Pseudosaturation and Negative Differential Conductance in Graphene Field-Effect Transistors

We study theoretically the different transport behaviors and the electrical characteristics of a top-gated graphene field-effect transistor where boron nitride is used as the substrate and gate insulator material, which makes the ballistic transport realistic. Our simulation model is based on the Green´s function approach to solving a tight-binding Hamiltonian for graphene, self-consistently coupled with Poisson´s equation. The analysis emphasizes the effects of the chiral character of carriers in graphene in the different transport regimes including the Klein and band-to-band tunneling processes. In particular, the Klein tunneling is shown to have an important role on the onset of the current saturation which is analyzed in detail as a function of the device parameters. Additionally, we predict the possible emergence of negative differential conductance and investigate its dependence on the BN-induced bandgap, the temperature, and the gate insulator thickness. Short-channel effects are evaluated from the analysis of transfer characteristics as a function of gate length and gate insulator thickness. They manifest through the shift of the Dirac point and the appearance of current oscillations at short gate length.