All‐thin‐film SrTiO3/YBa2Cu3O7−x/SrTiO3/Cu structures fabricated on sapphire substrates were used to study large electric field effects in YBCO superconducting films. The YBCO films (∼100 nm) were deposited by off‐axis radio‐frequency sputtering while the top SrTiO3films (∼200 nm) were reactively vacuum deposited at low temperatures to avoid significant deoxygenation of the superconducting YBCO underlayer films. Cu was used in the field‐effect transistor‐type YBCO/SrTiO3/Cu multilayered structures as the gate electrode and as Ohmic contacts for the source and drain electrodes. This avoided probable electroforming processes taking place in the SrTiO3insulating matrices under the large electric fields arising from the applied gate voltages. The YBCO films were polycrystalline withTconsets ∼72–82 K. The gate voltages were found to change both theTcandTcovalues and hence the resistivity of the relatively thick superconducting YBCO layers in the normal and superconducting states. ©1995 American Institute of Physics.