Graphene transistors for ambipolar mixing at microwave frequencies

This work presents a detailed study of the graphene RF mixer in the ambipolar configuration, using quasi-free-standing epitaxial graphene on SiC. Record high conversion gain is achieved through use of optimized growth and synthesis techniques, metal contact formation, and dielectric materials integration. Hydrogen intercalation is utilized to isolate the graphene from the underlying SiC substrate and improve transport properties. Low contact resistances at the metal-graphene interface are realized using an oxygen plasma pre-treatment, while dielectric seeding is achieved using a direct deposited layer of HfO₂ before ALD film growth. Output characteristics of the graphene transistor are analyzed and the effects on mixer performance are explained. A graphene RF transistor is designed with gate length 750nm, width 20μm, and equivalent oxide thickness ∼2.5nm in order to achieve record high conversion gain of -14 and -16dB at LO power 0dBm at 4.2 and 10GHz, respectively, 100× higher than previously reported ambipolar mixing.