Basic physics of semiconductor hydrogen sensors

Abstract

The most probable physical models of hydrogen sensors based on thin stannic oxide films, MOS-structures, and tunnel MOS-diodes are discussed. The emphasis is on the mechanisms of formation of sensor response to hydrogen. The analytical equations describing the dependence of the response on the hydrogen concentration n Н 2 are derived for all types of sensors. The relations describing the dependences of the SnO 2 -sensor conductivity and response on the absolute humidity of a gas mixture are given. It is shown that the relaxation time τ rel of the response of SnO 2 -and MOS-structure sensors is determined by the relaxation time τ а of hydrogen atom adsorption on the SnO 2 and SiO 2 surfaces, respectively. For the MOS-diodes, τ rel = τ а at n Н 2 <3⋅10 3 ppm and τ rel = τ d at n Н 2 ≥ 7.5⋅10 3 ppm, where τ d is the relaxation time of hydrogen atom diffusion through an SiO 2 layer.

Grain-size-dependent gas-sensing properties of TiO2 nanomaterials

“…For this purpose, the theoretical model proposed earlier by Gaman [14,15] for SnO 2 was applied. This approach seems to be justified by the fact that all assumptions of Gaman's model are fulfilled in the case of TiO 2 .…”

“…Modelling sensor responses has been performed mainly for SnO 2 [10][11][12][13][14][15][16]. The existing models [10][11][12][13][14][15][16] take into consideration the relation between grain size (d) and Debye length (λ D ), which corresponds to the width of the near-surface depletion or accumulation layer, and can be calculated according to the formula:…”

Studies of changes in electrical resistance of zinc oxide nanostructures under the influence of variable gaseous environments

“…The adsorption of gas leads to changes of the concentration of the charge carriers in semiconductor -an atom or particle of gas may add or remove an electron from the conduction band and also leads to filling up of trap surface states. In case of porous layers, a double Schottky barrier between the grains is supposed to exist [11]. The potential barrier at the connection of the grains, as well as the amount of free carriers and their spatial distribution, depend on the gas affecting the surface of the semiconductor.…”

“…The interaction between the semiconductor structure and the oxidizing gases differs from its reaction with the reducing gases [11,13]. Reducing gases react with the oxygen, which was earlier adsorbed by the structure.…”

“…: through the potential barrier in the case of grains (nanoparticles) contact and by tunnelling, when the grains do not contact each other directly, but are in close proximity [11]. In the theory of semiconductor sensors, the notion "receptor function" is used (describing the response of single crystallite to gas), as well as the notion "transducer function" (describing the impact of the common interaction of the nanostructures with the gases on the connections between crystallites) [12].…”