NO2 adsorption effects on p+–n silicon junctions surrounded by a porous layer

In this work, nitrogen dioxide (NO2) detection by using p+–n silicon diodes surrounded by a porous silicon (PS) layer is demonstrated. The effect of the NO2 (at concentrations of hundreds ppb) on the sensor current was investigated for both reverse and forward polarization voltages, using relative humidity (RH) and ethanol at different levels as interfering species. Adsorption of NO2 in the PS layer modifies the electrical properties of the PS/crystalline silicon interface and, in turn, the p+–n diode current. The device shows a high selectivity to NO2 with respect to ethanol, at any polarization voltage and relative humidity level: for instance, a NO2 concentration as low as 100 ppb produces a current variation of about one order of magnitude, while 100 ppm of ethanol do not significantly affect the diode current. For a given NO2 concentration, the current variation depends on the diode bias, so that the sensor response can be tuned by changing the polarization voltage. Finally, the relative humidity, one of the most important interfering species for gas sensors, shows a negligible effect on the sensor behaviour.