In a described example, an apparatus includes a transistor formed on a semiconductor substrate, the transistor including: a transistor gate and an extended drain between the transistor gate and a transistor drain contact; a transistor source contact coupled to a source contact probe pad; a first dielectric layer covering the semiconductor substrate and the transistor gate; a source field plate on the first dielectric layer and coupled to a source field plate probe pad spaced from and electrically isolated from the source contact probe pad; and the source field plate capacitively coupled through the first dielectric layer to a first portion of the extended drain.