Variations in I-V characteristics of oxide semiconductors induced by oxidizing gases

Current (I)-voltage (V) characteristics of porous sintered discs of semiconductive ZnO- and SnO2-based materials have been investigated in air, 100 ppm NO2, 800 ppm O-3 and 1% H-2 in the temperature range 200-700 degrees C. All the oxide discs tested exhibited nonlinear I-V characteristics of varistor-type in every atmosphere. The breakdown voltage shifted to a high electric field upon exposure to NO2 and O-3, compared with that in air, presenting a striking contrast to the behavior in 1.0% H-2. It was revealed that pure ZnO and SnO2 themselves exhibited a relatively large shift in breakdown voltage when exposed to the oxidizing gases. Addition of Bi2O3 to SnO2 resulted in a slight increase in the sensitivity to NO2 and O-3 but in a relatively large increase-in the H-2 sensitivity at 600 degrees C. It was also found that the breakdown voltage increased with increasing the sweep rate of the electric field, suggesting some influence of the Joule's heating of a specimen during measurement on the I-V characteristics at slow sweep rates. However, the magnitude of the gas-induced shift in breakdown voltage was not so affected by the sweep rate, but rather increased at 12.3 V s(-1). From these results, it was confirmed that the behavior of chemisorbed species on the surface of oxide particles near grain boundaries determined by the height of the double Schottky barrier and then the gas-sensitive non-linear I-V characteristics.