  A fitting model is developed for accounting the asymmetric ambipolarities in
the I-V characteristics of graphene field-effect transistors (G-FETs) with
doped channels, originating from the thermionic emission and interband
tunneling at the junctions between the gated and access regions. Using the
model, the gate-voltage-dependent intrinsic mobility as well as other intrinsic
and extrinsic device parameters can be extracted. We apply it to a top-gated
G-FET with a graphene channel grown on a SiC substrate and with SiN gate
dielectric that we reported previously, and we demonstrate that it can
excellently fit its asymmetric I-V characteristic.
