▎ 摘　　要

We theoretically calculate and compare the single particle relaxation time (tau(s)) defining the quantum level broadening and the transport scattering time (tau(t)) defining the Drude conductivity in two-dimensional (2D) graphene layers in the presence of screened charged impurity scattering and short-range defect scattering. We find that the ratio tau(t)/tau(s) strongly increases with increasing k(F)z(i) and kappa, where k(F), z(i), and kappa are, respectively, the Fermi wave vector, the separation of the substrate charged impurities from the graphene layer, and the background lattice dielectric constant. A critical quantitative comparison of the tau(t)/tau(s) results for graphene with those for the corresponding modulation-doped semiconductor structures is provided, showing significant differences between these two 2D carrier systems.