  Electron-beam deposition of an insulating granular aluminium or of an
off-stoichiometric amorphous alumina layer on a $SrTiO_3$ surface is a simple
way to get a metallic interface from insulating materials. No heating nor
specific preparation of the $SrTiO_3$ surface are needed. In this paper, we
investigate how the electrical properties of this interface can be tuned by the
use of a back gate voltage (electrical field through the $SrTiO_3$ substrate).
We demonstrate that the slow field-effect observed at room temperature can be
used to tune reversibly and in a controlled way the low temperature electrical
properties of the interface. In particular, important parameters of a
transistor such as the amplitude of the resistance response to gate voltage
changes or the existence of an "on" or an "off" state at zero gate voltage and
at low temperature can be adjusted in a single sample. This method should be
applicable to any $SrTiO_3$-based interface in which oxygen vacancies are
involved and might provide a powerful way to study the metal or superconductor
insulator transition observed in such systems.
