  Perovskite oxide based thin film gas sensors have long been considered as
potential alternatives to commonly investigated binary metal oxides based
sensors. BiFeO3, which is a prototype of p-type perovskite based semiconducting
oxides, has recently drawn significant attention for its promising gas sensing
characteristics. In the present work, the hydrogen sensing characteristics of
calcium doped BiFeO3 has been reported by varying the film thickness, doping
concentration, operating temperature, and test gas concentration. The films
were deposited on glass substrates by sol-gel route using spin coating. X-ray
diffraction analyses confirmed formation of phase pure films and scanning
electron microscopy confirmed their uniform and dense microstructure. The
Ca-doped BiFeO3 sensors exhibit higher sensitivity compared to pure BiFeO3
sensors. It is reported that the film thickness and Ca doping concentration
play major role to control hydrogen sensing characteristics of the deposited
films. The sensor based on 15% Ca-doped BiFeO3 sensor exhibited very high
sensitivity (~212 % at 500 ppm H2), and excellent selectivity towards hydrogen
at a moderate operating temperature (~250 {\deg}C).The enhanced gas sensing
response of the doped BiFeO3 films has been attributed to the higher oxygen
vacancy concentration induced by incorporation of aliovalent Ca2+.
