  The Mott transistor is a paradigm for a new class of electronic
devices---often referred to by the term Mottronics---, which are based on
charge correlations between the electrons. Since correlation-induced insulating
phases of most oxide compounds are usually very robust, new methods have to be
developed to push such materials right to the boundary to the metallic phase in
order to enable the metal-insulator transition to be switched by electric
gating.
  Here we demonstrate that thin films of the prototypical Mott insulator
LaTiO$_3$ grown by pulsed laser deposition under oxygen atmosphere are readily
tuned by excess oxygen doping across the line of the band-filling controlled
Mott transition in the electronic phase diagram. The detected insulator to
metal transition is characterized by a strong change in resistivity of several
orders of magnitude. The use of suitable substrates and capping layers to
inhibit oxygen diffusion facilitates full control of the oxygen content and
renders the films stable against exposure to ambient conditions, making
LaTiO$_{3+x}$ a promising functional material for Mottronics devices.
