  We report the study of metal-STO-metal memristors where the doping
concentration in STO can be fine-tuned through electric field migration of
oxygen vacancies. In this tunnel junction device, the evolution of the Density
Of States (DoS) can be followed continuously across the Metal-Insulator
Transition (MIT). At very low dopant concentration, the junction displays
characteristic signatures of discrete dopants levels. As the dopant
concentration increases, the semiconductor band gap fills in but a soft Coulomb
gap remains. At even higher doping, a transition to a metallic state occurs
where the DoS at the Fermi level becomes finite and Altshuler-Aronov
corrections to the DoS are observed. At the critical point of the MIT, the DoS
scales linearly with energy $N(\varepsilon) \sim \varepsilon$, the possible
signature of multifractality.
