  Resistive solid state sensors are widely used in multiple applications,
including molecular and gas detection. Absorption or intercalation of the
target species varies the lattice parameters and an effective thickness of thin
films, which is usually neglected in analyzing their transport properties in
general and the sensor response in particular. Here, we explore the case of
palladium-based thin films absorbing hydrogen and demonstrate that expansion of
thickness is an important mechanism determining the magnitude and the very
polarity of the resistance response to hydrogenation in high resistivity films.
The model of the resistance response that takes into account modifications of
thickness was tested and confirmed in three Pd-based systems with variable
resistivity: thin Pd films above and below the percolation threshold, thick
Pd-SiO2 granular composite films with different content of silica, and Pd-rich
CoPd alloys where resistivity depends on Co concentration. Superposition of the
bulk resistivity increase due to hydride formation and decrease of film
resistance due to thickness expansion provides a consistent explanation of the
hydrogenation response in both continuous and discontinuous films with
different structures and compositions.
