Improvement in sensitivity and selectivity of InP-based gas sensors: pseudo-Schottky diodes with palladium metallizations

The possibility of using single resistive n-type InP semiconductor gas sensors to perform accurate measurements of ozone or nitrogen dioxide concentration in air comes up against their low sensitivity and the inability to discriminate between the influence of each gas on the sensors without any exterior apparatus. To improve these two fundamental aspects of gas sensors, the sensitive n-InP layers have been included in more complex devices, called pseudo-Schottky diodes. Made by successive evaporation of metallic thin layers on p-InP substrates, their Schottky metallization schemes (Pd/Ge/Pd) satisfy a double objective: the creation of the necessary n-InP gas sensitive layer by activation of Ge dopants and the ozone catalytic conversion by palladium layers. Comparisons between the sensing performances of the two gas sensors (resistive and Schottky diode-type ones) show that sensitivity of the laters is largely higher than that of single resistive gas sensors. On the other hand, a good selectivity toward ozone is achieved with Pd/Ge/Pd/p-InP gas sensors, resulting from different reaction kinetics between O₃ or NO₂ and the sensitive layer. These differences can be attributed to the palladium metallization catalytic activity.