  In graphene devices with a varying degree of disorders as characterized by
their carrier mobility and minimum conductivity, we have studied the
thermoelectric power along with the electrical conductivity over a wide range
of temperatures. We have found that the Mott relation fails in the vicinity of
the Dirac point in high-mobility graphene. By properly taking account of the
high temperature effects, we have obtained good agreement between the Boltzmann
transport theory and our experimental data. In low-mobility graphene where the
charged impurities induce relatively high residual carrier density, the Mott
relation holds at all gate voltages.
