  Transport in graphene nanoribbons with an energy gap in the spectrum is
considered in the presence of random charged impurity centers. At low carrier
density, we predict and establish that the system exhibits a density
inhomogeneity driven two dimensional metal-insulator transition that is in the
percolation universality class. For very narrow graphene nanoribbons (with
widths smaller than the disorder induced length-scale), we predict that there
should be a dimensional crossover to the 1D percolation universality class with
observable signatures in the transport gap. In addition, there should be a
crossover to the Boltzmann transport regime at high carrier densities. The
measured conductivity exponent and the critical density are consistent with
this percolation transition scenario.
