  The effect of Coulomb scattering on graphene conductivity in field effect
transistor structures is discussed. Inter-particle scattering
(electron-electron, hole-hole, and electron-hole) and scattering on charged
defects are taken into account in a wide range of gate voltages. It is shown
that an intrinsic conductivity of graphene (purely ambipolar system where both
electron and hole densities exactly coincide) is defined by strong
electron-hole scattering. It has a universal value independent of temperature.
We give an explicit derivation based on scaling theory. When there is even a
small discrepancy in electron and hole densities caused by applied gate voltage
the conductivity is determined by both strong electron-hole scattering and weak
external scattering: on defects or phonons. We suggest that a density of
charged defects (occupancy of defects) depends on Fermi energy to explain a
sub-linear dependence of conductivity on a fairly high gate voltage observed in
experiments. We also eliminate contradictions between experimental data
obtained in deposited and suspended graphene structures regarding graphene
conductivity.
