Semiconductive sensors, metal oxide

Metal oxide semiconductive sensors are an important class of chemical sensors particularly for gaseous sensing. Among the metal oxide semiconductive materials. 

Metal oxide semiconductive sensors are not limited to tin oxide only. Many other metal oxides, such as zinc oxide, tungsten oxide, and others can also be used for chemical and gas sensing. It is understandable that an incorporation of a selective catalyst or a dopant may enhance the selectivity of the MOS sensors. Palladium, platinum, and others have been used as catalytic dopants for these sensors. The processes... 

In this entry, we focus on the discussion of the platform technology for electrochemical sensors, metal oxide semiconductive (MOS) sensors, and piezoelectric based quartz crystal microbalance (QCM) sensors. There are other types of chemical sensors, such as optical sensors, Schottky diode based sensors, calorimetric sensors, field-effect transistor (FET) based sensors, surface acoustic wave sensors, etc. Information of these specific sensors can be found elsewhere and in current journals on sensor technologies. Because of the increasing importance of microfabricated sensors, a brief discussion of microsensors is also given. 

Metal oxide semiconductive materials exhibit a relatively low conductivity at ambient temperature. Thus, it will be very difficult to observe a small conductivity change because of its reaction with a reducing gas. Therefore, it is common to operate a MOS based sensor at elevated temperature. At higher temperature, the conductivity of the MOS increases substantially, and a change caused by the reaction with reducing gas is now observable. 

Chapters 7-11 continue the theme and explore different types of chemical sensors. Chapter 7 describes the application of metal oxide semiconducting resistive sensors, and then Chapters 8-11 cover mainly recent developments of electrochemical sensors. 

In the next section, a variety of solid state environment gases sensors (NO,, CO2, CO, SO2, O2, etc.) are reviewed, and attention is also paid to semiconducting metal oxide type. Also discussed are the extension of the operating temperature to the near-human temperature regimes and better sensing properties derived from the nanostructured semiconducting metal oxide gas sensors. 

The basic mechanism of the semicondncting metal oxide CO sensors relies on the condnctivity changes experienced by the n-type semiconducting metal oxide material when snrface chemisorbed oxygen reacts with reducing gases such as carbon monoxide (CO) or methane (CH4) at elevated temperatures. The overall reactions... 

Various types of solid-state NO2 sensors have been proposed based on semiconducting metal oxides (including heterocontact materials) [42-50,58,59,234-238], solid electrolytes [1,239,240], metal phthalocyanine [241], and SAW devices [242]. Among these NO2 sensors, the semiconducting metal oxides and solid electrolytes appear to be the best. Specifically, semiconducting metal oxide gas sensors are most attractive because they are compact, sensitive, of low cost, and have low-power consumption. Their basic mechanism is that the NO2 gas is adsorbed on the surface of the material this decreases the free electron density into the space-charge layer and results in a resistance increase [243]. 

General discussion about sensing with semiconducting metal oxide gas sensors... 

The operation of a metal-oxide gas sensor relies on the change in resistance of an n- or p-type semiconducting layer - mainly Sn02 - when exposed to reducing or oxidizing gases. 

Kanan, S.M., El-Kadri, O.M., Abu-Yousef, LA., and Kanan, M.C. 2009. Semiconducting metal oxide based sensors for selective gas pollutant detection. Sensors 9 8158-8196. 

Wetchakun K, Samerjai T, Tamaekong N, Liewhiran C, Siriwong C, Kruefu V, Wisitsoraat A, Tuantranont A, Phanichphant S (2011) Semiconducting metal oxides as sensors for environmentally hazardous gases. Sens Actuators B 160 580-591... 

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