  The recent fabrication of graphene nanoribbon (GNR) field-effect transistors
poses a challenge for first-principles modeling of carbon nanoelectronics due
to many thousand atoms present in the device. The state of the art quantum
transport algorithms, based on the nonequilibrium Green function formalism
combined with the density functional theory (NEGF-DFT), were originally
developed to calculate self-consistent electron density in equilibrium and at
finite bias voltage (as a prerequisite to obtain conductance or current-voltage
characteristics, respectively) for small molecules attached to metallic
electrodes where only a few hundred atoms are typically simulated. Here we
introduce combination of two numerically efficient algorithms which make it
possible to extend the NEGF-DFT framework to device simulations involving large
number of atoms. We illustrate fusion of these two algorithms into the
NEGF-DFT-type code by computing charge transfer, charge redistribution and
conductance in zigzag-GNR/variable-width-armchair-GNR/zigzag-GNR two-terminal
device covered with a gate electrode made of graphene layer as well. The total
number of carbon and edge-passivating hydrogen atoms within the simulated
central region of this device is ~7000. Our self-consistent modeling of the
gate voltage effect suggests that rather large gate voltage might be required
to shift the band gap of the proposed AGNR interconnect and switch the
transport from insulating into the regime of a single open conducting channel.
