Investigations of conduction mechanism in Cr2O3 gas sensing thick films by ac impedance spectroscopy and work function changes measurements

Modeling of sensing and transduction for p-type semiconducting metal oxide based gas sensors

“…Figure 1 right describes the situation after the exposure to reducing gases has decreased the concentration of oxygen ions: the decrease of the surface negative charge, described in the energy bands representation as an downward band bending (qΔV S ¼ ΔΦ in the Figure), determines a decrease of the hole concentration resulting in the increase of the resistance of the accumulation layer. As already demonstrated in [13], the sensor resistance will be the result of the combination between the contributions of the resistances of the surface accumulation layer, bulk and contacts between the electrodes and the semiconductor. The particular manner in which those different contributions are combined depends on the morphology of the sensitive layer; moreover, in the case of the experimental results presented in [13] it was possible to identify the contribution of the electrode-semiconductor contact as a parallel (RC) element in the equivalent circuit that fits the AC impedance spectra; its identification was made possible by the fact that the values of the resistance and capacitance do not change upon exposure to gases.…”

“…Here, we want to devise a conduction model that links the changes of the surface charge to the measured resistance of the sensor. For doing so, we will consider that, as already discussed in [13], the main effect of the exposure to reducing gases is the decrease of the negative charge trapped at the surface of the semiconductor in the form of oxygen ions. Figure 1 middle, depicts what happens at the surface of a p-type semiconducting metal oxide when electrons from the valence band are captured on the surface traps considered to be associated to the adsorption of ambient oxygen as oxygen ions: one records an increase of the concentration of holes in the vicinity of the surfacebuild-up of an accumulation layer-described in the energy bands representation as an upward band bending; as a consequence, the electrical resistance of that layer decreases in comparison with the flat bands situation (case depicted in Fig.…”

“…sensing and transduction, is focused on the n-type case [12]. Recently, [13], we investigated the way in which surface reactions-induced electrical changes are affecting the sensor signals of thick porous layers of Cr 2 O 3 , a p-type material, by using simultaneous DC and work function changes (Kelvin probe method) as well as AC impedance spectroscopy measurements; on their basis we developed a conduction model, which qualitatively explains the experimental data. The most important finding, the validity of which should apply to all p-type metal oxides used as gas sensitive materials, is that the use of the sensing layer resistance changes as sensor output downgrades the sensor performance for that type of materials.…”

“…The reason is the way in which the conduction takes place that adds to the measured gas sensitive resistance of the oxide's surface depleted layer the gas insensitive resistances of its bulk, in parallel, and of the contact resistance between the semiconductor and the electrode, in series. In [13] we were not able to quantitatively analyze the experimental data because the modeling of conduction in gas sensitive p-type oxides was not available. Here, we are proposing such a model and we are using it in order to extract the relevant material parameters.…”

Ethanol sensor development based on ternary-doped metal oxides (CdO/ZnO/Yb2O3) nanosheets for environmental safety

“…According to the charge accumulation reproduction of p-type semiconductors, the conduction occurs along the conductive as well as active sensor surface. [59][60][61] The conductivity of ZnO thin lms doped with CdO/Yb 2 O 3 is very sensitive to the exposure of the surface to various chemicals and hence it can be used as a toxic chemical sensor capable of detecting the toxicity of contaminants, due to the high sensitivity to selective chemicals present in the sensing layer.…”