Basics of semiconducting metal oxide–based gas sensors

Abstract This chapter reviews the basic knowledge required to understand gas sensing with semiconducting metal oxides. It reviews the significant characteristics of the semiconductors including concentration and mobility of free charge carriers, work function, and electronic affinity. The origin of semiconductivity in metal oxides examined for the most significant oxides, e.g. SnO 2 , In 2 O 3 , ITO, WO 3 , ZnO, Cr 2 O 3 , CuO, and Co 2 O 3 , is presented. Current modeling of gas sensing starts by examining surface reactions between the atmospheric gases and the oxides—the so-called reception function. The newest experimental findings as well as the relevant theoretical concepts are presented. The experimental investigations provide information about the structural properties, including the chemical state of the dopants and sensitizers and the impact of gas exposure on the sensor’s parameters. The latest developments of the operando characterization techniques are presented. Different theoretical concepts are presented. The understanding of gas sensing is mostly based on formalized quasichemical reactions. A deeper insight, however, is obtained by talking into account the charging of the surface and how this can be quantified with the help of the Poisson and electroneutrality equations. The transduction function of metal oxide sensors as well as the impact of bulk properties and sensing layer structure is described.