An integrated CMOS sensing chip for NO2 detection

This paper reports the fabrication, calibration and operation of an on-chip detection/alarm circuit for continuous monitoring of NO2 in the concentration range of hundreds ppb. The circuit consists of: (i) an APSFET (adsorption porous silicon FET) sensor; (ii) a transresistive amplifier (first electronic stage); (iii) a Schmitt trigger (second electronic stage). The APSFET yields a current signal with an amplitude depending on the NO2 concentration. The transresistive amplifier converts the sensor current to an amplified voltage signal related in a known way to the NO2 concentration, after a proper calibration is performed. The Schmitt trigger compares the output voltage of the first stage with a threshold voltage, the latter corresponding to a chosen NO2 concentration, and it sets/unsets an alarm whenever the pollutant concentration exceeds/lessens this chosen value. All the elements were included on an integrated CMOS sensing platform, apart from a few external resistors.