There is a growing interest in the development of solid state devices for use as chemical sensors, and following the significant progress made in hydrogenated amorphous silicon thin-film transistor technology, the possibility of using devices based upon this as sensors has become very attractive.

A recent letter from the Istituto di Elettronica dello Stato Solido, CNR, Italy, describes the development of a palladium-gate thin-film transistor as a hydrogen sensor (L. Mariucci, A. Pecora, C. Puglia, C. Reita, G. Petrocco and G. Fortunato, Jpn. J. Appl. Phys., Pan 2 Lett., 1990, 29, (12), L2357−L2359).

Although the electrical characteristics of a top-gate device are inferior to those of a bottom-gate one, the former was selected to meet the requirement of a gate electrode in direct contact with the environment. A 100 nm thick layer of palladium was sputter deposited onto the 9 nm thick silicon nitride employed as the gate insulator. This enables the device to respond in a relatively fast time and in a relatively stable and reproducible manner to hydrogen, and the investigation has shown that such devices are good enough for application as hydrogen sensors.