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

通过在栅极上沉积纳米辅助相和Au电极，制备了NO_2场效应晶体管（FET）传感器。该器件在空气中温度为130 ~ 180℃时表现出近乎理想的场效应管特性。在含NO_2的空气中，随着NO_2浓度的增加，阈值电压以良好的控制方式向上移动。在源漏电压固定（3.0 V）时，使漏极电流保持在一个小的恒定值（200 ~ 300 μA）所需的栅极源电压（V_<GS>）随硝态氮浓度的对数增加而线性增加。结果表明，辅助相的改变对NO_2的传感特性有较大的影响，特别是对NO_2的检测下限有较大的影响。例如，使用NANO_2 - wg3（摩尔比为5:1）可以检测到低至10 ppb（130℃）的NO_2，而NANO_2可以检测到低至500 ppb（180℃）的NO_2。测试了NaNO_2-WO_3辅助相组合的FET传感器在CO_2、NO或水蒸气存在下对稀释NO_2传感信号的稳定性。发现CO_2是完全无害的，允许传感器检测低至10 ppb的NO_2。另一方面，no2的灵敏度仅为no2的十分之一，在no2浓度超过100 ppb的情况下，no2仍对人体无害，但no2是一种敏感气体。最后发现水汽使信号输出基线明显下移。随着所用金电极的烧成温度的升高，这种效应会急剧降低。

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.