▎ 摘　　要

We have carried out Hall measurement on back-gated graphene field effect transistors (PET) with and without a top dielectric medium. The gate efficiency increases by up to 2 orders of magnitude in the presence of a high kappa top dielectric medium, but the mobility does not change significantly. Our measurement further shows that the back-gate capacitance is enhanced dramatically by the top dielectric medium, and the enhancement increases with the size of the top dielectric medium. Our work strongly suggests that the previously reported top dielectric medium-induced charge transport properties of graphene FETs are possibly due to the increase of gate capacitance, rather than enhancement of carrier mobility.