T. Contaret

Physical-Based Characterization of Noise Responses in Metal-Oxide Gas Sensors

The noise level in the gas microsensors is a tool for characterizing the electrical conduction under various gases and a means to improve selectivity. Metal-oxide gas microsensors with WO3 sensitive thin film are characterized using a low-frequency noise technique. The spectral form of the noise responses measured using our specific systems is similar for tested gases (ozone and nitrogen dioxide). We observe a clear Lorentzian behavior according to adsorption-desorption (A-D) noise theory. To identify the detected gas, a physical-based characterization model of A-D noise source is proposed and compared with the empirical flicker noise model. We show that the excess noise is due to the A-D processes on the surface of the sensors sensitive film. The Lorentzian parameters depend on the nature of the gases and the noise-level dependence with gas concentration is clearly demonstrated. This confirms the interest on noise spectroscopy to improve the selectivity of gas sensors.

Noise spectroscopy measurements in metallic oxide gas microsensors

Noise spectroscopy is one of the solutions to enhance the selectivity of metal oxide sensors. This experimental technique is based on analysis of the power spectral density of noise fluctuations measured at the terminals of sensors in the presence of one or more gas. A specific noise measurement system has been developed to characterize the low frequency noise behavior of SnO2 gas microsensors under various gases. The noise voltage spectral density can be interpreted in terms of number fluctuations of the adsorbed molecules called adsorption-desorption noise. Our experimental results confirmed that noise spectroscopy could be used as a mean of improving gas sensors selectivity.

Noise Modeling in MOX Gas Sensors

In this paper, we propose a new model of adsorption–desorption (AD) noise in chemoresistive gas sensors by taking into account the polycrystalline structure of the sensing layer and the effect of the adsorbed molecule’s density fluctuation on the grain boundary barrier height. Using Wolkenstein’s isotherm, in the case of dissociative and non-dissociative chemisorption, combined with the electroneutrality, we derive an exact expression for power density spectrum (PDS) of the AD noise generated around one grain. We show that the AD noise generated in the overall sensing layer is a combination of multi-Lorentzian components. The parameters of each Lorentzian depend on the nature of the detected gas, the grain size, and the gas concentration. Moreover, we show that, according to the sensing layer microstructure (distribution of grain sizes in the sensing layer), this combination can lead to a 1/fγ spectrum, and in this case the noise level of the 1/fγ spectrum depends on the nature of the detected gas. The noise modeling presented in this paper confirms that noise spectroscopy is a useful tool for improving the gas sensor selectivity.

Development of metal oxide gas sensors for very low concentration (ppb) of BTEX vapors

The control and analysis of air quality have become a major preoccupation of the last twenty years. In 2008, the European Union has introduced a Directive (2008/50/EC) to impose measurement obligations and thresholds to not exceed for some pollutants, including BTEX gases, in view of their adverse effects on the health. In this paper, we show the ability to detect very low concentrations of BTEX using a gas microsensor based on metal oxide thin-film. A test bench able to generate very low vapors concentrations has been achieved and fully automated. Thin metal oxides layers have been realized by reactive magnetron sputtering. The sensitive layers are functionalized with gold nanoparticles by thermal evaporation technique. Our sensors have been tested on a wide range of concentrations of BTEX (5 - 500 ppb) and have been able to detect concentrations of a few ppb for operating temperatures below 593 K. These results are very promising for detection of very low BTEX concentration for indoor as well as outdoor application. We showed that the addition of gold nanoparticles on the sensitive layers decreases the sensors operating temperature and increases the response to BTEX gas. The best results are obtained with a sensitive layer based on ZnO.

Noise analysis of metal-oxide gas microsensors response to a mixture of NO 2 and CO

The noise level often limits the performances of gas sensors, particularly in terms of sensitivity. However, it can also be an effective detection tool. Indeed, noise spectroscopy is one of the solutions to enhance the selectivity and sensitivity of metal oxide sensors. This experimental technique is based on analysis of the power spectral density of noise fluctuations measured at the terminals of sensors in the presence of one or more gas. Our study focuses on physical-based analysis of low frequency noise responses in metal-oxide gas microsensors under a gas mixture composed of CO and NO2 gases. We clearly show the existing relationship between the measured noise levels according to the type of gas in a gas mixture. The study of NO2+CO mixture enable us to validate the models based on Langmuir and Wolkenstein theories which considers the noise generated by free electrons density fluctuations in the sensing layer due to instantaneous fluctuations in the adsorbed molecules density.

Physical-based characterization of low frequency responses in metal-oxide gas sensors

The noise level in the gas microsensors is a tool for characterizing the electrical conduction under various gases and a means to improve selectivity. Metal-oxide gas microsensors with WO3 sensitive thin film have been characterized using a low frequency noise technique. The spectral form of the noise responses measured using our specific systems is similar for tested gases (ozone and nitrogen dioxide). We observe a clear Lorentzian behavior according to adsorption-desorption (A-D) noise theory. To identify the detected gas, a physical-based characterization model of A-D noise source is proposed and compared with the empirical flicker noise model. We show that the excess noise is due to the A-D processes on the surface of the sensors sensitive film. The Lorentzian parameters depend on the nature of the gases and the noise level dependence with gas concentration is clearly demonstrated. This confirms the interest of noise spectroscopy to improve the selectivity of gas sensors.