The present disclosure describes epitaxial oxide integrated circuits. In some embodiments, an integrated circuit comprises: a field effect transistor (FET), comprising: a substrate comprising a first oxide material; an epitaxial buried ground plane on the substrate and comprising a second oxide material; an epitaxial buried oxide layer on the epitaxial buried ground plane and comprising a third oxide material; an epitaxial semiconductor layer on the epitaxial buried oxide layer and comprising a fourth oxide material with a first bandgap; a gate layer on the epitaxial semiconductor layer and comprising a fifth oxide material with a second bandgap; electrical contacts; and a waveguide coupled to the field effect transistor. The waveguide can comprise: the epitaxial buried ground plane; the epitaxial buried oxide layer; and a signal conductor, wherein the epitaxial buried oxide layer is between the signal conductor and the epitaxial buried ground plane.