Palladium field effect sensors

There are three major classes of palladium-based hydrogen sensors [4], The most popular class of palladium-based sensors is based on palladium resistors. A thin film of palladium deposited between two metal contacts shows a change in conductivity on exposure to hydrogen due to the phase transition in palladium. The palladium field-effect transistors (FETs) or capacitors constitute the second class, wherein the sensor architecture is in a transistor mode or capacitor configuration. The third class of palladium sensors includes optical sensors consisting of a layer of palladium coated on an optically active material that transforms the hydrogen concentration to an optical signal. 

The hydrogen sensitivity of palladinm-oxide-semiconductor (Pd-MOS) strnctnres was first reported hy Lnndstrom et al. in 1975 [61]. A variety of devices can he nsed as field-effect chemical sensor devices (Fignre 2.6) and these are introdnced in this section. The simplest electronic devices are capacitors and Schottky diodes. SiC chemical gas sensors based on these devices have been under development for several years. Capacitor devices with a platinum catalytic layer were presented in 1992 [62], and Schottky diodes with palladium gates the same year [63]. In 1999 gas sensors based on FET devices were presented [64, 65]. There are also a few publications where p-n junctions have been tested as gas sensor devices [66, 67]. 

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