Nowadays, xylene is not only one major air pollutant which threats human health even if its concentration is lower than the human olfactory threshold of 470 ppb, but also one of the typical gases exhaled by the lung cancer patients with a criterion of 10-20 ppb. However, in situ detection of the ppb-level xylene for air quality monitoring and breath analysis remains challenging using the easily fabricated and low cost metal oxide semiconductor gas sensors. Herein, a synergetic p+ n field effect transistor (FET) amplification circuit is designed to detect the ppb level xylene. By optimizing the load resistor (RL) and the p- and n-FET coupling effect, a magnification factor (similar to 7.5) is obtained. This amplification circuit decreases the detection limit of TGS2602 sensor to similar to 10 ppb xylene with apparent response of about 2.3 and voltage change of >0.5 V, promising for air quality monitoring (the highest permissive limit of 42 ppb) and breath disease analysis (threshold of lung cancer 10-20 ppb). The mechanism is that the matched couple of p + n FETs work synchronically when their (R-L+ RFET)-I curves nearly coincide with each other. All those results show the prospect of ppb level gas detection with MOX sensors using the synergetic p + n FET amplification circuit.