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Although a graphene is a zero-gap semiconductor, band-gap energy values up to several hundred millielectronvolts have been introduced by utilizing quantum-mechanical confinement in nanoribbon structures or symmetry breaking between two carbon layers in bilayer graphenes (BLGs). However, the opening of a band gap causes a significant reduction in carrier velocity due to the modulation of band structures in their low-energy spectra. In this paper, we study intrinsic effects of the band-gap opening on ballistic electron transport in graphene nanoribbons (GNRs) and BLGs based on a computational approach, and discuss the ultimate device performances of FETs with those semiconducting graphene channels. We have shown that an increase in the external electric field in BLG-FETs to obtain a larger band-gap energy degrades substantially its electrical characteristics because of deacceleration of electrons due to a Mexican hat structure; therefore, GNR-FETs outperform in principle BLG-FETs.