  We have used the electric--field effect to modulate the resistivity of the
surface of underdoped Sr$_{0.88}$La$_{0.12}$CuO$_{2+x}$ thin films, allowing
opposite modifications of the electron and hole density in the CuO$_2$ planes,
an original situation with respect to conventional chemical doping in
electron-doped materials. When the Hall effect indicates a large contribution
of a hole band, the electric--field effect on the normal state resistivity is
however dominated by the electrons, and the superconducting transition
temperature increases when carriers are transfered from holes to electrons.
