Real-time detection of negative ions in air using nanoFET sensor. Ids increasing rate depends on the distance of the sensor from the ion generator which is related to the anion concentration. The closer distance corresponds to the higher concentration. After the ionizer is turned off, Ids decreases slowly as the discharging of the charged negative ions occurs. (a) Real-time detection of negative ions in air using the nanoFET sensor. The blue line is the conductance change over time and the red line corresponds to the rate of change at each sampling time. Gradual increase and saturation of Ids in the p-type nanoFET device at a constant Vbg of 0 V represents the negative ion charging on the top gate surface. The sensitivity of the sensor reaches over 700. The Ids by back-gate voltage modulation up to -15 V of Vgs could be compared with the Ids caused by the amount of adsorbed charges by negative ions in air. Therefore, under the condition that the electric field applied to the nano channel by the amount of adsorbed charges on the sensing area is equivalent to the electric field driven by the backgate voltage applied through the back-gate MOS structure, the Vbg = -15 V equals to approximately 5.7×105 electron charges adsorbed on the sensor to form the same conductance value of Ids. (b) Real-time detection of negative ions according to the distance of the nanoFET from the anion generator which is related to the anion concentration in the air. (c) The slope of Ids change rate versus time according to the distance of the nanoFET from the anion generator. The inset shows the maximum value of the slope versus the distance which is related to negative ions concentration in air. Note that the anion concentration near the sensor reduces as the ionizer is placed farther away.