Semiconductor gas sensors

The temporal change of the surface electron density in response to the presence of a reducing gas is  

Under equilibrium conditions the excess electron density is  

Calculating the total electrical surface conductivity Js of the titanium oxide as a function of the gas concentration [X] yields  

As is evident from Eqs (7.21) and (7.23), the excess surface conductivity is directly proportional to the square root of the partial pressure [X] of the gas if, per mole of the gas, only one oxygen atom at the surface will be consumed that is, only one lattice vacancy V will be created. However, if two oxygen atoms are being consumed per mole of the monitored gas (V ), then the excess surface conductivity will be increased with the cube root of [X] (Heywang, 1984). Also, the signal of the sensor will be maximized by minimizing the surface electron density that is related to defects at the surface of the sensing metal oxide. 

Titania-based gas sensors may also be developed for breathalyzer purposes to detect alcohol in the breath of inebriated drivers. For ejample, Nb-Pt co-doped Ti02 thin films can detect ethanol at concentrations below 300ppm, with a response and recovery time of less than 2 minutes (Le et al., 2005). 

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In conduction models of semiconductor gas sensors, surface barriers of intergranular contacts dominate the resistance. Electrons must overcome this energy barrier, eV., in order to cross from one grain to another. For these... 

Construction In tin dioxide semiconductor sensors, the sensing material is small sintered particles. For the sensor current flow, particle boundaries form potential energy barriers, which act as a random barrier netw ork. Different types t)f semiconductor gas sensors are shown in Fig. 13..54. 

Applicability of Semiconductor Gas Sensors Research into the applications of this type of sensor has mainly been concerned with measuring carbon monoxide concentration in flue gases. Tests show that sensors follow the concentration of carbon monoxide in the flue gas. Improvement in sensor performance has resulted with the introduction of a catalytic additive (palladium or... 

FfCURE 13.54 Semiconductor gas sensors (o) tubular, (b) thick film, (e) bulk-type one-electrode sensor where a thin Pt wire spiral is embedded Inside a sintered oxide semiconductor button. ... 

Golovanov, V., J. L. Solis, V. [.antto, and S. Leppavnori. Different Thick-Film Methods in Prinr-mg ot one-Electrode. Semiconductor Gas Sensors. Sensors Actuators B34 (1996), pp. 410-416. Lantto, V., and V. Golovanov. A Comparison of Conductance Behaviour between SnO> and CdS Gas-Sensitive Films, Sensors Actuators B24-25 (1995), pp. 614-618. 

Lantto, V., P. Romppainen, and S. Leppiivuori. Response Studies of Some Semiconductor Gas Sensors under Different Experimental Conditions. Sensors Actuators 15 (1988), pp.. 347-357. 

Thus, the major conclusions of tiie early studies by Volkenshtein and his colleagues applicable to the theory of the method of semiconductor gas sensors are the following a) chemisorption of particles on a semiconductor surface can be accompanied by a charge transfer between adsorption-induced surface levels and volume bands of adsorbent and b) only a certain fraction of absorbed particles is charged, the fraction being dependent on adsorbate and adsorbent. 

The experimental studies of the surface properties of monocrystals of oxides of various metals recently conducted at well-controlled conditions [32, 210] enable one to proceed with detailed analysis of separate effects of various factors on characteristics of semiconductor gas sensors. In this direction numerous interesting results have been obtained regarding the fact of various electrophysical characteristics of monocrystalline adsorbents on the value of adsorption-related response. Among these characteristics there are crystallographic orientation of facets [211], availability of structural defects, the disorder in stoichiometry [32], application of metal additives, etc. These results are very useful while manufacturing sensors for specific gases with required characteristics. 

The lifetime of metal oxide gas sensors in general is several years. Nevertheless poisoning effects can occur when exposed to silicones. An example of commercial Ga203-semiconductor gas sensors is shown in Fig. 5.32. 

Yamazoe, N. Miura, N. Some Basic Aspects of Semiconductor Gas Sensors. Chemical Sensor Technology, Vol. 4, Ed. Yamauchi, S., Kodansha Ltd. Tokyo,... 

G. Heiland and D. Kohl. Physical and Chemical Aspects of Oxidic Semiconductor Gas Sensors , Chemical Sensor Technology, T. Seiyama, (Ed.), Elsevier, Amsterdam, Netherlands (1988). 

T. Takada. Temperature drop of semiconductor gas sensor when exposed to reducing gases - simultaneous measurement of changes in sensor temperature and in resistance . Sensors and Actuators B66 (2000), 1-3. 

M. Graf, D. Barrettino, S. Taschini, C. Hagleitner, A. Hierlemann, and H. Baltes. Metal-Oxide-Based Monohthic Complementary Metal Oxide Semiconductor Gas Sensor Microsystem , Analytical Chemistry 76 (2004), 4437-4445. 

Kim YS, Ha SC, Kim K et al (2005) Room-temperature semiconductor gas sensor based on nonstoichiometric tungsten oxide nanorod film. Appl Phys Lett 86(21) 213105-1-213105-3... 

Aishima, T. (1991) Aroma discrimination by pattern recognition analysis of responses from semiconductor gas sensor array. J. Agric. Food Chem. 39 752-756. 

Metal oxide semiconductor gas sensors. The production of semi-... 

The application of semiconductor gas sensors (mainly the Sn02 sensor) is ever expanding into various fields, domestic and industrial, as shown in Figure 3. Examples are the CO selective sensor and combustion monitor sensor, which are applicable to micro wave ovens or ventilation fans, kerosene or gas stoves, hot water supply systems, and so on. 

The resistivity of certain semiconductors such as tin oxide (SnOx) and zinc oxide (ZnO) can be strongly modulated by the presence of certain gaseous species in the ambient. Several gas sensors have been developed based on such material characteristics (1-5). The principal advantages of semiconductor gas sensors are (a) relative simplicity of fabrication (b) relative simplicity of operation (c) low cost (fabrication and maintenance). However, the major drawback of these sensors is their low sensing selectivity among various gases. 

Fig. 8.10 (a) Response of semiconductor gas sensors toward ethylene, ethane... 

Fig. 8.11 Comparisons of ethylene concentration in effluent by semiconductor gas sensor with that by conventional gas chromatograph, (a) Effluents obtained by changing reaction temperature from 300 to 475 °C and (b) effluents from Ni-Fe-mixed oxide with different composition at 450°C (reproduced by permission of Elsevier from [19]).

Lantto, V. (1992) Semiconductor gas sensors based on Sn02 thick films. In Gas Sensors, G. Sberveglieri (ed.), Kluwer Academic Publishers, Dordrecht, pp. 117-67. 

Shimizu, Y. and Egashira, M. (1999) Basic aspects and challenges of semiconductor gas sensors. MRS Bull, 18-24. 

Vernat-Rossi, V., Garcia, C., Talon, R., Denoyer, C., and Berdague, J.L., Rapid discrimination of meat products and bacterial strains using semiconductor gas sensors Sens. Actuators B, B37 (1-2), 43 48, 1996. 

Aronova, M. A. Chang, K. S. Takeuchi, I. Jabs, H. Westerheim, D. Gonzalez-Martin, A. Kim, J. Lewis, B., Combinatorial libraries of semiconductor gas sensors as inorganic electronic noses, Appl. Phys. Lett. 2003, 83, 1255-1257... 

Since the discovery of oxide semiconductor gas sensors in the 1960s, many sensing materials, such as Sn02, ZnO, W03, Fe203, and Ti02, have been explored to achieve the requirements of (1) high sensitivity, (2) selective detection of a specific gas, (3) rapid response time, (4) long-term stability, (5) cost efficiency.1-5... 

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