Hall effect measurements to calculate the conduction control in semiconductor films of SnO2

Hall effect measurement is one of the most powerful methods for obtaining information about transport mechanisms in polycrystalline semiconductor compounds that constitute the basis for understanding the sensing function of semiconductor gas sensors. The presence of grain boundaries represents the essential difference between single-crystal and polycrystalline semiconductors. The boundaries are important because they generally contain fairly high densities of interface states which trap free carriers from the bulk of the grains. In this paper the grain size of the semiconductor (calculated by the XRGA technique) and Hall effect measurements are used in order to obtain conduction-band profiles. Depending on the preparation method (reactive sputtering, electron beam, serigraphy), three types of conduction control can be distinguished. Similar results are obtained from analysis of the material microstructure

Properties of polycrystalline gas sensors based on d.c. and a.c. electrical measurements

Electrical properties of polycrystalline gas sensors are analyzed by d.c. and a.c. measurements. d.c. electrical conductivity values compared with those obtained by admittance spectroscopy methods help to obtain a detailed 'on line' analysis of conductivity-modulated gas sensors. The electrical behaviour of grain boundaries is obtained and a new design of sensors can be achieved by enhancing the activity of surface states in the detecting operation. A Schottky barrier model is used to explain the grain boundary action under the presence of surrounding gases. The height of this barrier is a function of gas concentration due to the trapping of excess charge generated by gas adsorption at the interface. A comparison between this dependence, and a plot of the real and imaginary components of the admittance versus frequency at different gas concentrations, provides information on the different parameters that play a role in the conduction mechanisms. These methods have been applied to the design of a CO sensor based on tin oxide films for domestic purposes, the characteristics of which are presented