Capacitive-type gas sensors combining silicon semiconductor and NaNO2-based solid electrolyte for NO2 detection

A capacitive-type NO2 gas sensor, developed by replacing metal in metal-insulator-semiconductor structure with solid electrolyte as sensing material, was investigated to improve the response and recovery rates and the operating temperature by modifying NaNO2-sensing material. Binary sensing materials of NaNO2-Ca-3(PO4)(2) and NaNO2-WO3 were tested for auxiliary phase of the device. The Ca-3(PO4)(2)-containing phase showed a response closer to ideal Nernstian behavior at 140 degrees C as compared to WO3-containing phase. On the other hand, the WO3-containing phase showed faster response and recovery rates at 160 degrees C, while the response deviated from Nernstian correlation. In order to obtain optimum sensor response, a ternary phase of NaNO2-Ca-3(PO4)(2)-WO3 was used as sensing phase. The device with ternary auxiliary phase was found to show more stable and faster response and recovery rates at 130 degrees C, as compared to previous devices attached with binary phases, corresponding Nernstian correlation in the concentration range of 20-500 ppb NO2. Furthermore, the effects of modification process on the auxiliary phase were investigated through SEM observation of the surface structures and measurement of ionic conductivity. (C) 2005 Elsevier B.V All rights reserved.