A compound semiconductor device includes a plurality of high-resistance crystalline silicon epitaxial layers and a plurality of activated dopant regions disposed in a same region of at least some of the epitaxial layers so that the activated dopant regions are aligned in a vertical direction perpendicular to a main surface of the epitaxial layers. The compound semiconductor device further includes an III-nitride compound semiconductor device structure disposed on the main surface of the epitaxial layers. The III-nitride compound semiconductor device structure has a source, a drain and a gate. An electrically conductive structure is formed from the activated dopant regions. The electrically conductive structure extends in the vertical direction through the epitaxial layers with the activated dopant regions toward the III-nitride compound semiconductor device structure, and is electrically connected to the source.