We present a quantum transport simulation of graphene field-effect transistors based on the self consistent solution of 2D-Poisson solver and Dirac equation within the non-equilibrium Green's function formalism. The device operation of double gate 2D-graphene field effect transistors is investigated. The study emphasizes the band-to-band and Klein tunneling processes of massless carriers and the resulting features of the electrostatic modulation of I-V characteristics. A transconductance as high as a few hundreds of μS/μm is observed, despite low on/off current ratios. The model is also extended to massive carriers, which allows us to show the on/off current ratio enhancement due to finite bandgap. The obtained results suggest the feasibility of 2D-graphene devices for analogue applications.