Harmonic modulation of impurity in gas flow as a method to study the semiconductor sensors

The method was developed, involving the harmonic modulation of impurity con centration in a gas flow, in order to characterize semiconductor sensor sur faces. The method is similar to any spectroscopic technique and consists of the analysis of the conductivity response of semiconductor sensor to the h armonic modulation of impurity introduced into the gas flow. The anomalous behavior of amplitude of the harmonic signal dependency on frequency of the driving concentration wave was found if the thick film tin dioxide sensors were used to analyze propane in air mixtures. The theoretical model of the process was suggested assuming two simultaneous reactions running concurre ntly with the time constants of the same order of magnitude, thus providing quasi-resonance dependence of conductivity in the frequency domain. The si mulation was done based on the differential equation, obtained from a set o f expressions, describing the tin dioxide surface reactions kinetics. The r esults of the simulation correspond to the experimental data obtained. (C) 1999 Elsevier Science S.A. All rights reserved.