Dielectrically isolated semiconductor devices are producible through a relatively convenient fabrication procedure. In this fabrication procedure, a substrate having regions of single crystal silicon and regions of silicon oxide is employed. Such substrate is expeditiously produced by methods which leave the surface of the single crystal regions below those of the silicon oxide regions. Silicon is deposited by CVD onto the structure with its regions of silicon dioxide and single crystal silicon. Initially, the conditions of the CVD procedure are controlled so that epitaxial silicon grows on the regions of single crystal silicon but essentially no growth is induced on the silicon oxide regions. When the growth of the single crystal regions has proceeded sufficiently to produce a substantially planar structure, advantageously the deposition conditions are adjusted so that silicon is also deposited on the surface of the silicon oxide. The polycrystalline or amorphous silicon layer overlying regions of silicon oxide produced from this growth is then converted into single crystal silicon.