The influence of heterovalent doping of LaF3 with SrF2 on the electrical behavior of a silicon-based chemical sensor and ion conductivity of the doped LaF3 layer were investigated. An improved quality of field effect devices was achieved using a La-(0.964)Sr-(0.036)F-(2.96) layer instead of the insulator. Capacitance voltage curves of metal/ionic conductor/semiconductor (MICS) devices are much steeper than for conventional metal/insulator/semiconductor (MIS) devices. It was proven that interface, not insulator-bulk, capacitances dominate the behavior of the field effect device. The results obtained can be applied to field effect devices other than chemical sensors.

A chemical semiconductor sensor based on a silicon carbide substrate was investigated for the high temperature measurements of fluorine up to 350°C. The use of the structure SiC/SiO2/LaF3/Pt at room temperature leads to results comparable to the silicon based sensor. The influence of the temperature on the sensor response time was smaller than expected but the increased desorption rate and a high signal to noise ratio improved the detection limit. An impulse method using the initial slope of the response curve was shown to be advantageous.

A new Sensor for the determination of fluorine has been developed using a simple structure Si/LaF3/Pt. A thin layer of the ionic conductor LaF3 is in direct contact to the semiconductor without intermediate insulator. The field effect in the semiconductor leads to much steeper capacitance-voltage curves than usually found for conventional MIS-structures. It was shown that a special treatment of the sensor is necessary before first use to achieve a stable response. Different ionic conductors and gate materials were tested and the preparation conditions varied. The lower limit of detection of the optimised sensor was shown to be < 0.1 ppm. It was proven that potential formation takes place at the three phase boundary LaF3/Pt/gas. Oxygen can be determined with a similar sensor system but does not interfere for the fluorine sensor.