NO₂ Sensing Properties of WO₃ Gate-fitted FET Device

Abstract

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 (V_GS) 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.