**1. Introduction**

There are several natural and artificial chemical species in the air, some of which are toxic and combustible gases, which can be considered as potential hazard to the health [1–3]. Gas sensors helps to prevent these hazards and play an important role in places with the risk of potentially harmful substances specifically in industrial processes and manufacturing plants.

Depending on the principal of the operation, gas sensor devices can be classified into two distinct categories: chemical gas sensors and physical gas sensors [3]. Merely, considering transduction mechanism, chemical gas sensors are based on chemical reaction between gas and sensing materials, resulting in change of conductivity of the detector material. In this method, gases in different combinations could produce the same net-change in conductance and as a result, distinguishing between a gas and its mixtures is impossible. These types of sensors are very

sensitive to changes in moisture, temperature and gas pressure. Their other drawback is that chemical reactions could cause irreversible changes in detector materials [3]. Physical type sensors have overcome the disadvantages of chemical type sensors. There are several physical type gas sensors, according to the mechanism of operation, including surface plasmon resonance (SPR) based [4] gas sensors, fiber optic based gas sensors [5–8] and gas ionization sensors [9].

Surface plasmon resonance (SPR) is the resonant oscillations of surface electrons, which are stimulated by incident illumination at the interface between a metal and dielectric [10, 11]. SPR is very sensitive to the refractive index of the medium close to the metal film. The resonance spectral response of the SPR will change when the conditions of the medium are changed, which can reflect certain properties of the system. Kretschmann geometry (prism coupler) is widely used to study SPR. In this configuration, optical wave is totally reflected at prism-metal interface [12]. Evanescent field wave may penetrate the metal layer and excite surface plasmon at the metal-dielectric boundary. As excitation of surface plasmon significantly reduces the intensity of reflected light, reflectivity of the sensor as a function of either wavelength or incident angle is considered as the sensor response [12].

Agbor et al. [13] reported the SPR gas sensing measurements in Kretschmann configuration using nickel/silver coated glass microscope slides. According to their results, the SPR curves were influenced by 50 ppm of NO2 and H2S at room temperature. Maharana et al. [14] reported a numerical study on a high performance SPR sensor based on graphene coated silver on wide range of refractive indices of gases. Graphene is widely used in SPR based gaseous detection systems, as its refractive index is highly sensitive to the absorbed gas molecules. Furthermore, graphene is robust against the oxidation and the layer of graphene in SPR sensors (in presence of noble metals) prevents oxidation of the silver layer. Nooke et al. [15] studied the SPR gas sensing measurements in Kretschmann configuration using gold (Au) coated glass for combustible, toxic and greenhouse gases. They also reported that the gas detection limit is related to the rate of gas adsorption, which is defined by polarizability of the gases.

SPR-based fiber optic sensors are designed by replacing the cladding with a thin layer (in nm range) of metal. In these sensors it is hard to reach the sensitivity similar to Kretschmann SPR configuration due to complexity in controlling the incidence angle of light, impossibility to control the wave polarization and an excessive number of reflections. In these sensors the spatial-frequency bandwidth of their angular spectrum is wider in comparison with other types of SPR sensors [16, 17]. However, some noticeable advantages like low cost, flexibility, real-time monitoring, compatibility with human tissue and blood vessels, remote sensing, small sample volume, reusability, and simple structure have made the SPR fiber optic approach very attractive.

Gas sensing application of the fiber optic sensor was developed in 1980 [1] and the sensing measurements are essentially based on changing the features of transmitted light along the fiber. Transmitted light can be modified in response to external medium properties. According to the principal of the operation, fiber optic gas sensor devices can be classified into two distinct categories: extrinsic and intrinsic [1, 6]. In extrinsic fiber optic gas sensors, light exits the fiber and interacts with the medium before continuing propagation inside the core again. In these sensors, light propagates through the input fiber optic toward a microcell containing the unknown gas. The output signal is guided to a spectrometer using the output fiber optic, which is accurately aligned with the input one. This provides the unknown gas detection by comparing the input interrogating wavelength and the absorption spectrum of the gas [6]. This technique can be only used for the gases which spectral absorption is in the range of telecommunication window so the fiber can be successfully employed. Stewart et al. [18] reported a design of fiber optic methane sensor using a microcell and DFB laser source. The theoretical modeling of the designed

**101**

coefficient (β = Eloc/Eapp) [27].

**2. Geometrical field enhancement**

will enhance field ionization tunneling phenomenon.

*Miniaturized Gas Ionization Sensor Based on Field Enhancement Properties of Silicon…*

sensor was in a good agreement with experimental results and both showed a single

In intrinsic fiber optic gas sensors, light propagates inside the core continuously without any external interaction. When a light is propagating through fiber optics, at the core-cladding boundary it undergoes total internal reflection (TIR). During each TIR it penetrates into the cladding region, which is known as evanescent wave. The amplitude of the evanescent wave decays exponentially in the cladding region. So the cladding (with lower refractive index) absorbs a small portion of propagating light energy. This process is known as attenuated total reflection (ATR). If an absorbing chemical or testing sample is present with the evanescent field region, the propagating light will be attenuated (as the reflection coefficient is less than unity) as it travels along the fiber. Since the energy levels associated with an atom or molecules are unique, the absorption spectrum serves as a "fingerprint" identification of the chemical species [1]. To increase the sensitivity of such sensors the cladding can be manufactured to be sensitive to specific organic vapors [21] or an unclad fiber can be coated with sensitive coatings [22]. Another alternative way is diminishing the cladding thickness, which results in a more fragile but more sensitive sensor [6, 20]. Gas field ionization sensors identify unknown gases by their unique ionization characteristics [23]. Calibration of these devices is based on fingerprinting breakdown voltage of the target gases. The gas sensor is made of two parallel planar electrodes. The two plates are separated by pieces of insulating films such a way that there is enough opening between the two electrodes to allow the gas flow between them. Applied voltage across the device is swept and insulation-toconduction transformation of the gas (known as breakdown), using I-V characteristics of electrical discharge, is recorded [23]. This technique has improved selectivity property of the gas sensors, as it is approved that at a constant pressure and temperature, each gas has unique breakdown voltage (or breakdown electric field) [9]. An applied voltage required to generate breakdown electric field (Eapp), depends on the separation gap between the plates and typically is over several hundred volts [24–26]. As applying this voltage is not practical, to scale down the breakdown voltage of the gases, 1D nanostructures could be applied as one of the electrodes. Enhanced local electric field (Eloc) is created at the tip of the nanostructures due to non-uniform distribution of charged carriers [27, 28]. Field enhancement factor (β) characterizes the level of influence of the 1D nanostructures onto the electrostatic field distribution and can be defined as the field gain

In this work we have explored an example of gas ionization sensor that is fabricated based on p-type silicon (Si) nanostructures. The ionization characteristics of several gases are reported. The effect of applying these structures on the field enhancement factor of the sensor, compared to a parallel plate system, is described.

Nanowires amplify Eloc regardless of their bias direction. However depending on the type of the materials of the nanowires some promote field emission, and others

Shemshad et al. [19, 20] investigated the absorption band of methane and stated that absorbance spectrum of methane is between 1620 and 1700 nm which is suitable for fiber optic detection sensors. As methane is released during coal extractions, they have also studied the cross sensitivity of methane with other gases emitted from mine. Their results showed that the absorbance spectrum of methane

*DOI: http://dx.doi.org/10.5772/intechopen.84264*

does not interfere with other gases in the mine.

absorption line for the methane gas.

#### *Miniaturized Gas Ionization Sensor Based on Field Enhancement Properties of Silicon… DOI: http://dx.doi.org/10.5772/intechopen.84264*

sensor was in a good agreement with experimental results and both showed a single absorption line for the methane gas.

Shemshad et al. [19, 20] investigated the absorption band of methane and stated that absorbance spectrum of methane is between 1620 and 1700 nm which is suitable for fiber optic detection sensors. As methane is released during coal extractions, they have also studied the cross sensitivity of methane with other gases emitted from mine. Their results showed that the absorbance spectrum of methane does not interfere with other gases in the mine.

In intrinsic fiber optic gas sensors, light propagates inside the core continuously without any external interaction. When a light is propagating through fiber optics, at the core-cladding boundary it undergoes total internal reflection (TIR). During each TIR it penetrates into the cladding region, which is known as evanescent wave. The amplitude of the evanescent wave decays exponentially in the cladding region. So the cladding (with lower refractive index) absorbs a small portion of propagating light energy. This process is known as attenuated total reflection (ATR). If an absorbing chemical or testing sample is present with the evanescent field region, the propagating light will be attenuated (as the reflection coefficient is less than unity) as it travels along the fiber. Since the energy levels associated with an atom or molecules are unique, the absorption spectrum serves as a "fingerprint" identification of the chemical species [1]. To increase the sensitivity of such sensors the cladding can be manufactured to be sensitive to specific organic vapors [21] or an unclad fiber can be coated with sensitive coatings [22]. Another alternative way is diminishing the cladding thickness, which results in a more fragile but more sensitive sensor [6, 20].

Gas field ionization sensors identify unknown gases by their unique ionization characteristics [23]. Calibration of these devices is based on fingerprinting breakdown voltage of the target gases. The gas sensor is made of two parallel planar electrodes. The two plates are separated by pieces of insulating films such a way that there is enough opening between the two electrodes to allow the gas flow between them. Applied voltage across the device is swept and insulation-toconduction transformation of the gas (known as breakdown), using I-V characteristics of electrical discharge, is recorded [23]. This technique has improved selectivity property of the gas sensors, as it is approved that at a constant pressure and temperature, each gas has unique breakdown voltage (or breakdown electric field) [9]. An applied voltage required to generate breakdown electric field (Eapp), depends on the separation gap between the plates and typically is over several hundred volts [24–26]. As applying this voltage is not practical, to scale down the breakdown voltage of the gases, 1D nanostructures could be applied as one of the electrodes. Enhanced local electric field (Eloc) is created at the tip of the nanostructures due to non-uniform distribution of charged carriers [27, 28]. Field enhancement factor (β) characterizes the level of influence of the 1D nanostructures onto the electrostatic field distribution and can be defined as the field gain coefficient (β = Eloc/Eapp) [27].

In this work we have explored an example of gas ionization sensor that is fabricated based on p-type silicon (Si) nanostructures. The ionization characteristics of several gases are reported. The effect of applying these structures on the field enhancement factor of the sensor, compared to a parallel plate system, is described.
