Disclosed are isolation techniques for bulk FinFETs. A semiconductor device includes a semiconductor substrate with a fin structure on the semiconductor substrate. The fin structure is perpendicular to the semiconductor substrate and has an upper portion and a lower portion. Source and drain regions are adjacent to the fin structure. A gate structure surrounds the upper portion of the fin structure. A well contact point is provided in the semiconductor substrate. The lower portion of the fin structure includes a sub-fin between the region surrounded by the gate structure and the semiconductor substrate. The sub-fin directly contacts the semiconductor substrate. The upper portion of the fin structure and an upper portion of the sub-fin are undoped. A lower portion of the sub-fin may be doped. Electrical potential applied from the well contact point to the lower portion of the sub-fin reduces leakage currents from the upper portion of the fin structure.