Detecting NO2 with APSFET, an adsorption porous silicon FET

APSFET (adsorption porous silicon FET) is a porous silicon-based device constituted of a FET structure with a porous adsorbing layer between drain and source. Adsorbed gas molecules in the porous layer induce an inverted channel in the crystalline silicon under the porous silicon itself. The mobile charge per unit area in the channel depends on the molecular concentration in the sensing layer, so that adsorbed gas plays a role similar to the charge on the gate of a FET. NO₂ detection using the APSFET is demonstrated for the first time. NO₂ concentration as low as 100 ppb was detected. Unfortunately, such high sensitivity is obtained only for fresh APSFET samples, due to a drift of sensor properties with sample aging. A low thermal oxidation was performed on fresh samples in order to passivate the porous silicon surface. The oxidation step positively affects the sensor in term of stability of electrical performance, keeping the high response. to nitrogen dioxide.