  We successfully tuned an underdoped ultrathin YBa2Cu3O7-x film into the
overdoped regime by means of electrostatic doping using an ionic liquid as a
dielectric material. This process proved to be reversible. Transport
measurements showed a series of anomalous features compared to chemically doped
bulk samples and a different two-step doping mechanism for electrostatic doping
was revealed. The normal resistance increased with carrier concentration on the
overdoped side and the high temperature (180 K) Hall number peaked at a doping
level of p$\sim$0.15. These anomalous behaviors suggest that there is an
electronic phase transition in the Fermi surface around the optimal doping
level.
