  A computationally efficient mode space simulation method for atomistic
simulation of a graphene nanoribbon field-effect transistor in the ballistic
limits is developed. The proposed simulation scheme, which solves the
nonequilibrium Green's function coupled with a three dimensional Poisson
equation, is based on the atomistic Hamiltonian in a decoupled mode space. The
mode space approach, which only treats a few modes (subbands), significantly
reduces the simulation time. Additionally, the edge bond relaxation and the
third nearest neighbor effects are also included in the quantum transport
solver. Simulation examples show that, mode space approach can significantly
decrease the simulation cost by about an order of magnitude, yet the results
are still accurate. This article also demonstrates that the effects of edge
bond relaxation and third nearest neighbor significantly influence the
transistor's performance and are necessary to be included in the modeling.
