Field Effect Transistor Type Gas Sensor Coated with NaNO_2-based Auxiliary Phase for NO_2 Detection

点击翻译按钮获取中文摘要

A filed effect transistor (FET) sensor for NO_2 was fabricated by depositing a NaNO_2-based auxiliary phase together with an Au electrode over the gate area. The resulting device exhibited almost ideal FET characteristics at 130- 180 ℃ in air. In NO_2 containing air, the threshold voltage was found to shift upward in a well controlled manner as the NO_2 concentration increased. At a fixed source-drain voltage (3.0 V), die gate-source voltage (V_<GS>) necessary to keep the drain current at a small constant value (200-300 μA) was found to increase linearly with an increase in the logarithm of NOr concentration. It was found that the modification of the auxiliary phase resulted in considerable changes in NO_2 sensing characteristics, especially in the lower detection limit of NO_2. For example, the use of NaNO_2-WG3 (5:1 in molar ratio) allowed to detect NO_2 down to 10 ppb (130 ℃), while NANO_2 did down to 500 ppb (180 ℃). The FET sensor combined with NaNO_2-WO_3 auxiliary phase was tested for the stability of its sensing signal to dilute NO_2 in the presence of CO_2, NO or water vapor. It was found that CO_2 is totally unharmful, allowing the sensor to detect as low as 10 ppb NO_2. On the other hand, NO was found to be an active gas responded by the sensor, although its sensitivity is one tenth the NO_2 sensitivity so that it would remain unharmful under its concentration exceeds 100 ppb. Finally, water vapor was found to cause the base line of signal output to shift downward significantly. The effect could be reduced drastically with a rise in firing temperature of the gold electrode used.