Depletion Region
A zone near the surface of a semiconductor experiences a measurable reduction in mobile charge carriers due to the adsorption of ionized gas species. Formation of a space charge layer provides the physical basis for the sensing mechanism in metal oxide devices. Its depth is determined by the balance between the surface charge density and the bulk doping level.
Potential Profile
The distribution of electric potential within the layer creates a barrier that electrons must overcome to move between grains. This profile is non linear and extends from the surface into the bulk of the material.
Debye Length
Calculations of the characteristic thickness of the region often compare the result to the physical dimensions of the sensing grains. When the grain size is small, the layer can extend through the entire volume of the material.
Instrument Sensitivity
Modulation of the layer width by ambient gases directly translates into a change in the macroscopic electrical resistance of the sensor. Higher sensitivity is achieved when the surface to volume ratio is maximized, as in the case of nanowires or nanoparticles. Metrological analysis focuses on the relationship between the carrier concentration and the sensor response at various temperatures.
Drift in the baseline resistance often indicates a change in the permanent charge trapped at the surface or the grain boundaries.
