[en] Electrical properties of Au/ and YBa₂Cu₃O_7-x/SrTi_1-yNb_yO₃ heterojunctions were studied by measuring their capacitance-voltage, current-voltage, and conductance-voltage characteristics. The heterostructures were made by depositing Au or YBa₂Cu₃O_7-x films on SrTi_1-yNb_yO₃ substrates. The results of the capacitance-voltage measurement indicated that there was an interfacial layer having a dielectric constant lower than that of bulk SrTiO₃ at the Au/SrTiO₃ and YBa₂Cu₃O_7-x/SrTiO₃ interfaces. The current-voltage characteristics of the Au/SrTi_1-yNb_yO₃ diodes with substrate Nb concentrations of 0.05 and 0.005 wt. % matched characteristics normally associated Schottky junctions and had a large ideality factor, n, consistent with the low-dielectric-constant interfacial layers. When the carrier concentration of the n-SrTiO₃ substrate was 2x10¹⁹ cm^-3, the Au and YBa₂Cu₃O_7-x junctions showed interfacial-layer tunneling characteristics. The YBa₂Cu₃O_7-x junctions exhibited two peaks in their conductance-voltage relations whose peak structures relies on the superconducting state density in YBa₂Cu₃O_7-x films