Basic approach to the transducer function of oxide semiconductor gas sensors

Abstract

Transducer function was explored for oxide semiconductor gas sensors in conjunction with the receptor function of small semiconductor grains recently revealed. In these sensors, grains are in contact with their neighbors to establish a network of electron conduction paths through the contacts. Under biased conditions, applied voltage and drift current are distributed most densely at the points of contacts; the contacts are the narrowest passages for electrons while the other portions of grains are loose ones. In case the grains are uniform in every aspect of size, shape, donor density and kind of semiconductor, each contact is shown to have a conductance proportional to the surface density of electrons for the grains. The resistance of the whole device is thus inversely proportional to the surface density of electrons, in agreement with the transducer function assumed previously. In case the grains are not uniform, however, additional factors participate in determining the conductance of each contact. The energy band diagram of contacting grains is featured by the presence of a difference in conduction band edge and the generation of contact potential across the contact. It is shown that the contact potential plays a role to modulate the mobility of electrons, giving rise to an additional effect to increase the resistance of the contacts, while the conduction band edge difference is to establish exchange current in between under the non-biased condition. The influence of the contact potential on the transducer function is rather complex, being almost extinct for usual packing structures of grains, while it can be very striking for some extreme packing structures.