AlGaN/GaN shows great promise as a generic platform for (bio-)chemical sensing because of its robustness and intrinsic sensitivity to surface charge or dipoles. Here, we employ the two-dimensional electron gas (2DEG) formed at the interface of AlGaN/GaN layers grown on Si substrates for the ultra-sensitive detection of NOx. In the presence of humidity, the interaction of NOx with the naturally adsorbed water layer at the surface changes the surface polarity, thereby instantaneously affecting the 2DEG conductivity. We show that this concept can be used for ultra-low-power continuous air quality monitoring, at room temperature and humidity levels down to 10%. The extremely low current noise level in our structures enables the reproducible detection of variations in the NOx concentration of 1 ppb. Furthermore, we show that these devices can be functionalized with metalloporphyrins resulting in sensitivity to NO and NO2 concentrations below 50 ppb even in dry conditions. © 2011 Published by Elsevier Ltd.