  An electrostatic field, which is applied to a gated high-temperature
superconducting (HTSC) film, is believed to affect the film similar to charge
doping. Analyzing the pairing in terms of a t-J model, we show that a coupling
to electric dipoles and phonons at the interface of film and dielectric gate
localizes the injected charge and leads to a superconductor-insulator
transition. This results in a dramatic modification of the doping dependent
phase diagram close to and above the optimal doping which is expected to shed
light on recent electric field-effect experiments with HTSC cuprates.
