size, the sensitivity of the gas sensor is dependent on the nanocrystallite size for D > 2L.

As the nanocrystallite size reduces below twice the spacecharge layer thickness (D < 2L), the entire nanocrystallite becomes depleted of electrons. The space-charge layer then penetrates into the nanocrystallite completely, Figure 14c. The electrical resistance of the gas sensor increases abruptly and is controlled mainly by the grain resistance under this situation. In this region, the ratio of electron concentration in gas (ng) to that in air (na) within a nanocrystallite decreases as the distance from its surface increases. As a result, when D < 2L, the sensitivity of the gas sensor increases with decreasing nanocrystallite size in this range.

Thus, the nanocrystallite size (D) relative to the spacecharge depth (L) is one of the most important factors affecting the sensing properties of a semiconductor oxide gas sensor. For a thin sputtered SnO2 film and sintered powders, the space-charge depth has been calculated to be ~3 nm at 250 °C [85, 86]. Hence, for the sol-gel derived nano-crystalline SnO2 gas sensor, very high sensitivity is theoretically expected just above and below the nanocrystallite size of ~6 nm.

Figure 15 shows the effect of the calcination temperature on the gas sensitivity for the sol-gel derived nanocrystalline SnO2 thin-film sensor [5, 8, 9, 12-14, 17, 21, 22, 25]. The maximum gas sensitivity is observed to be reported at the calcination temperature of 450 °C. Above and below this calcination temperature range, the gas sensitivity decreases. The sol-gel derived nanocrystalline SnO2 thin film is always amorphous at room temperature. Hence, the calcination treatment at higher temperature is necessary to crystallize

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