Introduction

In recent years, there has been increasing interest and development efforts in miniaturizing gas sensors and systems. Particularly strong efforts have been made to monitor environmentally relevant gases like carbon-monoxide (CO), methane (CH₄) and ozone (O₃). Commonly used chemically sensitive materials for these target gases are wide-bandgap semiconducting oxides such as tin oxide, tungsten oxide or indium oxide, which are operated at elevated temperatures of 200–400 °C [1–3]. At those high temperatures, these oxides show considerable resistance changes upon exposure to a multitude of inorganic gases and volatile organics. The most prominent example is tin oxide (SnO₂), which shows large electrical resistance changes upon exposure to the above-mentioned gases at operating temperatures between 250 °C–350 °C and has been engineered to provide sufficient long-term stability [4–6]. The miniaturization efforts in the field of metal-oxide-based gas sensors follow several major trends: (a)the development of micromachined sensor platforms [7–9],(b)the micro- and nanotechnological fabrication of the sensing materials [10, 11], and(c)the design and co-integration of application-specific circuits with the transducer leading to smart sensor systems [8, 12–14].