  We show that silicon nitride can provide uniform coverage of graphene in
field-effect transistors while preserving the channel mobility. This insulator
allowed us to study the maximum channel resistance at the Dirac (neutrality)
point as a function of the strength of a perpendicular electric field in
top-gated devices with different numbers of graphene layers. Using a simple
model to account for surface potential variations (electron-hole puddles) near
the Dirac point we estimate the field-induced band-gap or band-overlap in the
different layers.
