Bulk Conductivity Sensor Mode

In a mode 1 sensor, Eq. (17) applies throughout the system. To be more specific, let us formulate the overall introduction of E atomistically as 

SrTi03 may serve as a well-investigated material for such a bulk conductivity sensor. Its defect thermodynamics and also the relevant kinetic parameters have been discussed in detail in Part I.2 In particular at low temperatures and at small sample thicknesses L, the kinetics of oxygen incorporation becomes surface reaction controlled, and ks the decisive kinetic parameter. 

Important advantages of the bulk conductivity sensor are, besides selectivity, its simplicity (no reference needed) and its selectivity an important drawback is the T-dependence which can be quite significant (see above). Improvement via doping or by using a T-reference is straightforward, but partly at the cost of sensitivity, simplicity, or range of application. 

Obviously sensitivity is highest in the acceptor doped case. Here, however, the temperature dependence is also highest, namely 

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The emphasis on the thickness of the selective layer is linked to the mode of interaction between the analyte and the selective layer. If this interaction takes place exclusively at the surface of the selective layer, then the bulk conductivity does not contribute and represents only a shunt which decreases the signal-to-noise ratio. This is a typical case of chemiresistors based on inorganic materials (Fig. 8.6). On the other hand in chemiresistors based on organic semiconductors, the signal usually originates in the bulk of the selective layer. In that case, the response time of the sensor is affected by its thickness. 

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