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As an introduction to this book, a variety of semiconductor gas sensors is presented. They are classifi ed into fi ve types, according to the transducers used: resistor, diode, MIS (metal-insulator-semiconductor) capacitor, MIS FET (metal-insulator-semiconductor fi eld effect transistor) and oxygen concentration cell. The structure, working principle and sensing mechanism are described for each type. As the only type successfully on the market at present, the resistor is allotted considerably more space than the alternative semiconductor gas sensors, with experimental knowledge and the theory of gas response described in detail. The receptor function of metal oxides recently found in resistors is also of critical importance in other types incorporating the same oxides; the contact potential generated between oxide and metal is likely a main origin of the gas response of those devices. Finally, observations on the prospects for and problems with semiconductor gas sensors are made.

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SnO 2 nanotubular materials were prepared by using a natural cellulosic substance (filter paper) as template, and their morphologies were determined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Cellulose fibers were first coated with SnO 2 gel layers by the surface sol-gel process using Sn(O i Pr) 4 as precursor, followed by calcination in air to give SnO 2 nanotubular materials as hollow replicas of natural cellulose fibers. The nanotubes obtained by calcination at 450 °C were amorphous-like and composed of fine particles with sizes smaller than ca. 5 nm. The outer diameters are tens to two hundred nanometers, and wall thicknesses are 10-15 nm. Calcination at 1100 °C yielded tubelike polycrystalline SnO 2 nanocages (outer diameter 100-200 nm), which were composed of rutile-phase SnO 2 nanocrystallites with sizes of 10-20 nm. The thermal behavior and the crystalline property of the powder obtained from calcination of the as-prepared SnO 2 sheet were examined in the temperature range of 300-900 °C. The sizes of the nanoparticle obtained by calcination at 300 and 900 °C were 2.0 and 9.2 nm, respectively, in fair agreement with TEM observation. Calcination temperatures above 500 °C are needed to obtain pure SnO 2 . A sensor setup was fabricated from the SnO 2 nanotube sheet, and the sensor performance was measured for H 2 , CO, and ethylene oxide. The sensor signal, S, was 16.5 at 450 °C to 100 ppm H 2 , and was comparable to that of the conventional SnO 2 sensor. Finally, the sensor characteristics were discussed in relation to the morphology of the nanotube sheet.