Device with a conductive feature formed over a cavity and method therefor

A device comprising:

• a semiconductor substrate with a first substrate surface and a second substrate surface, wherein the semiconductor substrate includes a host substrate and a group-III nitride semiconductor layer disposed over the host substrate;
• a transistor comprising a drain contact, a source contact, and a gate electrode, wherein the gate electrode is formed over the semiconductor layer in an active region and is electrically coupled to a channel, and wherein an electrical signal applied to the gate electrode controls electrical current flow in the channel between the drain contact and the source contact;
• a first conductive feature formed over the first substrate surface in an isolation region wherein the first conductive feature is electrically coupled to a transistor electrode selected from the gate electrode, the drain contact, and the source contact;
• a second conductive feature coupled to the second substrate surface, wherein the second conductive feature includes an opening through which a portion of the second substrate surface is exposed to define a first conductor-less region along the second substrate surface;
• a cavity within the first conductor-less region and extending into the semiconductor substrate, wherein the cavity is at least partially defined by an inner cavity surface that is recessed into the semiconductor substrate from the second substrate surface, and the inner cavity surface is vertically aligned below the first conductive feature;
• and
• a dielectric medium disposed within the cavity and in contact with the inner cavity surface that terminates on a dielectric medium surface parallel to the second substrate surface, the dielectric medium having a dielectric constant less than a dielectric constant of the semiconductor substrate, wherein the dielectric medium surface does not contact a conductive material.