▎ 摘　　要

We report on a theoretical study of the electronic-structure and transport properties of single and bilayer graphene with vacancy defects, as well as N-doped graphene. The theory is based on first-principles calculations as well as model investigations in terms of real-space Green's functions. We show that increasing the defect concentration increases drastically the conductivity in the limit of zero applied gate voltage, by establishing carriers in originally carrier-free graphene, a fact which is in agreement with recent observations. We calculate the amount of defects needed for a transition from a nonconducting to a conducting regime (i.e., a metal-insulator transition) and establish the threshold of the defect concentration where the increase in impurity scattering dominates over the increase in carrier-induced conductivity.