  In this work, we clarify the physical mechanism for the phenomenon of
negative output differential resistance (NDR) in short-channel graphene FETs
(GFETs) through non-equilibrium Green's function (NEGF) simulations and a
simpler semianalytical ballistic model that captures the essential physics.
This NDR phenomenon is due to a transport mode bottleneck effect induced by the
graphene Dirac point in the different device regions, including the contacts.
NDR is found to occur only when the gate biasing produces an n-p-n or p-n-p
polarity configuration along the channel, for both positive and negative
drain-source voltage sweep. In addition, we also explore the impact on the NDR
effect of contact-induced energy broadening in the source and drain regions and
a finite contact resistance.
