  Semiconducting metal oxide (SMOX)-based gas sensors are indispensable for
safety and health applications, e.g. explosive, toxic gas alarms, controls for
intake into car cabins and monitor for industrial processes. In the past, the
sensor community has been studying polycrystalline materials as sensors where
the porous and random microstructure of the SMOX does not allow a separation of
the phenomena involved in the sensing process. This lead to conduction models
that can model and predict the behavior of the overall response, but they were
not capable of giving fundamental information regarding the basic mechanisms
taking place. The study of epitaxial layers is the definite prove to clarify
the different aspects and contributions of the sensing mechanisms that are not
possible to do by studying a polycrystalline sample. A detailed analytical
model for n and p-type single-crystalline/compact metal oxide gas sensors was
developed that directly relates the conductance of the sample with changes in
the surface electrostatic potential. Combined DC resistance and work function
measurements were used in a compact SnO2 (101) layer in operando conditions
that allowed us to check the validity of our model in the region where
Boltzmann approximation holds to determine surface and bulk properties of the
material.
