Theory of gas-diffusion controlled sensitivity for thin film semiconductorgas sensor

Influences of gas transport phenomena on the sensitivity of a thin film sem iconductor gas sensor were investigated theoretically, A diffusion equation was formulated by assuming that an inflammable gas (target gas) moves insi de the film by Knudsen diffusion, while it reacts with the adsorbed oxygen following a first-order reaction kinetic. By solving this equation under st eady-state conditions, the target gas concentration inside the film was der ived as a function of depth (x) from the film surface, Knudsen diffusion co efficient (D-K) rate constant (k) and film thickness (L). The gas concentra tion profile thus obtained allowed to estimate the gas sensitivity (S) defi ned as the resistance ratio (R-a/R-g), under the assumption that the sheet conductance of the film at depth x is linear to the gas concentration there with a proportionality constant (sensitivity coefficient), a. The derived equation shows that S decreases sigmoidally down to unity with an increase in L rootk/D-K. Further by assuming that the temperature dependence of rate constant (k) and sensitivity coefficient (a) follows Arrenius type ones wi th respective activation energies, it was possible to derive a general expr ession of S involving temperature (T). The expression shows that, when the activation energies are selected properly, the S versus T correlation resul ts in a volcano-shaped one, its height increasing with decreasing L. The de pendence of S on L at constant T as well as on T at constant L can thus be simulated fairly well based on the equation. (C) 2001 Elsevier Science B.V. All rights reserved.