We show that a ballistic quantum transport model based on the effective mass approximation can fairly well describe the I-V characteristics of armchair graphene nanoribbon FETs at all bias conditions, including regimes dominated by direct or band-to-band tunneling, provided first-order non-parabolic corrections be included in the simulation. This is achieved by means of an energy (position) dependent effective mass. The analysis is supported by comparisons with an atomistic tight-binding model. (C) 2009 Elsevier Ltd. All rights reserved.