A CHEMFET gate has been fabricated using epitaxially deposited LaF3 as the gate dielectric on Si(111). The LaF3 layer is used both as the electrical insulator and the ionic conductor. Platinum or RuO2 was sputtered on top of LaF3 as the gate metal catalytically active to the reduction of oxygen. The epitaxial film and substrate surface was characterized in-situ by RHEED. Further characterization of the film was performed ex-situ by X-ray diffraction methods. Due to the greater dielectric constant of LaF3, the epitaxial construction has a steeper CV curve than constructions which use additional insulating layers of SiO2 and/or Si3N4. Oxygen activity above the gate was modulated between 0.10 and 1.00 while applying a constant bias on the gate. Capacitance versus time plots revealed the Nernst response to be the closest to a single electron reaction.