Abstract
Characteristics for 
sensing of a 
oxide capacitor were studied. The capacitances of 
mixed with
, 
, 
, and 
are dependent on the 
concentration, similar to 
. In particular, the mixed oxide, 
, is more sensitive to 
than 
, but requires a rather long time to recover. On the other hand, the mixed oxide capacitor, 
, exhibits high 
sensitivity, and a monotonic and approximately logarithmic relationship exists between the sensitivity and the concentration of 
from 100 ppm to 50%. Furthermore, the capacitance of 
increased rapidly within 30 s after exposure to 2% 
, and returned rapidly to the original level after reexposure to air. Therefore, the system, 
is promising for sensing 
over a wide range of concentrations. From a complex impedance measurement, the equivalent circuit of the sensor element can be expressed by a series connection of three parallel resistors and capacitors which correspond to the intragrain region, grain interface between
and 
, and electrode interface. Since exposure of the element to 2% 
varied the capacitance of the grain interface, the grain interface between 
and 
has an important role for 
detection.