In an embodiment, a device includes: a gallium nitride device on a substrate, the gallium nitride device including an electrode; a dielectric layer on and around the gallium nitride device; an isolation layer on the dielectric layer; a semiconductor layer on the isolation layer, the semiconductor layer including a silicon device; a through via extending through the semiconductor layer, the isolation layer, and the dielectric layer, the through via electrically and physically coupled to the electrode of the gallium nitride device; and an interconnect structure on the semiconductor layer, the interconnect structure including metallization patterns electrically coupled to the through via and the silicon device.