Graphene field effect transistors on SiC with T-Shaped gate: Homogeneity and RF performance

In this letter we present our work on top-gated graphene field-effect transistors GFET. The Transistors have been processed on epitaxial graphene synthesized on the Si-face of a SiC substrate, using a high-k dielectric Al₂O₃ as a gate oxide. In order to reduce the gate access resistance, we chosed a T-gate geometry. The Hall mobility is on the order 780 cm²/vs at room temperature, for an intrinsic carrier concentration of the order of 8.7 10⁺¹² /cm². We used Nickel as a contact material, The contact resistance was measured by the Transmission Line Method (TLM) where the value of ρ_c is 2 × 10^-6 Ω.cm². We characterized our devices from DC up to 67 GHz. Cut-off frequency f_t as high as 90 GHz has been obtained for the 170 nm gate length transistors. The maximum value of f_t is 43 GHz and the maximum value of f_max is 23 GHz. Finally, we studied the homogeneity of the GFETS realized across the whole sample surface (1/4 of inch).