▎ 摘　　要

Using the semiclassical Boltzmann theory, we calculate the conductivity as a function of the carrier density. We include the scattering from charged impurities but conclude that the estimated impurity density is too low in order to explain the experimentally observed mobilities. We thus propose an additional scattering mechanism involving midgap states, which leads to a similar k dependence of the relaxation time as charged impurities. The proposed scattering mechanism can account for the experimental findings such as the sublinear behavior of the conductivity versus gate voltage and the increase of the minimal conductivity for clean samples. We also discuss temperature dependent scattering due to acoustic phonons.