Biosensing Based on Gas Sensitive Semiconductor Devices

Abstract

The combination of catalytic metals and semiconductor devices has led to chemical sensors which appear to have both technical and medical applications.^1–5 More particularly, metal insulator semiconductor structures with the metal gate consisting of thin, discontinuous, iridium or platinum layers, have a large sensitivity to molecular ammonia.^6–9 These structures are so-called field effect devices which can be constructed, e.g., in the form of capacitors (Fig.1(a)) or transistors (Fig. 1(b)). It was observed several years ago that hydrogen gas could shift the electrical characteristics of such devices along the voltage axis if the metal gate was made of a catalytic metal, namely palladium.¹⁰ For these devices where the metal gate was thick enough to be continuous and non-porous, the voltage shift is due to hydrogen atoms adsorbed at the metal-insulator interface where they give rise to a dipole layer changing the work function of the metal at the metal-insulator interface (see Fig.2(a)). It was found, however, that this type of device was only to a very small extent sensitive to ammonia, although ammonia molecules can be dehydrogenated on a number of catalytic metal surfaces. Since ammonia molecules (or rather ammonium ions) are produced in a large number of biochemical reactions, we found it of interest to develop a field effect structure sensitive to ammonia. It was discovered that gates of catalytic metal films thin enough (of the order of 10 nm) to be discontinuous gave the field effect devices a large ammonia sensitivity. The details behind the ammonia sensitivity are not fully understood yet.