Electrostatic Threshold
The energy required for a charge carrier to cross the interface between two adjacent grains determines the electrical conductivity of a polycrystalline material. Development of the surface potential barrier arises from the trapping of electrons by adsorbed oxygen ions. The height of the barrier is sensitive to the presence of reducing or oxidizing gases in the atmosphere.
Conduction Regulation
Electrons must possess sufficient thermal energy to jump over the barrier via thermionic emission. As a result, the resistance of the sensor follows an exponential relationship with temperature.
Gas Modulation
Reducing gases react with the adsorbed oxygen, releasing electrons back into the semiconductor and lowering the barrier height. This action causes a sharp increase in the observed conductance of the device.
Qualification Standard
Measuring the barrier height requires a plot of the logarithm of conductance versus the inverse of temperature, known as an Arrhenius plot. The slope of this line yields the activation energy associated with the barrier. Precision sourcing of sensors involves verifying that this activation energy remains consistent across a production lot.
Variations can be caused by impurities at the grain boundaries or differences in the annealing temperature during fabrication. Calibration software uses these values to compensate for ambient temperature fluctuations.
