New analytical drain current model for the sub-linear region of output characteristics of graphene field-effect transistors in the low carrier density limit.

A new analytical drain current model for a quantitative description of output characteristics of graphene field-effect transistors (GFETs) in the sub-linear region is derived from a previously-developed diffusion-drift theory for GFETs. To simplify calculation, the diffusion-to-drift current ratio is assumed to be constant along the graphene channel, and a reasonable representative value of the ratio is used instead. In addition, the analytical modeling is conducted in the low-carrier-density limit where carrier velocity at the source end is lower than saturation velocity caused by optical phonon emission. This limit facilitates correct explanation of the peculiar behavior of output characteristics that has been attributed to an ambipolar property of graphene. For realistic simulation, extrinsic series resistances are considered, and carrier mobility degraded by vertical electric field is calculated from a modified classical formula. The output characteristics of GFETs in the sub-linear region can properly be reproduced by the new model, and good agreement between simulation results and several sets of experimental data taken from previous literatures is obtained in this region.