  The mechanism of field-effect doping in the "123" high critical temperature
superconductors (HTS) has been investigated by x-ray absorption spectroscopy in
the presence of an electric field. We demonstrate that holes are created at the
CuO chains of the charge reservoir and that field-effect doping of the CuO2
planes occurs by charge transfer, from the chains to the planes, of a fraction
of the overall induced holes. The electronic properties of the charge reservoir
and of the dielectric/HTS interface determine the electric field doping of the
CuO2 planes
