  We develop a semianalytical model for monolayer graphene field-effect
transistors in the ballistic limit. Two types of devices are considered: in the
first device, the source and drain regions are doped by charge transfer with
Schottky contacts, while, in the second device, the source and drain regions
are doped electrostatically by a back gate. The model captures two important
effects that influence the operation of both devices: (i) the finite density of
states in the source and drain regions, which limits the number of states
available for transport and can be responsible for negative output differential
resistance effects, and (ii) quantum tunneling across the potential steps at
the source-channel and drain-channel interfaces. By comparison with a
self-consistent non-equilibrium Green's function solver, we show that our model
provides very accurate results for both types of devices, in the bias region of
quasi-saturation as well as in that of negative differential resistance.
