Significance Semiconductor interfaces are among the most important in use in modern technology. The properties they exhibit can either enable or disable the characteristics of the materials they connect for functional performance. While much is known about important junctions involving conventional semiconductors such as Si and GaAs, there are several unsolved mysteries surrounding interfaces between oxide semiconductors. Here we resolve a long-standing issue concerning the measurement of anomalously low dielectric constants in SrTiO ₃ films with record high electron mobilities. We show that the junction between doped and undoped SrTiO ₃ required to make dielectric constant measurements masks the dielectric properties of the undoped film. Through modeling, we extract the latter and show that it is much higher than previously measured.