  The ability to control electronic properties of a material by externally
applied voltage is at the heart of modern electronics. In many cases, it is the
so-called electric field effect that allows one to vary the carrier
concentration in a semiconductor device and, consequently, change an electric
current through it. As the semiconductor industry is nearing the limits of
performance improvements for the current technologies dominated by silicon,
there is a constant search for new, non-traditional materials whose properties
can be controlled by electric field. Most notable examples of such materials
developed recently are organic conductors [1], oxides near a superconducting or
magnetic phase transition [2] and carbon nanotubes [3-5]. Here, we describe
another system of this kind - thin monocrystalline films of graphite - which
exhibits a pronounced electric field effect, such that carriers in the
conductive channel can be turned into either electrons or holes. The films
remain metallic, continuous and of high quality down to a few atomic layers in
thickness. The demonstrated ease of preparing such films of nearly macroscopic
sizes and of their processing by standard microfabrication techniques, combined
with submicron-scale ballistic transport even at room temperature, offer a new
two-dimensional system controllable by electric-field doping and provide a
realistic promise of device applications.
