Semiconductor type

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Oxide semiconductors such as SnO2, ZnO, In2O3, and TiO2 show the large changes in resistance at the exposure to reducing(CO, H2, CH4, C2H6) or oxidiging gases(NO, NO2) at 300-400oC. This is called as 'semiconductor-type' gas sensors.
The gas sensing mechanim is being explained by the reaction of reducing (or oxidizing) gases with surface oxygen. The followings are sensing mechanism of SnO2. SnO2 is n-type semiconductor with oxygen deficiency. The lattice oxygen is evaporated in the form of gas, which makesthe doubly ionized oxygen vacancy and electrons. This electrons play the role in conduction. If tin dioxide is heated at 300-400oC, the oxygen in the atmosphere are adsorbed at the surface of SnO2 with the negative charge. Because electron is provided from the surface of crystal, the surface of tin oxide becomes electron depletion layer. This means the formation of potential barrier near the grain boundary. The core with yellow tone shows the conducting phase and the red tone of surface layer indicates the resistive shell layer. If CO is present in the atmosphere, the CO oxidizes into CO2 with the reaction of adsorbed oxygen and remaining electron returns to the SnO2 crystal. This increases the electrical conductivity to a great extent. So, we can detect CO by the remarkable resistance decrease. The similar mechanism can be applied to the detection of other reducing gases. Electrochemically speaking, the reduction in resistance occurs mainly at the surface and can be explained by the reduction of potential barrier.

Semiconductor type

Fig. The CO sensing mechanism of semiconductor-type gas sensor.

Oxide nanostructures with hierarchical, hollow, and 1-D structures are very promising to achieve both of high gas response and fast gas response kinetics due to their high surface area and less agglomerated structures. We have prepared various oxide nanostructures for gas sensor applications by thermal evaporation, hydrothermal/solvothermal method, thermal oxidation, hydrazine method, and polyol route.

Semiconductor type

Fig. Various oxide hollow spheres preprared by our group.

Semiconductor type

Fig. Various oxide hierarchical nanostructures preprared by our group.

Semiconductor type

Fig. Various oxide nanowires preprared by our group.

References

1 Chang Sup Moon, Hae-Ryong Kim, Graeme Auchterlonie, John Drennan, and Jong-Heun Lee,* "Highly Sensitive and Fast Responding CO sensor using SnO2 Nanosheets," Sensors and Actuators B, (accepted)
2 Pyeong-Seok Cho, Ki-Won Kim, and Jong-Heun Lee,* "Improvement of dynamic gas sensing behavior of SnO2 acicular particles by microwave calcination," Sensors and Actuators B, 123, 1034-1039 (2007)
3 Ki-Won Kim, Pyeong-Seok Cho, Sun-Jung Kim, Jong-Heun Lee,* Chong-Yun Kang, Jin-Sang Kim, and Seok-Jin Yoon, "The selective detection of C2H5OH using SnO2-ZnO thin film gas sensors prepared by combinatorial solution deposition," Sensors and Actuators B, 123, 318-324 (2007)
4 Pyeong-Seok Cho, Ki-Won Kim, and Jong-Heun Lee, "NO2 sensing characteristics of ZnO nanorods prepared by hydrothermal method," J. Electroceram., 17, 975-978 (2006)