A vertical field-effect transistor (FET) device is fabricated with a self-aligned bottom insulating spacer for improved electrostatic control. A semiconductor fin is formed on a semiconductor substrate. A lower source/drain region, which is formed of a first type of epitaxial semiconductor material, is epitaxially grown on a surface of the substrate in contact with a bottom portion of the semiconductor fin. A sacrificial epitaxial semiconductor layer is epitaxially grown on top of the lower source/drain region, wherein the sacrificial epitaxial semiconductor layer is formed of a second type of epitaxial semiconductor material which is different from the first type of epitaxial semiconductor material. The sacrificial epitaxial semiconductor layer is selectively oxidized to form a self-aligned bottom insulating spacer comprising an oxide layer. A gate structure is formed contact with sidewalls of the semiconductor fin. The self-aligned bottom insulating spacer electrically insulates the gate structure from the lower source/drain region.