An isolator device is provided comprising a body of first dielectric material between the first and second conductors, such as primary and secondary coils of a micro-transformer. A region of second dielectric material is provided between the body of first dielectric material and at least one of the first and second electrodes, wherein the second dielectric material has a higher relative permittivity than the first dielectric material. This provides enhances ability to withstand the Electric fields generated at the edge of a conductor. The body of the first dielectric can be tapered to provide stress relief to prevent the second dielectric material developing stress cracks.