  We analyse doping of graphene grown on SiC in two models which differ by the
source of charge transfered to graphene, namely, from SiC surface and from bulk
donors. For each of the two models, we find the maximum electron density
induced in monolayer and bilayer graphene, which is determined by the
difference between the work function for electrons in pristine graphene and
donor states on/in SiC, and analyse the responsivity of graphene to the density
variation by means of electrostatic gates.
