  Boltzmann diffusive transport including relaxation time and mobility in
graphene limited by ionized impurity scattering is investigated. The relaxation
time is evaluated with two different methods, first one directly use Boltzmann
transport equation via scattering matrix, second one is based on scattering
cross-section. Two methods yield the same relaxation time results for graphene.
Assume linear Thomas-Fermi screening and a reasonable electron carrier density,
relaxation time and mobility can be calculated and plotted, which demonstrates
graphene is a very promising high mobility material for ultra fast electronic
device applications.
