the measurement results. Finally, the FOMs of the GFET balun are benchmarked and compared to the state of the art conventional baluns designed in silicon, GaAs, and GaN technologies.