Electrolyte-insulator-semiconductor (EIS) sensor is commonly considered for chemical and biosensing applications due to its small size and simple fabrication method. Here, we demonstrate a fluoride-sensitive EIS sensor using thermally-deposited polycrystalline lanthanum fluoride (poly LaF₃) film as sensing membrane, which is cheaper than single-crystal LaF₃. The sensing characteristics are analyzed for poly LaF₃ layers deposited at different temperatures, and the EIS sensors with the sensing membrane formed at 500 °C exhibit excellent sensing response to fluoride ions with a high sensitivity of 52.3 mV/pF and low limit of detection of 1.9 ppb. This limit of detection is lower than previously reported values in the literatures. In addition, the poly LaF₃ film deposited at 500 °C has good stability with a low hysteresis voltage of 5.1 mV and a small drift rate of 0.67 mV/h. These superior metrics come from a rather well crystallized LaF₃ structure including denser surface grains, enhanced preferential crystalline (002) plane, and improved stoichiometric composition. Furthermore, the sensors show a good selectivity over other ions such as NO₃⁻ and SO₄²⁻.