Metal oxide semiconductor chemical sensors

Metal oxide semiconductor chemical sensors in combination with MDA have been shown to be useful to estimate the oxidative stability of polypropylene during processing instead of traditional melt flow index analysis (50). An array of sensors was used to receive a detailed analysis of volatiles. At quality measurements of different poly(butylene adipate)s the use of indicator products has been proven better than analyses of the decrease in molecular weight or mass loss for early degradation detection. Adipic acid, quantified using gas chromatography, was then used as the indicator product [51]. 

A variety of chemical gas sensors are or could be used in electronic nose instruments. So far, successful results have been reached with conductive polymer (CP) sensors, metal oxide semiconductor (MOS) sensors, metal oxide semiconductor field effect transistor (MOSFET) sensors, quartz crystal microbalance (QCM) sensors, and infrared sensors. 

When confronted with non-linearly responding chemical sensors, forms of linearization are exploited to make the regression easier. The most common is the technique of log transforms. If a non-linear equation can be transformed into a linear one, it is said to be intrinsically linear. Using the response characteristics of a metal oxide semiconductor gas sensor as an example, an examination of transformation, linear regression, and confidence intervals can be performed. The response equation for metal oxide sensors has been theoretically determined as the following [10] ... 

Binions, R. (2007) A comparison of the gas sensing prop>erties of solid state metal oxide semiconductor gas sensors produced by atmospheric pressure chemical vapour deposition and screen printing. Measurement Science and Technology 18. 

Kanazawa, E., Sakai, G., Shimanoe, K., Kanmura, Y, Teraoka, Y, Miura, N. and Yamazoe, N. (2001) Metal oxide semiconductor N2O sensor for medical use. Sensors and Actuators B Chemical 77,72-7. 

There are, however, several fields of current research in which a corresponding level of understanding would be of interest also for large molecular adsorbates. For example, adsorbate-substrate interactions are relevant in the general areas of biocompatibility [51] and chemical sensors [52]. The requirement of dye-sensitization of metal oxide semiconductors also makes this an important aspect of many molecular photovoltaic devices. In fact, a good interfacial contact between dye and substrate, characterized by long-term stability and intimate electric contact, is vital for the efficiency of e.g. the dye-sensitized solar cells which have been at the center of our attention for the last five years. 

The attractiveness of silicon as a semiconductor material for ICs derives in part from the feet that this important material forms a naturally insulating surface oxide. Use is made of this fact, for example, in metal-oxide-semiconductor (MOS) field-effect transistors (FET), where the oxide serves as the gate insulator. No such naturally insulating oxide occurs with any of the compound semiconductors that offer improved performance over silicon in many device apphcations. Roberts et al. (38) demonstrated the feasibiUty of such metal-insulator-semiconductor (MIS) structures as FETs and chemical sensors shown schematically in Figure 1.23. These researchers... 

A variety of applications of chemical sensor arrays coupled with multivariate data analysis for quantitative measurements have been studied [26-32]. Each study investigated different types of sensors in the array, such as quartz crystals, ion-selective electrodes, metal oxide semiconductors, and chemFETs, with various types of modeling techniques as described above. 

Recently, organic conducting polymers have become the focus of much of the materials research in chemosensing devices. Synthetic flexibility allows the chemical and physical properties of polymers to be tailored over a broad range of values for any given application. In addition, polymers exhibit tunable specificity to volatile organic compounds, which makes them ideal candidates for replacing canonical sensor materials such as metal oxide semiconductors. 

To test this novel architecture as a tool for classification, a simulated experiment was performed. The case of chemo-resistive sensors was considered because of the simple involved electronics. This class of sensors is rather wide and can include sensors based either on inorganic (e.g. metal-oxide semiconductors) or organic (e.g. conducting polymers) sensitive materials. The concepts here illustrated can be extended, with a proper modification of the AORN architecture, to different kinds of chemical sensors. Actually, the features of the olfactive epithelium define the following structure of the AORN. 

Baraton M.-L, Metal oxide semiconductor nanoparticles for chemical gas sensors, lEEJ Trans. Sensors Micromach., 126, 553-559, 2006. 

Another most interesting approach proposed to improve chemical interactions and reduce the operating temperature is optical excitation. High temperatures limit the application of chemical sensors to nonexplosive and inflammable environments. As photons above the bandgap are absorbed by metal oxide semiconductors, free carriers are produced in the space charge area. The excess electrons are swept away from the surface, while excess holes are swept towards it due to the electrical field in the space charge... 

Thermoelectric Conductivity. The electrical conductivity of certain materials can be strongly modulated following surface adsorption of various chemicals. Heated metal oxide semiconductors and room-temperature conductive polymers are two such materials that have been used commercially. The change in sensor conductivity can be measured using a simple electronic... 

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