Modelling of gas-sensitive conducting polymer devices

Recent studies have shown that conducting polymers are sensitive to a wide range of gases and vapours and may be used in gas-sensing microelectronic devices. The authors present a basic model for polymer gas sensors which consists of a thin uniform polymer film lying on top of a pair of either semi-infinite or finite coplanar electrodes supported by an insulating substrate. It is assumed that the gas, or vapour, diffuses into the film and is, simultaneously, adsorbed at sites randomly distributed throughout the film. The diffusion and adsorption equations are presented in terms of several fundamental dimensionless parameters which describe the underlying chemical and physical properties of the system. Numerical solutions to the equations are calculated for both the gas and adsorbate profiles within the films at various times. These numerical solutions are compared with approximate analytical expressions previously derived for diffusion-rate limited, reaction-rate limited and intermediate cases, and show good agreement. Finally, a semiconductor model of electronic conduction in gas-sensitive polymer films is developed to calculate the theoretical device response to the sorption of organic vapours. This model can be used to investigate the effects of device geometry on sensor response and is therefore a useful design tool for evaluating novel device structures. The model may also be extended to cover other types of device, such as capacitive or mass balance.

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