**6.1 Estimation of βtol**

To calculate the total field gain coefficient (βtol) based on Eq. (3), I-V characteristics of a PPL sensor is compared to that of the GITS. The parallel-plates sensor consists of two plates, one perfectly smooth p-type Si wafer as the anode and an aluminum coated Si wafer as the cathode, separated by 100 μm gap. GITS sensor is fabricated as explained in Section 5 with the same separation gap of 100 μm between the electrodes.

Ohmic regions of both PPL sensor and fabricated GITS, under vacuumed air (10<sup>−</sup><sup>5</sup> Torr), are shown in **Figure 8**. The total enhancement factor of the device is calculated using Eq. 3. Accordingly, the enhancement factor of the device is equal to:

$$\beta\_{tol} = \frac{Slope\_{GIS}}{Slope\_{PPL}} \approx \frac{3 \times 10^{-5}}{8 \times 10^{-9}} = 3,750 \,\tag{5}$$

Theoretically, no tunneling occurs at electric fields less than 109 V/m. The electric filed strength in the parallel plate sensor is in the range of 105 –106 V/m according to applied voltage (100 V is the maximum voltage of HP4155-SMU). This value should at least increases by 1000 times to make the tunneling possible. Our estimated value of βtol, indicates that the geometrical field enhancement of fabricated GITS induces the required electric field for the field ionization tunneling.

#### **6.2 Characterization of the gas sensor**

Field ionization tunneling tests were performed at 10<sup>−</sup><sup>2</sup> Torr for several gases, while separation gap between the electrodes was set at 200 μm. A 20 mA current compliance is applied to both SMUs in order to protect nanowires from burning.

**Figure 9** shows the same trend for I-V characteristics of all the gases. Below 20 V the gas discharge is in its ohmic region, in which current is low and is due to the movement of radiation-generated charged particles. At the voltages above the ohmic region, contrary to ionization discharge curve of a PPL sensor (shown in

#### **Figure 8.**

*Ohmic regions of the PPL sensor and the fabricated GITS at 10<sup>−</sup><sup>5</sup> Torr vacuumed air. The slopes of the curves are used to calculate the enhancement factor of the system.*

**109**

is explained earlier.

the gases.

**Figure 9.**

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

**Figure 2**) which reaches to a saturated current, the discharge current of fabricated GITS increases sharply as the applied voltage is increased. In this region the electric field is higher than ionization threshold and the valence electrons of the gases are able to tunnel through the potential barrier of the gas atom, into the unoccupied

*the gases. The transition from ohmic region to tunneling region can be seen at 20 V, where the tunneling current* 

Each gas reaches to a quasi-breakdown (Vqbr) at a specific voltage. At Vqbr, an abrupt rise in the tunneling current is due to the cathode bombardment by positive ions, which results in gas amplification due to secondary emissions of the Al layer. Large number of positive ions is generated during the gas amplification and move toward the cathode to get neutralized. These positive ions create an internal electric field in opposite direction to the applied electric field. Consequently the total electric field is reduced, which reduces the rate of ionization and as a result the tunneling current reaches a plateau. After neutralization of the accumulated positive ions,

the tunneling current is increased again until reaching current compliance.

The Vqbr and threshold of plateau can be used as calibrating data to distinguish

**Figure 10** represents the room temperature I-V characteristics of the fabricated GITS for He (**Figure 10a**) and Ar (**Figure 10b**) at different pressures. According to the results, no correlation between the tunneling current and pressure can be addressed. However, the Vqbr for the both gases is raised as the pressure is increased. This increase in Vqbr can be explained by reduced mean free path (the average distance traveled by a particle to make successive ionizing collisions). As the mean free path is reduced in higher pressures, the electrons acquire enough energy to create

The experiments are extended to test the effect of separation gap between the electrodes on I –V characteristics of the sensor. For this reason, the same device

shown in **Figure 11**, by reducing the separation gap to 50 μm, the sensor showed a complete breakdown at 50 V due to the high electric field strength. At 100 μm a complete breakdown was observed at 60 V. Lower electric field in the devices with 200 μm separation gap, resulted in quasi breakdown and a threshold of plateau that

Torr. As it is

 *Torr show distinctive tunneling properties of* 

with 50, 100 and 200 μm separation gaps was tested for Ar at 10<sup>−</sup><sup>2</sup>

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

energy states of the anode electrode.

*Room temperature I-V characteristics of Ar, N2, He and O2 at 10<sup>−</sup><sup>2</sup>*

*increased steeply (reproduced from Abedini Sohi & Kahrizi [38]).*

impact ionizations at higher electric fields.

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

#### **Figure 9.**

*Nanostructures*

**6. Results and discussions**

**6.1 Estimation of βtol**

between the electrodes.

β*tol* = \_\_\_\_\_\_\_

**6.2 Characterization of the gas sensor**

(10<sup>−</sup><sup>5</sup>

from 0 to 100 V, with steps of 1 V and 1 second sweep delay. The sensor was tested for oxygen (O2), argon (Ar), Nitrogen (N2) and helium (He) at low pressures.

To calculate the total field gain coefficient (βtol) based on Eq. (3), I-V characteristics of a PPL sensor is compared to that of the GITS. The parallel-plates sensor consists of two plates, one perfectly smooth p-type Si wafer as the anode and an aluminum coated Si wafer as the cathode, separated by 100 μm gap. GITS sensor is fabricated as explained in Section 5 with the same separation gap of 100 μm

Ohmic regions of both PPL sensor and fabricated GITS, under vacuumed air

*SlopeGIS SlopePPL*

Theoretically, no tunneling occurs at electric fields less than 109

electric filed strength in the parallel plate sensor is in the range of 105

Field ionization tunneling tests were performed at 10<sup>−</sup><sup>2</sup>

 Torr), are shown in **Figure 8**. The total enhancement factor of the device is calculated using Eq. 3. Accordingly, the enhancement factor of the device is equal to:

<sup>≈</sup> <sup>3</sup> <sup>×</sup> <sup>10</sup>−5 \_\_\_\_\_\_

according to applied voltage (100 V is the maximum voltage of HP4155-SMU). This value should at least increases by 1000 times to make the tunneling possible. Our estimated value of βtol, indicates that the geometrical field enhancement of fabricated GITS induces the required electric field for the field ionization tunneling.

while separation gap between the electrodes was set at 200 μm. A 20 mA current compliance is applied to both SMUs in order to protect nanowires from burning. **Figure 9** shows the same trend for I-V characteristics of all the gases. Below 20 V the gas discharge is in its ohmic region, in which current is low and is due to the movement of radiation-generated charged particles. At the voltages above the ohmic region, contrary to ionization discharge curve of a PPL sensor (shown in

<sup>8</sup> <sup>×</sup> <sup>10</sup>−9 <sup>=</sup> 3, <sup>750</sup> (5)

V/m. The

V/m

–106

Torr for several gases,

**108**

**Figure 8.**

*Ohmic regions of the PPL sensor and the fabricated GITS at 10<sup>−</sup><sup>5</sup>*

*are used to calculate the enhancement factor of the system.*

 *Torr vacuumed air. The slopes of the curves* 

*Room temperature I-V characteristics of Ar, N2, He and O2 at 10<sup>−</sup><sup>2</sup> Torr show distinctive tunneling properties of the gases. The transition from ohmic region to tunneling region can be seen at 20 V, where the tunneling current increased steeply (reproduced from Abedini Sohi & Kahrizi [38]).*

**Figure 2**) which reaches to a saturated current, the discharge current of fabricated GITS increases sharply as the applied voltage is increased. In this region the electric field is higher than ionization threshold and the valence electrons of the gases are able to tunnel through the potential barrier of the gas atom, into the unoccupied energy states of the anode electrode.

Each gas reaches to a quasi-breakdown (Vqbr) at a specific voltage. At Vqbr, an abrupt rise in the tunneling current is due to the cathode bombardment by positive ions, which results in gas amplification due to secondary emissions of the Al layer. Large number of positive ions is generated during the gas amplification and move toward the cathode to get neutralized. These positive ions create an internal electric field in opposite direction to the applied electric field. Consequently the total electric field is reduced, which reduces the rate of ionization and as a result the tunneling current reaches a plateau. After neutralization of the accumulated positive ions, the tunneling current is increased again until reaching current compliance.

The Vqbr and threshold of plateau can be used as calibrating data to distinguish the gases.

**Figure 10** represents the room temperature I-V characteristics of the fabricated GITS for He (**Figure 10a**) and Ar (**Figure 10b**) at different pressures. According to the results, no correlation between the tunneling current and pressure can be addressed. However, the Vqbr for the both gases is raised as the pressure is increased. This increase in Vqbr can be explained by reduced mean free path (the average distance traveled by a particle to make successive ionizing collisions). As the mean free path is reduced in higher pressures, the electrons acquire enough energy to create impact ionizations at higher electric fields.

The experiments are extended to test the effect of separation gap between the electrodes on I –V characteristics of the sensor. For this reason, the same device with 50, 100 and 200 μm separation gaps was tested for Ar at 10<sup>−</sup><sup>2</sup> Torr. As it is shown in **Figure 11**, by reducing the separation gap to 50 μm, the sensor showed a complete breakdown at 50 V due to the high electric field strength. At 100 μm a complete breakdown was observed at 60 V. Lower electric field in the devices with 200 μm separation gap, resulted in quasi breakdown and a threshold of plateau that is explained earlier.

**Figure 10.**

*Room temperature I – V characteristics of He (a) and Ar (b) at a wide range of pressures (0.01–10 Torr) (part (b) is reproduced from Abedini Sohi & Kahrizi [38]).*

**Figure 11.**

*Fabricated GITS is tested for Ar at 10<sup>−</sup><sup>2</sup> Torr for different separation gaps. By increasing the separation gap between the electrodes, the transition from complete breakdown to quasi breakdown is observed.*

**111**

provided the original work is properly cited.

Parsoua Abedini Sohi and Mojtaba Kahrizi\* Concordia University, Montreal, QC, Canada

\*Address all correspondence to: mojtaba.kahrizi@concordia.ca

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

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

A novel fabrication technique, based on consecutive chemical and electrochemical etching techniques, is used to fabricate Si nanostructures. Si surface is textured by pyramidal hillocks through anisotropic etching in TMAH based solution. Electrochemical etching (anodic etching) of the textured Si was carried out in a HF-based solution in an electrochemical cell. Non-uniform distribution of the electric field induces different level of etching rate over the anode, which results in formation of the arrow shape structures. Mechanism of the developed structures is investigated by modeling and simulation by COMSOL multiphysics. Fabricated structures were applied as the anode in GITS. The total field enhancement coefficient (βtol) of the GITS is estimated based on the ohmic region of the gas discharge characteristics, as compared to a parallel-plates sensor. Field penetration and band bending at the surface of p-type nanostructures lead to tunneling current in the range of mA in low voltages and as a result, the fabricated Si nanostructure based GITS showed

the capability to distinguish the unknown gases as well as the gas pressure.

This work was partially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) and by the Gina Cody school of Engineering

Parsou Abedini Sohi and Mojtaba Kahrizi declare that this article does not

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

**7. Conclusion**

**Acknowledgements**

**Conflict of interest**

**Author details**

contain any conflict of interest.

and Computer Science at Concordia University.

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