**2. Geometrical field enhancement**

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 will enhance field ionization tunneling phenomenon.

*Nanostructures*

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

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

optic based gas sensors [5–8] and gas ionization sensors [9].

**100**

In field emission based gas sensors, metallic or highly doped n-type semiconductor nanostructures are applied as the cathode (negatively biased), in which enhanced local electric field induces the emission of the electrons from the tip of the nanostructures [9]. In this phenomenon, as the name suggested, under a strong electric field electrons will overcome the deformed potential barrier of the vacuum and will be emitted from the surface of the cathode [29]. The diagram shown in **Figure 1a** qualitatively illustrates the electron emission from a metallic cathode. The shape and width of the metal-surface potential barrier is influenced by the strength of the applied electric field [30]. When the barrier is narrow enough electrons can escape through the barrier, thereby forming a field electron emission. In a gas sensor, field emission dominates the breakdown mechanism of the gases and reduces the breakdown voltage significantly. This phenomenon occurs as field emitted electrons contribute in impact ionization of gas atoms. There are several materials reported for field emission applications. Among these materials carbon nanotubes have been actively studied as field emitters [31]. The use of multi-walled carbon nanotubes is frequently reported in field emission gas ionization sensors, which reduced the breakdown voltage of the gases compared to planar parallel plate [32–34]. Metallic nanowires such as gold and silver have also shown field emission properties and are employed as the cathode in gas ionization sensors [35, 36].

In field ionization tunneling sensors, p-type semiconductor nanostructures are applied as the anode (positively biased). Due to enhanced electric field at the tip of the nanostructures, valence electrons of gas atoms can escape from the atom by tunneling through the nucleus potential barrier into available energy states of the p-type nanostructures (as shown in **Figure 1b**). In these sensors tunneling currents can be used as the calibrating data as it occurs at lower voltages compared to complete breakdown [36, 37]. However in some cases (depending on the separation gap distance, gas pressure and the material of the cathode) bombardment of the cathode by the released positive ions may result in secondary emissions, which can contribute to either quasi or complete breakdown [38].

There are several analytical studies reported in literature to calculate field enhancement factor (β) of the nanostructures [39, 40]. These studies mathematically predicted the field enhancement factor of a single protrusion; however, the interpretation of the average field gain coefficient (βtol), considering constructive/ destructive interferences of the local electric field of thousands of nanowires in the whole structure, is desired to optimize the design and structure of the gas sensors.

#### **Figure 1.**

*(a) Field electron emission mechanism from metallic cathode. At large electric fields the potential barrier is bended, which allows electrons escape through the barrier toward the vacuum, (b) valence electrons of gas atom, tunnels through the potential barrier of the nucleus into the available energy states of p-type anode (part b is reproduced from Abedini Sohi & Kahrizi [38]).*

**103**

**Figure 2.**

*I-V characteristics of gas discharge in a uniform electric field.*

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

emission current and can be experimentally calculated by plotting ln (J/V2

increased. In this region the discharge current density is expressed as:

(1/V) where J is the emission current density and V is the applied voltage [41, 42].

In field emission based gas sensors, Fowler Nordheim (FN) theory of electron emission from a metallic or semi-metallic roughly predicts βtol based on the field induced

In the second approach, this factor can be estimated practically using the slope of the gas discharge graph (I-V characteristics) in the ohmic region of the curves (for field emission gas sensors as well as field ionization tunneling sensors). **Figure 2** shows different regions of discharge characteristic of gases in a uniform electric field generated between planer parallel plates. Any gas contains free-floating negative electrons and positive ions due to the ionization of the gas atoms by cosmic radiation. In region I of the discharge curve, when the voltage is applied to the electrodes, the traveling radiation-generated charged particles produce current as electrons migrate to the anode and the ions move toward the cathode. The current in this region shows strong electric field dependence and increases as the applied voltage is

*JGIS* = *σGas* × *Eapp* = (*ne μe* + *ni μi*) × *e* × *Eapp* (1)

where σGas is the gas conductivity, *e* the electron charge, *Eapp* the applied electric field, *ne* and *ni* the electron and ion concentrations respectively, and *μe*, and *μi* are

In saturation region (II), the current reaches its saturated value which means that all radiation-generated particles are attracting to the electrodes. At the last phase the electric field strength would be enough to accelerate the electrons, resulting ionizing collisions. At breakdown voltage (Vb) the discharge current is self-sustainable and is

*JGIS* = *σGas*×*Eeff* = (*neμe* + *niμi*)×*e*×*βtol*×*Eapp* (2)

maintained due to ionizing collisions without any external ionization source. According to Eq. (1) in the ohmic region, the traveling radiation-generated charged particles produce current proportional to applied field. As in this region there is no ionization-induced current, the current density exclusively depends on the strength of the electric field. The current density of nanowire based gas ioniza-

tion sensor (*JGIS*) due to enhanced electric field can be expressed as

) against

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

the electron and ion mobility respectively.

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

In field emission based gas sensors, Fowler Nordheim (FN) theory of electron emission from a metallic or semi-metallic roughly predicts βtol based on the field induced emission current and can be experimentally calculated by plotting ln (J/V2 ) against (1/V) where J is the emission current density and V is the applied voltage [41, 42].

In the second approach, this factor can be estimated practically using the slope of the gas discharge graph (I-V characteristics) in the ohmic region of the curves (for field emission gas sensors as well as field ionization tunneling sensors). **Figure 2** shows different regions of discharge characteristic of gases in a uniform electric field generated between planer parallel plates. Any gas contains free-floating negative electrons and positive ions due to the ionization of the gas atoms by cosmic radiation. In region I of the discharge curve, when the voltage is applied to the electrodes, the traveling radiation-generated charged particles produce current as electrons migrate to the anode and the ions move toward the cathode. The current in this region shows strong electric field dependence and increases as the applied voltage is increased. In this region the discharge current density is expressed as:

$$J\_{\rm GLS} = \sigma\_{\rm Gats} \times E\_{\rm app} = \{ n\_{\epsilon} \mu\_{\epsilon} + n\_{i} \mu\_{i} \} \times e \times E\_{\rm app} \tag{1}$$

where σGas is the gas conductivity, *e* the electron charge, *Eapp* the applied electric field, *ne* and *ni* the electron and ion concentrations respectively, and *μe*, and *μi* are the electron and ion mobility respectively.

In saturation region (II), the current reaches its saturated value which means that all radiation-generated particles are attracting to the electrodes. At the last phase the electric field strength would be enough to accelerate the electrons, resulting ionizing collisions. At breakdown voltage (Vb) the discharge current is self-sustainable and is maintained due to ionizing collisions without any external ionization source.

According to Eq. (1) in the ohmic region, the traveling radiation-generated charged particles produce current proportional to applied field. As in this region there is no ionization-induced current, the current density exclusively depends on the strength of the electric field. The current density of nanowire based gas ionization sensor (*JGIS*) due to enhanced electric field can be expressed as

$$J\_{\rm GLS} = \sigma\_{\rm Ga} \times E\_{\rm eff} = \{n\_{\epsilon}\mu\_{\epsilon} + n\_{i}\mu\_{i}\} \times \epsilon \times \beta\_{\rm tal} \times E\_{\rm app} \tag{2}$$

**Figure 2.** *I-V characteristics of gas discharge in a uniform electric field.*

*Nanostructures*

In field emission based gas sensors, metallic or highly doped n-type semiconductor nanostructures are applied as the cathode (negatively biased), in which enhanced local electric field induces the emission of the electrons from the tip of the nanostructures [9]. In this phenomenon, as the name suggested, under a strong electric field electrons will overcome the deformed potential barrier of the vacuum and will be emitted from the surface of the cathode [29]. The diagram shown in **Figure 1a** qualitatively illustrates the electron emission from a metallic cathode. The shape and width of the metal-surface potential barrier is influenced by the strength of the applied electric field [30]. When the barrier is narrow enough electrons can escape through the barrier, thereby forming a field electron emission. In a gas sensor, field emission dominates the breakdown mechanism of the gases and reduces the breakdown voltage significantly. This phenomenon occurs as field emitted electrons contribute in impact ionization of gas atoms. There are several materials reported for field emission applications. Among these materials carbon nanotubes have been actively studied as field emitters [31]. The use of multi-walled carbon nanotubes is frequently reported in field emission gas ionization sensors, which reduced the breakdown voltage of the gases compared to planar parallel plate [32–34]. Metallic nanowires such as gold and silver have also shown field emission properties and are employed as the cathode in gas ionization sensors [35, 36].

In field ionization tunneling sensors, p-type semiconductor nanostructures are applied as the anode (positively biased). Due to enhanced electric field at the tip of the nanostructures, valence electrons of gas atoms can escape from the atom by tunneling through the nucleus potential barrier into available energy states of the p-type nanostructures (as shown in **Figure 1b**). In these sensors tunneling currents can be used as the calibrating data as it occurs at lower voltages compared to complete breakdown [36, 37]. However in some cases (depending on the separation gap distance, gas pressure and the material of the cathode) bombardment of the cathode by the released positive ions may result in secondary emissions, which can

There are several analytical studies reported in literature to calculate field enhancement factor (β) of the nanostructures [39, 40]. These studies mathematically predicted the field enhancement factor of a single protrusion; however, the interpretation of the average field gain coefficient (βtol), considering constructive/ destructive interferences of the local electric field of thousands of nanowires in the whole structure, is desired to optimize the design and structure of the gas sensors.

*(a) Field electron emission mechanism from metallic cathode. At large electric fields the potential barrier is bended, which allows electrons escape through the barrier toward the vacuum, (b) valence electrons of gas atom, tunnels through the potential barrier of the nucleus into the available energy states of p-type anode (part* 

contribute to either quasi or complete breakdown [38].

**102**

**Figure 1.**

*b is reproduced from Abedini Sohi & Kahrizi [38]).*

where *Eeff* is the enhanced electric field and βtol is average field gain coefficient.

By comparing Eqs. (1) and (2) and considering a constant *σGas*, βtol of gas ionization sensors can be estimated by dividing the slopes of I-V characteristics of the device with a parallel-plates in the ohmic region.

$$\mathfrak{R}\_{tol} = \frac{\text{Slope}\_{GIS}}{\text{Slope}\_{PPL}} \tag{3}$$

where *SlopeGIS* is the slope of I-V discharge graph of the nanowire based GIS and *SlopePPL* is the slope of the I-V discharge graph of the parallel plate in their ohmic regions.
