A semiconductor field effect structure Si/SiO2/Si3N4/LaF3/Pt can be used for the detection of fluorine in air. Depending on the preparation, two different sensor types were investigated having advantages at low or high fluorine concentrations, respectively. Combining these sensors concentrations between 0.01 and 1000 ppm can be detected. The mechanism of the sensor was shown to be an electrochemical charge transfer at the three-phase boundary LaF3/Pt/gas. Differences in the mechanism of the two sensor types are due to partial fluorination of the platinum at concentrations higher than 10 ppm. (C) 2000 Elsevier Science Ltd. All rights reserved.