We report on the fabrication and electrical characterization of epitaxial metal-semiconductor junctions between Y1 Ba2 Cu3 O7-δ (YBCO) (optimally doped and Zn doped) and (001) Nb:SrTi O3 with different Nb concentrations (0.05%, 0.1%, and 0.5%). The current-voltage characteristics of such epitaxial junctions are nonlinear and rectifying, and these are dramatically enhanced with decreasing Nb concentration and Zn doping. Indeed, for the case of 0.05% Nb:STO, reverse breakdown voltage as high as -18 V (-28 V) is realized for optimally doped (Zn doped) YBCO. These data are analyzed within the framework of thermionic emissiondiffusion models for Schottky and metal-insulator- semiconductor-type junctions. © 2006 American Institute of Physics.