Surface Reaction
Reversible adsorption of oxygen ions on a metal oxide semiconductor surface establishes a balanced distribution of charge states. This steady state, known as oxygen species equilibrium, involves molecular and atomic ions that capture electrons from the conduction band of the semiconductor. The dominant ion species on the surface changes depending on the environmental conditions.
Temperature Dependency
Thermal energy dictates which ionic species dominates the semiconductor surface at any moment. Below one hundred and fifty degrees Celsius, molecular oxygen ions are the primary surface charge species. Above this temperature, atomic oxygen ions become dominant because the thermal energy is sufficient to dissociate the oxygen molecules.
At even higher temperatures, double-charged oxygen ions become the prevailing species. This progression illustrates how temperature dictates the steady-state composition of the surface layer.
Electronic Modulation
Electron extraction from the semiconductor bulk creates a depletion region near the surface. The oxygen species equilibrium determines the thickness of this depletion layer, which directly controls the electrical resistance of the sensor. Introducing reducing gases disrupts this balance, releasing electrons and lowering the resistance.
Interference Source
Atmospheric humidity represents a common source of disturbance to this ionic balance. Water molecules compete with oxygen for the same active adsorption sites on the metal oxide surface. This competitive adsorption changes the baseline resistance of the sensor and leads to drift in gas measurement.
