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Introduction

Gas sensors, which are a subset of the broader family of chemical sensors, allow inferring on the chemical species present in the surrounding environment. An ideal gas sensor should have high sensitivity, high specificity, small limit of detection, high resolution, high accuracy, high precision, large dynamic range, null offset, high linearity, null hysteresis, short response time, and long operation life (see Korotcenkov (2013)). Of course, no current gas sensors meet all these requirements simultaneously, which are, on the other hand, neither achievable nor necessary at the same time in real-world applications.

Fig. 1
figure 1

Sketch of a multilayered optical device (on the left) and a FET-like electrical device (on the right) representative of the last-generation architecture of porous silicon gas sensors

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The analysis of the state of the art of porous silicon gas sensors highlights how the early studies were mostly focused on the development of suitable readout approaches with the aim of investigating the sensing properties of porous silicon layers in sensor structures featuring both basic architecture (e.g., capacitor, monolayer) and simple surface chemistry (e.g., native surface or oxidized surface). On the other hand, the latest studies are mostly focused on the integration of porous silicon layers in sensor structures featuring advanced architectures (e.g., FET-like transistors, stacked rugate filters, etc.) and sophisticated surface chemistry (e.g., silanization, carbonization, metallization, etc.) with the aim of improving sensitivity, selectivity, and reliability performance. Finally, optical and electrical sensing platforms integrating array of porous silicon sensors with advanced features on the same chip are envisaged in the near future, though still in their infancy today (see handbook chapter “Porous Silicon for Microdevices and Microsystems”) (Fig. 1).

Gas Sensing with Porous Silicon

In the last two decades, micro-, meso-, and nanostructured forms of silicon, namely, porous silicon, have been demonstrated to be very effective for the fabrication of integrated gas sensors with low-cost process and room temperature operation (see Mizsei (2007), Ozdemir and Gole (2007), Saha (2008)). The increased specific surface (by definition, accessible surface to volume ratio) of porous silicon, up to 107 times larger than bulk materials, ensures a stronger interaction between material surface and gas molecules and allows high sensitivity and good limit of detection to be achieved for a large number of gaseous species (e.g., NOx and other inorganic gases, organic compounds, among which explosives, hydrocarbons, alcohols, halides, amines, ketones, etc.). Optical, electrical, and electrochemical approaches have been established to be valuable for the detection of a multitude of different gas species (both inorganic and organic). Changes in optical (e.g., refractive index, radiative recombination processes, etc.) and electrical (e.g., dielectric constant, conductivity, etc.) properties of the porous silicon material upon interaction (adsorption and/or condensation processes) with the specific gas species have been demonstrated through quantitative monitoring of the variation of different parameters (e.g., photoluminescence spectrum; reflected, transmitted, and diffracted optical power; capacitance; current; resistance; etc.) as a function of the gas concentration. A number of architectures have been proposed for both electrical (e.g., metal-based devices, among which capacitor, resistor, and Schottky diode, and pn junction-based devices, among which diode, transistor, etc.) and optical (e.g., monolayer-based devices, among which waveguide, and multilayer-based devices among which Bragg mirror, resonant cavity, rugate filter, etc.) gas sensors with the aim of improving sensor performance (e.g., improve sensitivity, limit of detection and selectivity, compensate for baseline drift, compensate for measurement angles, etc.). An equivalent large number of readout approaches (e.g., spectrometry, interferometry, ellipsometry, birefringence, conductometry, impedance spectroscopy, etc.) have been reported for monitoring the sensor parameters, by using both single-parameter and multiparameter approaches. Besides, a number of functionalization schemes (e.g., oxidation, hydrosilylation, carbonization, metallization, etc.) of the porous silicon surface have been proposed with the aim of reducing aging and interfering effects while improving reliability, sensitivity, and selectivity. See, for example, the handbook chapter “Silicon-Carbon Bond Formation on Porous Silicon.” Tables 1 and 2 provide the main architectures of porous silicon electrical and optical gas sensors, respectively, since 1990. The porous silicon physical parameters that mainly account for the variation of the sensed quantity for the specific sensor architecture are also indicated along with the gas species sensed (Table 3).

Table 1 Main architectures of porous silicon electrical gas sensors since 1990
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Table 2 Main architectures of porous silicon optical gas sensors since 1990
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Table 3 Sensed species with electrical and optical porous silicon gas sensor architectures of Tables 1 and 2
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Sensing Platforms

More recently, in addition to mere sensor devices exploiting either single-parameter or multiparameter monitoring, miniaturized sensing platforms integrating a number of porous silicon optical sensors featuring different surface chemistry on the same chip, as well an array of porous silicon electrical sensors together with CMOS electronic driving/readout circuits on the same chip, have been demonstrated to be feasible, thus envisaging the realization of a new class of electronic and photonic system-on-chips with gas sensing capability.

Table 4 provides study examples on up-to-date sensing platforms exploiting porous silicon either electrical or optical gas sensors.

Table 4 Electrical and optical sensing platforms with porous silicon
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Summary

Over the last two decades, electrical and optical gas sensors based on porous silicon have been tremendously improved, in terms of architectures, performance, and sensed species. On the one hand, single devices with high sensitivity, low limit of detection, and good selectivity have been achieved for different analytes, although reliability and lifetime still remain among the major challenges for both electrical and optical sensors. On the other hand, multiparametric sensing on single devices and multisensor integration in a single chip have been very recently reported for both optical and electrical approaches, thus pushing porous silicon gas sensors to a new generation of miniaturized sensing platforms. As to the latter, tremendous improvement due to simultaneous integration of sensors with electronic and photonic silicon circuits is expected for both approaches, respectively, in the next future.