Search

Abstract: This paper reports the fabrication, calibration and operation of an on-chip detection/alarm circuit for continuous monitoring of NO2 in the concentration range of hundreds ppb. The circuit consists of: (i) an APSFET (adsorption porous silicon FET) sensor; (ii) a transresistive amplifier (first electronic stage); (iii) a Schmitt trigger (second electronic stage). The APSFET yields a current signal with an amplitude depending on the NO2 concentration. The transresistive amplifier converts the sensor current to an amplified voltage signal related in a known way to the NO2 concentration, after a proper calibration is performed. The Schmitt trigger compares the output voltage of the first stage with a threshold voltage, the latter corresponding to a chosen NO2 concentration, and it sets/unsets an alarm whenever the pollutant concentration exceeds/lessens this chosen value. All the elements were included on an integrated CMOS sensing platform, apart from a few external resistors. [Copyright &y& Elsevier]

Fabrication and optical testing of high-aspect-ratio 1D photonic crystals, obtained by electrochemical micromachining of silicon, are discussed in this paper. The devices consist of high-aspect-ratio periodic (P=4 mum) arrays of 1.22-mum-thick silicon walls separated by 2.78-mum-wide air gaps, with 100 mum etching depth. They were designed as hybrid quarter-wavelength reflectors with photonic bandgaps in the near-IR region, one in particular centered at lambda=1.55 mum. The fabrication process was improved to increase structure uniformity and strength. Thermal oxidation of the silicon structures was exploited to tune the wavelength position and width of the bandgaps. Fabricated devices, also with different silicon dioxide thicknesses, were optically tested by measuring their spectral reflectivity in the wavelength range of 1.0-1.7 mum. Experimental results were found in good agreement with the calculated spectra. [ABSTRACT FROM PUBLISHER]

Adsorption porous silicon FET (APSFET) is a porous silicon (PS)-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 PS itself. The mobile charge per unit area in the channel depends on the molecular gas concentrations in the sensing layer so that adsorbed gas molecules play a role similar to the charge on the gate of a FET. In this work, NO2 detection by using the APSFET is demonstrated for the first time. NO2 concentration as low as 100 ppb was detected. Devices with both as-grown and oxidized PS layers were fabricated and compared in order to investigate the effect of a low-temperature thermal oxidation on the electrical performances of the sensor. Nonoxidized sensors show a high sensitivity only for fresh devices, which reduces with the aging of the sample. Oxidation of the PS layer improves the electrical performance of sensors, in terms of stability, recovery time, and interference with the relative humidity level, keeping the high sensitivity to nitrogen dioxide. [ABSTRACT FROM PUBLISHER]