A field effect transistor (FET) device fitted with a WO3 layer over the gate area exhibited almost ideal FET behavior at 150 and 180°C in air containing various concentrations of NO2. Under the conditions of fixed source-drain voltage (3.0 V) and fixed drain current (450 μA), the gate-source voltage (VGS) was found to increase linearly with an increase in the logarithm of NO2 concentration over the range of several tens to 700 ppb NO2, proving its potentiality to work as an environmental NO2 sensor. However the times of 90%-response and -recovery to switching-on and -off 50 ppb NO2 were as long as 10 and 25 min even at 180°C, respectively. Cross sensitivity test revealed that the device was totally insensitive to CO2, but rather sensitive to NO. Water vapor was found to give a serious disturbance to the device: The VGS response to NO2 as well as its dependence on NO2 concentration changed with a change in relative humidity.
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