A FinFET has a structure including a semiconductor substrate, semiconductor fins and a gate spanning the fins. The fins each have a bottom region coupled to the substrate and a top active region. Between the bottom and top fin regions is a middle stack situated between a vertically elongated source and a vertically elongated drain. The stack includes a top channel region and a dielectric region immediately below the channel region, providing electrical isolation of the channel. The partial isolation structure can be used with both gate first and gate last fabrication processes.