Design and analysis of electrostatic-charge plasma based dopingless IGZO vertical nanowire FET for ammonia gas sensing

Accepted Manuscript Design and analysis of electrostatic-charge plasma based dopingless IGZO vertical nanowire FET for ammonia gas sensing Neha Jayaswal, Ashish Raman, Naveen Kumar, Sarabdeep Singh PII:

S0749-6036(18)31620-3

DOI:

https://doi.org/10.1016/j.spmi.2018.11.009

Reference:

YSPMI 5950

To appear in:

Superlattices and Microstructures

Received Date: 9 August 2018 Revised Date:

23 October 2018

Accepted Date: 11 November 2018

Please cite this article as: N. Jayaswal, A. Raman, N. Kumar, S. Singh, Design and analysis of electrostatic-charge plasma based dopingless IGZO vertical nanowire FET for ammonia gas sensing, Superlattices and Microstructures (2018), doi: https://doi.org/10.1016/j.spmi.2018.11.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Design and Analysis of Electrostatic-Charge Plasma based Dopingless IGZO Vertical Nanowire FET for Ammonia Gas Sensing Neha Jayaswal, Ashish Raman, Naveen Kumar, Sarabdeep Singh

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Department of Electronics and Communication Engineering, NIT Jalandhar, India

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Abstract: In this paper, Dopingless Gate All Around (GAA) Vertical Nanowire Field Effect Transistor (VNWFET) is designed with artificial material Indium Gallium Zinc Oxide (IGZO) as a channel material. IGZO channel has high electron mobility compared to more traditional amorphous semiconductors. In VNWFETs, since the channel length (Lch) is characterized vertically, it can be relaxed without area penalty on-chip, which in turn also allows some relaxation in the nanowire diameter while keeping optimum shortchannel-effects control. Electrostatic-Charge Plasma technique is used to form a source-drain region on an intrinsic body of IGZO material. At the source side, the N+ region is formed by selecting the appropriate work function of the metal electrode, and at the drain side, the N+ region is formed by giving biasing to the metal electrode. N+ channel dopingless VNWFET with the catalytic metal gate is proposed for ammonia gas sensing. Cobalt, Molybdenum, and Ruthenium are used as a gate electrode in ammonia gas detection due to their high reactivity towards ammonia. Also, we have compared their ON and OFF sensitivity of the proposed device toward the gas adsorption. Due to the presence of gas on the gate, the metal work function of gate metal changes which varies the OFF-current (IOFF), ON-current (ION) and Threshold voltage (Vth) as these are considered as sensitivity parameters for sensing the ammonia gas molecules. The dimensional parameters (radius, and length) and dielectric materials are varied to check the change in device sensitivities. Results show that as the work function varies increases 50, 100, 150, 200meV and 250meV for catalytic metal at the gate, the sensitivity is increased. Keywords—Vertical nanowire FET (VNWFET), Ammonia Gas sensor, Indium Gallium Zinc Oxide (IGZO), Electrostatic-Charge Plasma (E-CP)

I.

INTRODUCTION

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Over the past decades, aggressive and continuous transistor scaling according to Moore’s law has provided ever-increasing device performance and density [1] [2], but the short channel is a serious issue due to scaling. Therefore, a nanowire structure is designed to occur with better performance. To keep the development pace, a few alternatives can be considered as far as material decisions, device models, and circuits design. However, more scaling utilizing ordinary 2D formats has limitations by key factors, such as the physical limits on gate length/metal, metal-contact placement, and interconnect routing congestion. A Vertical Nanowire Field Effect Transistor (VNWFET) is a better device to conquer a portion of these constraints [1]. In VNWFETs, since the channel length (Lch) is characterized vertically, the base area can be changed without affecting the channel length, which additionally permits some changing in the Nanowire (NW) thickness, so Short Channel Effects (SCEs) can be reduced to negligible values. The proposed device is designed with artificial material Indium gallium zinc oxide (IGZO) because of its transparency, high mobility, and temperature stability. The most significant advantage of using IGZO is due to its transparency at visible wavelengths, which have shown to possess surprisingly high electron mobility compared to more traditional amorphous semiconductors such as Silicon and organic semiconductors. IGZO material prevents mobility degradation due to external photon absorption. Amorphous silicon has the low mobility of 1cm2V-1s-1, is sensitive to visible light, and has required extensive development to overcome dangling bond passivation and & stability issues that can lead to troublesome threshold voltage shifts. IGZO transistors do not require dangling bond passivation strategies. In addition to the low OFF current of IGZO transistors, their speed and power consumption is improved [2]. Continuous downscaling of CMOS innovation has significantly enhanced the performance, functionality, and packaging density at the cost of ascending in power dissipation and complex manufacture procedure of nanoscale CMOS gadgets. The ascent in power dissipation is because of faster switching and subthreshold [3]. Manufacture process with the high thermal budget requirement is complex because of ion-implantation and thermal annealing techniques. There is another problem of random dopant fluctuations (RDFs), subsequently; process variations related issues could degrade the sensitivity of a FET-based sensor [4]. To remove these issues, a dopingless device was proposed. The dopingless device employs intrinsic doping through whole body structure without externally doped source/drain (S/D) regions. Intrinsic nature of silicon nanowire provides less susceptibility towards process parameter and temperature variations. For making the p+ and n+ S/D regions, the polarity gate (PG) concept was employed [5]. There are two strategies for plan dopingless device, Electrostatic technique, and Charge Plasma. In the electrostatic technique, appropriate bias is connected at polarity gates for inducing the desired type of carriers in the desired level [6]. This process is referred to as electrostatic doping (ED). The ED disposes of necessities of external doping;

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thereby yields, low thermal budget and simplified manufacture process [7]. Apart from that, it also facilitates dynamic reconfigurability.

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In charge plasma technique, a “p” source and “n” drain are formed on an intrinsic body by using a suitable work function of metal electrodes. This device provides the uniform distribution of electron concentrations at both the top and bottom of the source/drain electrode, which gives better control and a large hike in ON state current [8]. The major concern of the electrostatic technique is power consumption due to the use of extra biasing at the source side. In charge Plasma devices [9], fabrication requires an extra metallization process step due to the need of metals with different work functions. Electrostatic-Charge Plasma technique is used to design dopingless VNWFET to form source-drain region on an intrinsic body. At the source side n region is formed by selecting the appropriate work function of the metal electrode at drain side, N region is formed by giving biasing at meal electrode considering charge plasma technique. The electrostatic technique is used at a drain side; the metal deposited over oxide layer is used for drain side polarity gate with applied Vds. Therefore, the fabrication process becomes simpler as compared to individual dopingless techniques. Charge plasma technique is used on the source side, where appropriate work function metal is used for source metal to develop n-type doping in the source. Therefore, no requirement of the extra biasing source at the source is needed and overall power consumption is reduced.

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Indium gallium zinc oxide (IGZO) is an artificial semiconducting material consisting of indium (In), Gallium (Ga), zinc (Zn) and oxygen (O). High-performance amorphous IGZO transistors were fabricated with an extracted field effect mobility of 11-15 cm2V-1s-1, ON/OFF current ratio > 107, subthreshold gate voltage swing of 0.20-0.25 V/decade, low OFF-state current and good saturation. Low and tunable threshold voltages of 1-2V were achieved for IGZO transistor [2]. Ammonia (NH3) detection has gotten impressive consideration in the fields of agricultural, environmental monitoring, chemical and pharmaceutical processing, and disease diagnosis. Nowadays MOSFET based gas sensors and chemical sensors are mostly used because of their low cost [10]. Due to the large volume to ratio, this sensor shows the better sensitivity with higher selectivity and fast response at room temperature with the lowest power consumption [11]. Vertical nanowire with catalytic metal gate film is designed for ammonia gas detection. Cobalt, Molybdenum, and Ruthenium are used as a catalytic metal for high sensitivity [12], [13]. The sensitivity of the proposed device has a critical role in the designing of the ammonia gas sensor. This paper is divided into 5-sections; the Structure and parameter of IGZO based VNWFET with 20nm channel length explains in section-2. The working principle of the ammonia gas sensor is explained in section-3. Performance analysis and comparison of the sensor are given in section-4 and conclusion of this paper is summarized in the section-5. II. DEVICE STRUCTURE AND SIMULATION PARAMETERS

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For designing Electrostatic-Charge plasma-based dopingless IGZO-VNWFET, the Simulation was performed through Silvaco Atlas tool considering as three-dimensional system framework [14]. 3D Cubicle geometry is defined for design IGZO VNWFET shown in Fig 1(a), and the 2D cross-sectional structure is shown in Fig 1 (b). IGZO is a high mobility material, which is used as a body material. The length (Lch) and thickness (Tch) of the channel are taken as 20nm and 10nm respectively. Drain and source length is 20nm respectively. Drain and source length is 20nm. HfO2 is used as an oxide layer, and the thickness is 1.1nm. To implement electrostatic-charge plasma technique NW source, drain and gate are kept intrinsic.

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(b)

(c)

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Fig. 1. (a) Simulated 3D Cubicle structure of IGZO VNWFET (b) 2D cross-sectional view of IGZO VNWFET (c) Electron conc. across the proposed device at ON & OFF state

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Electrostatic technique (E-CP) is used at a drain side, so that N+ Si Metal is used for drain side metal, and Vds is applied at it. Charge plasma (CP) technique is used for source side, so low work function (3.9eV) metal (hafnium) is used for source side polarity gate to develop N+ type doping in the source. The drain metal is polysilicon with default work function of 4.17. Polarity gate length is 10nm at both side, and spacer thickness is 10nm. For designing of ammonia gas sensor high sensitivity, in IGZO VNWFET catalytic metal as a gate metal is proposed of 1nm thickness. Co, Mo, and Ru are good catalytic reactive metal for ammonia gas sensor. Drain to source voltage (Vds) and gate voltage (Vgs) values are 1V and 1.5V respectively. Figure 1(c) shows the electron concentration across the device confirming the effect of source metal (work function=3.9) and drain bias. The increase in electron conc. shows the proper working of the proposed device within the limits of Fermi-Dirac statistics. TABLE I.

Parameters used for Device Simulations

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Parameters Channel length Channel Thickness Spacer thickness Gate oxide thickness Doping Drain voltage Work function of Gate Work function of Drain Work function of Source

JL-NWFET Lch Tch tspac Tox ND Vds φmg φmd φms

CP-NWFET 20nm 10nm 10nm 1.1nm 1015/cm3 1V 4.8eV 4.17eV 3.9eV

Alignment of simulated IGZO VNWFET has been performed by the analytical outcomes got by [1] utilizing distinctive models representing FLDMOB (Field-dependent-mobility), CONMOB (Concentration-dependent-mobility) used as mobility model and SRH (Shockley–Read–Hall model) used as generation/recombination model. BQP.NALPHA and BQP.NGAMMA are also used to invoke alpha and gamma value of electrons with default values 0.5 and 1.2 respectively. III. DEVICE CHARACTERISTICS AND SIMULATION The simulation study of the energy band diagram, carrier concentration, electric field and channel potential helps in understanding the physics involved in the conduction mechanism of the proposed device structure of IGZO VNWFET.

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0.9 0.5 0.1 -0.7

Drain

Gate

-1.1

Cobalt

-1.5 -1.9

Source

Ruthenium Molybdenum

(a)

(b)

-2.3 -2.7 -3.1 0.005

0.015 0.025 0.035 0.045 X Coordination (µm)

0.055

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Energy (eV)

-0.3

Fig. 2. Conduction band energy and Valance band energy in (a) OFF state (b) ON state

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Fig 2 (a) shows conduction and valence band energy of the proposed device in OFF state. The higher work function of cobalt (Cu) as compared to Ruthenium (Ru) and Molybdenum (Mo) shifts the energy band higher due to a larger concentration of induced p-type charges. Source metal has a low work function, which induces n-type charge carriers within the source region [15]. Higher applied voltage at drain metal also induces n-type charge carriers within the drain region, but charge plasma generated in the source region have higher n-type charge induction as compared to the drain region. Fig 2 (b) show conduction and valence band energy of the proposed device in ON state. When Vds applied at drain and Vgs applied at the gate, energy band shifts down from the drain side due to positive biasing [15]. In ON state, the electrons drift from source to drain that is responsible for the higher drain current.

18.9

18.7

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0.015 0.025 0.035 0.045 X Coordination (µm)

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18.3 0.005

Cobalt Ruthenium Molybdenum

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Hole Concentration (cm-3)

-3

(a)

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Electron concentration (cm-3)

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(b)

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-11 -15 -19 -23 -27

-31 0.005

Cobalt Ruthenium Molybdenum 0.015 0.025 0.035 0.045 X Coordination (µm)

0.055

Fig. 3. Electron Concentration (b) Hole Concentration of VNWFET at ON-state (Vds=1V, Vgs=1.5V)

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In Fig 3(a), source region have a higher concentration of electrons due to low work function but in spacer area electron is exponentially decreased due to diffusion of electrons from source to spacer region. In channel electron concentration increases due to inversion of charge carriers with positive gate voltage. At the channel-source junction, electron concentration is decreased from both sides. In the drain region, the concentration of electrons is high due to the positive drain voltage. Electrons are more in drain than channel due to the attraction of n-type charge carriers towards the drain (near drain-channel interface). In spacer region electron conc. is less, concentration is decreased from the drain and channel region interface. At the edge of drain e- is decreased due to the formation of the depletion layer due to the presence of Schottky contact with drain metal and drain semiconductor [16]. In Fig 3(b) hole concentration in the channel is less than source and hole conc. is less in drain region as compared to the channel due to lower work function & repulsion of holes from drain metal in ON state condition [16]. Cobalt having higher work function repels the p-type charge carriers from the channel region, which makes the channel have lowest hole conc. as compared to ruthenium and molybdenum.

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2.25 Cobalt Molybdenum

2.05

(e)

3.5E+5 2.5E+5

(f)

1.85 1.75 1.65

1.5E+5 5.0E+4 0.005

1.95

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5.5E+5 4.5E+5

Cobalt Ruthenium Molybdenum

2.15

Ruthenium

6.5E+5

Potential (V)

Electric Field (V/m)

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1.55 0.015 0.025 0.035 0.045 X Coordination (µm)

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1.45 0.005

0.015 0.025 0.035 0.045 X Coordination (µm)

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Fig. 4. Electric Field (b) Channel potential profile of VNWFET at ON-state (Vds=1V, Vgs=1.5V)

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Fig 4(a) shows the variation of the electric field across the device. The electric field can be depicted as the gradient of Fermi level across the length of the proposed device. When energy near the drain channel interface increases, electric field increase and near spacer/channel interface, the change in energy is low due to which electric field decreases. In gate region due to linear Fermi level, the electric field becomes constant. At gate-drain junction again change in Fermi energy increase then decrease and finally constant. Hence the electric field follows the same pattern. Fig 4(b) shows potential. Potential is the negative integral of the electric field, in the source region, the slope of the electric field is more but it decreases in spacer region, and it becomes constant in the gate region. At gate-drain junction slope again increases then decrease and it becomes constant in the source region [17].

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Calibration of simulated cuboidal JLNWFET has been performed according to the analytical results and is shown in fig. 5. The calibrated results validate the correct use of physical models for the simulated results. Ming-Hung Han et al. [18] for simulated parameters of channel length Lch=15nm, the thickness of channel, Tch=10nm, oxide thickness, Tox=1nm, and doping concentration, ND= 2*1019cm-3. The proposed device uses the same physical models required for the simulation of the device used for calibration, without affecting the operating limit of the defined models.

Drain Current, Id (µA/µm)

Analytical

5.E+0

Vds=1V Lch=15nm Tch=10nm Tox=1nm ND=2*1019cm-3

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-1

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-0.6 -0.4 Gate Voltage, Vgs (V)

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Fig. 5. Log Transfer characteristics of simulated device calibrated with ref. [18]

IV. IGZO VNWFET WORKING PRINCIPLE For the ammonia gas sensor, Cobalt, Molybdenum and Ruthenium metal are used as a gate electrode. These metals are highly sensitive towards ammonia gas molecules [12] [13]. Certain approximations are taken into consideration to simulate the gas sensor such as constant pressure and constant temperature. The gas flow rate may also affect the sensitivity of the device, therefore subtle environment with the uniform composition of ammonia gas in environment suits conveniently for the proper working of the device. The principle of ammonia gas sensor is based on the adsorption of ammonia gas molecules on the catalytic gate electrode, a chemical reaction occurs between ammonia gas atoms and the catalytic electrode, because of this gate metal work function is changed. The work function variation of the gate electrode exposed to ammonia gas affects the device

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characteristics and changes the ON-state current Ion, OFF-state current Ioff, and threshold voltage Vth. Therefore, for sensitivity estimation, the parameter mentioned above can be used for detecting the ammonia gas.

Fig. 6. Proposed IGZO VNWFET ammonia gas sensor with exposed ammonia gas on the gate metal

The proposed structure of the ammonia gas sensor based on IGZO VNWFET are shown in Fig 6. When the ammonia gas is adsorbed over the gate metal, Sensitivity can be estimated with respect to variation in OFF current, ON current and threshold voltage [20]. Equation (1) denotes the change in work function because of the reaction of the catalytic metal gate with gas particles [21]. ∆Φm= cont – [(

)* ln(P)]

(1)

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Where ‘∆Φm’ is variation in work function, ‘R’ is gas constant, ‘F’ is Faraday’s constant, ‘T’ is temperature, and ‘P’ is gas partial pressure. Work function depends on the molar concentration, partial pressure of the gas and any other variation. In this way, we can experience the presence of ammonia gas and can measure the concentration of ammonia gas by calculating the changes in OFF current, ON current and subthreshold voltage.

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The difference of work function can be measured from the eqn. (1). Due to a varying work function (∆Φm), Flat-band voltage (Vfb) also varies as shown in eqn. (2). Eqn. (3) tells us that flat band voltage (Vfb) variation correspond to the threshold voltage (Vth) variation of VNWFET [22] [23]. Equation (4) explains the subthreshold current variation due to the threshold voltage variation. This leads to change in the OFF-state current (Ioff) [24]. V = ϕ - ϕ ± ∆ϕ

V =V +φ

(2) (1- ) - 2√AB ( 1- )

+



(3)

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Where Vfb is flat-band-voltage, ϕ is metal-work-function ϕ is IGZO work function and ∆ϕ is gate-metal-work-function variation due to adsorption of ammonia gas [23]. I

!

"#

$%&

= I' [e

)* + ),.)/

Where I' =

][1-e

)0 )/

]

12. 345 6/ #7.9 :

V =

(4) ; <

[17]

′V ′ is the thermal voltage, ‘Vgs’ and ‘Vds’ are the gate to source voltage and drain to source voltage respectively, ‘Vth’ is threshold voltage, n is the subthreshold swing coefficient, W is the effective width and L is the length of the transistor respectively, Cox is gate oxide capacitance and μ is the mobility. From the equations mentioned above, it can be concluded that the presence of ammonia gas atoms on the catalytic electrode, changes the leakage current and threshold voltage.

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(a)

3.5E-6

1.E-09

3.0E-6

1.E-11

2.5E-6 1.E-13 2.0E-6 1.E-15

1.5E-6

1.E-17

Cobalt Ruthenium Molybdenium

1.E-19 1.E-21

1.0E-6 5.0E-7

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0.6 0.8 1 Gate Voltage, Vgs (V)

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Fig. 7. Output characteristics (Solid lines-Log scale, Dash lines-Linear scale) and (b) Transfer characteristics of IGZO VNWFET for Cobalt, Molybdenum, and Ruthenium as a catalytic gate electrode

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Id-Vgs characteristic of IGZO VNWFET is shown in Fig. 7(a). For higher work function energy band is increased and more opposite charge carrier is induced that increases the recombination rate and decreases the OFF-state current. In ON-state condition, required energy also increases to invert the space charge region leading to increase in ON-state current. Therefore, minimum OFF-state current and ON-state current occurred for cobalt due to a higher work function. As the proposed device is made up of IGZO material, which has a high number of n-type charge carriers and low conc. of p-type charge carriers, therefore such low minority charge carriers limit the OFF-state current. Id-Vds characteristic of IGZO VNWFET is shown in Fig 7(b). At Vds=0.2 V, sufficient charge carriers are induced in the drain side and after 0.2 V of Vds, drain current is saturated which shows that device can be used as a low power device. 4.0E-6

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1.E-05

∆wf_000meV ∆wf_050meV ∆wf_100meV ∆wf_150meV ∆wf_200meV ∆wf_250meV

1.E-09 1.E-11 1.E-13

3.5E-6 3.0E-6 2.5E-6

(a)

1.E-15

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1.E-17

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0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Gate Voltage, Vgs (V)

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1.E-17

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Drain Current, Id (A/µm)

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Drain Current, Id (A/µm) (Log)

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∆wf_000meV ∆wf_050meV ∆wf_100meV ∆wf_150meV ∆wf_200meV ∆wf_250meV

Drain Current, Id (A/µm)

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(b)

Fig. 8. Transfer characteristics with and without gas molecules for IGZO VNWFET ammonia gas sensor for (a) Cobalt, (b) Molybdenum and (c) Ruthenium use as a gate electrode

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Fig. 8 represent the change in drain current for IGZO VNWFET with a change in work function of 50meV, 100meV, 150meV, 200meV and 250meV of the catalytic gate electrode for the ammonia gas sensor. The variation in Id-Vgs characteristics of Cu, Mo, and Ru is shown in Fig. 8(a), Fig. 8(b) and Fig. 8(c) respectively. A significant reduction in OFF-state current and ON-state current can be seen as the work function is increased. Increased work function reduces the minority charge carriers to decrease the OFF current and confine the channel area for flow of majority charge carriers after saturation that reduces the ON current as well. Therefore, we can sense the ammonia gas molecules by measure the changes in Ioff and Ion. PERFORMANCE ANALYSIS AND COMPARISION

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The sensitivity with respect to Ion and Ioff are calculated for the proposed IGZO VNWFET based ammonia gas sensor by use of catalytic metal (Cobalt, Molybdenum, and Ruthenium) on the gate, and its sensitivity was compared to the initial state of catalytic metal. Effect of variation in channel length and radius of the nanowire is also observed on the sensitivity of the sensor for work function variation. Sensitivity is observed for varies type of dielectric material. Sensitivity equations with respect to OFF-state current and ON-state current for the composed sensor are given below [23]. S@ABB =

S@AK =

@4CC(EFC4GF * 0 4GH, 4.) @4CC(JC,FG * 0 4GH, 4.)

@4.(EFC4GF * 0 4GH, 4.) @4.(JC,FG * 0 4GH, 4.) @4CC(JC,FG * 0 4GH, 4.)

(5)

(6)

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Fig. 9. Transient analysis of proposed device in ON state with the 250meV change in gate electrode work function (a) Cobalt (b)

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Molybdenum (c) Ruthenium and (d) Combine

TABLE II. ∆Φm (eV)

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Figure 9(a) shows that the higher concentration of ammonia deposited over gate electrode makes the device more sensitive to the detection, which seems similar for molybdenum and ruthenium also as depicted in figure 9(b) & figure 9(c). Whereas, figure 9(d) shows that molybdenum provides much better transient variation in drain current with time for work function change of 250meV. The sensitivity factors for IGZO VNWFET based ammonia gas sensor for gate electrode as a Cobalt Molybdenum and Ruthenium with respect to on current Ion and off current Ioff, are listed in table no 1, 2 and 3 respectively. : SENSITIVITY CALCULATION OF IGZO VNWFET BASED AMMONIA GAS SENSOR FOR COBALT USE AS A GATE ELECTRODE Ioff (A/µm)

Ion (A/µm)

LMNOO

Ion/Ioff

LMNP

2.15354e-021

1.08719e-6

5.04839e+014

-

-

50meV

3.18394e-022

6.56458e-7

2.06178e+15

6.76375e+00

3.96e-01 7.04e-01

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Without gas

100meV

4.70739e-023

3.21907e-7

6.83833e+15

4.57480e+01

150meV

6.96265e-024

1.16256e-7

1.66971e+16

3.09301e+02

8.93e-01

200meV

1.04500e-024

2.83201e-8

2.71006e+16

2.06080e+03

9.74e-01

250meV

1.50576e-025

5.14207e-9

3.41493e+16

1.43020e+04

9.95e-01

TABLE III.

: SENSITIVITY CALCULATION OF IGZO VNWFET BASED AMMONIA GAS SENSOR FOR MOLYBDENUM USE AS A GATE ELECTRODE LMNOO

LMNP

∆Φm (eV)

Ioff (A/µm)

Ion (A/µm)

Ioo/Ioff

Without gas

2.02e-14

3.59e-6

1.78138e+8

50meV

2.61e-15

3.47e-6

1.32689e+9

7.73946e+00

3.37e-02

100meV

3.39e-16

3.33e-6

9.82561e+9

5.95870e+01

7.29e-02

150meV

4.38e-17

3.16e-6

7.20798e+10

4.61187e+02

1.20e-01

200meV

5.67e-18

2.96e-6

5.21774e+11

3.56261e+03

1.76e-01

-

-

ACCEPTED MANUSCRIPT 250meV TABLE IV. ∆Φm (eV)

7.33e-19

2.72e-06

3.71492e+12

10 2.75579e+04

2.46e-01

: SENSITIVITY CALCULATION OF IGZO VNWFET BASED AMMONIA GAS SENSOR FOR RUTHENIUM USE AS A GATE ELECTRODE Ioff (A/µm)

Ion (A/µm)

LMNOO

Ion/Ioff

LMNP

Without gas

1.39191e-16

3.02285e-6

2.17173e+10

-

50meV

2.06491e-17

2.8221e-6

1.36669e+11

6.74077e+00

6.64e-02

100meV

3.06102e-18

2.58334e-6

8.43947e+11

4.54720e+01

1.45e-01

4.53467e-19

2.29543e-6

5.06196e+12

3.06948e+02

6.71377e-20

1.92084e-6

2.86105e+13

2.07321e+03

3.65e-01

2.40e-01

250meV

9.93469e-21

1.46161e-6

1.47122e+14

1.40106e+04

5.16e-01

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150meV 200meV

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When ammonia gas diffused on the sensor, the increase in work function decreases the Ioff and Ion. If the concentration of ammonia gas is increased, the work function is further increased, and Ioff and Ion further decrease but the rate of change in Ioff and Ion decrease. The decrement in Rate of change for Ioff is more than Ion. Hence, Ion/Ioff increases with the change in work function. With variation in work function OFF sensitivity and ON sensitivity is increased, but Ioff sensitivity variation is more than Ion sensitivity with variation in work function. A. Analysis of the ammonia gas sensor

Cobalt

M AN U

2.5E+4

Ruthenium 2.0E+4

Molybdenum 1.5E+4

1.0E+4

5.0E+3

0.1 0.15 0.2 Change in Work Function, ∆ϕ (eV)

0.25

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0.0E+0 0.05

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OFF Current Sensitivity, SIoff

1) Comparison of OFF-state current Sensitivity for ammonia gas sensor Comparison of OFF-state current Sensitivity between different gate electrodes Cobalt, Molybdenum and Ruthenium for IGZO VNWFET ammonia gas sensor with respect to variation in work function is shown in Fig 10.

Fig. 10. OFF current Sensitivity comparison of ammonia gas sensor between Cobalt, Molybdenum and Ruthenium use as a gate electrode

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When ammonia gas molecules adsorbed on the catalytic gate electrode, the work function is changed, therefore Ioff decrease so sensitivity increases that are measured with equation (5). OFF current Sensitivity is more for Molybdenum due to a low work function in comparison of Cobalt and Ruthenium. 2) Comparison of ON-state current Sensitivity for ammonia gas sensor Comparison of ON current sensitivity between Cobalt, Molybdenum, and Ruthenium for IGZO VNWFET ammonia gas sensor with respect to work function variation is shown in Fig.11. Due to a change in work function, Ion decreases and sensitivity increases that is measured with equation (5). ON current Sensitivity is more for cobalt due to a higher work function. The change in ON current sensitivity for cobalt increases at first and then saturates to follow the pattern of ON-state current whereas the increase in ON sensitivity for Ru and Mo approximately linear for the subsequent increase in metal work function.

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Cobalt Ruthenium

0.8

Molybdenum

0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0.05

0.1 0.15 0.2 Change in Work Function, ∆ϕ (eV)

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ON Current Sensitivity, SIon

0.9

0.25

On current Sensitivity comparison of ammonia gas sensor between Cobalt, Molybdenum and Ruthenium use as a gate electrode

SC

Fig. 11.

11

M AN U

3) Comparison of the ON-OFF current ratio for ammonia gas sensors The ON-OFF current ratio comparison between different gate electrode Cobalt, Molybdenum, and Ruthenium for IGZO VNWFET ammonia gas sensor with respect to work function variation is shown in Fig 12. As the work function of gate metal increases, it generates a large number of charge carriers in the channel. This leads to increase in recombination, and both OFF current and ON current decrease, but OFF current varies more than ON current because the gate to source voltage is sufficient to give the energy to charge carrier to travel from source to drain, which results in increase of Ion/Ioff ratio. ON-OFF current ratio is measured with gas molecules e.g. 50meV, 100meV, 150meV, 200meV and 250meV work function variation. The ON-OFF current ratio is highest for cobalt followed by Ruthenium and Molybdenum depending on their work function. 1.E+17 ON/OFF Current Ratio, Ion/IOFF

1.E+16

1.E+14 1.E+13 1.E+12 1.E+11

EP

1.E+10

TE D

1.E+15

Cobalt Ruthenium

1.E+09

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1.E+08 0.05

Fig. 12.

Molybdenum 0.1

0.15

0.2

0.25

Change in Work Function, ∆ϕ (eV)

ON-OFF current ratio comparison of ammonia gas sensor between Cobalt, Molybdenum, and Ruthenium used as a gate electrode

4) Variation of channel length (Lch) on transfer characteristics Effect of Variation of channel length (Lch) (10nm, 20nm, 30nm, 40nm) on transfer characteristics for the ammonia gas sensor is shown in Fig 13 (a), Fig 13 (b) and Fig 13 (c) for Cobalt, Molybdenum, and Ruthenium respectively. When the gate channel length (Lch) is increased, the surface to volume ratio increases with an increase in recombination rate and it leads to a decrease in OFF-state current decreases. With the increase in channel length, higher controllability over channel increases the flow of a number of charge carriers across per unit area leading to a significant increase in ON-state current. In ammonia sensor; 20nm channel length is used for scaling, but if the channel is decreased below 20nm, OFF current is increased due to short channel effects.

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1.E-07

Drain Current, Id (A/µm) (Log)

(a)

1.E-09 1.E-11 1.E-13 1.E-15

Lch_10nm

1.E-17

Lch_20nm Lch_30nm

1.E-19

(b)

1.E-06 1.E-07 1.E-08 1.E-09 1.E-10

Lch_10nm

1.E-11

Lch_20nm

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1.E-05 Drain Current, Id (A/µm) (Log)

12

1.E-12

Lch_30nm Lch_40nm

1.E-13

Lch_40nm

1.E-14

1.E-21 0

0.2

0.4 0.6 0.8 1 Gate Voltage, Vgs (V)

1.2

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1.4

4.E-07

1.2

1.4

4.E-09 4.E-11 4.E-13

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Drain Current, Id (A/µm) (Log)

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(c)

0.2

Lch_20nm Lch_30nm

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4.E-17 0

0.2

0.4

0.6

0.8

1

1.2

1.4

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Gate Voltage, Vgs (V)

Fig. 13. Variation of channel length (Lch) for ammonia gas sensor for (a) for Cobalt, (b) molybdenum and (c) ruthenium use as a gate electrode

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5) Variation of Channel thickness on sensitivity for ammonia gas sensor Variation of channel thickness (Tch) (5nm, 10nm, 15nm) on transfer characteristics for the ammonia gas sensor is shown in Fig 14 (a), Fig 14 (b) and Fig 14 (c) for Cobalt, Molybdenum, and Ruthenium respectively. As channel thickness increased, because of the decrease of the surface to volume ratio, controllability is less and OFF-state current increases. Charge carrier density is also increased because of increase in Tch, therefore ON-state current is increased [8]. For the optimum value of OFF current and ON current, 10nm channel thickness is used in the ammonia gas sensor. 1.E-05 Drain Current, Id (A/µm) (Log)

1.E-07

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Drain Current, Id (A/µm) (Log)

1.E-05

1.E-09 1.E-11 1.E-13 1.E-15

Tch_05nm

1.E-17

Tch_10nm 1.E-19

(a)

Tch_15nm

1.E-06 1.E-07 1.E-08 1.E-09 1.E-10 1.E-11 Tch_05nm 1.E-12

Tch_10nm

1.E-13

(b)

Tch_15nm

1.E-14

1.E-21 0

0.2

0.4

0.6

0.8

1

Gate Voltage, Vgs (V)

1.2

1.4

0

0.2

0.4

0.6

0.8

1

Gate Voltage, Vgs (V)

1.2

1.4

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13

1.E-07 1.E-09 1.E-11 1.E-13

Tch_05nm Tch_10nm

1.E-15

(c)

Tch_15nm

1.E-17 0

0.2

0.4

0.6

0.8

1

1.2

Gate Voltage, Vgs (V)

1.4

Variation of channel thickness for ammonia gas sensor for (a) Cobalt, (b) molybdenum and (c) ruthenium use as a gate electrode

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Fig. 14.

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Drain Current, Id (A/µm) (Log)

1.E-05

M AN U

6) Variation of dielectric materials for ammonia gas sensor The sensitivity Variation for different dielectric materials for the different catalytic metal of the ammonia gas sensor is shown in Fig 15(a), Fig 15 (b) and Fig 15 (c). When gate dielectric material values increases, oxide capacitance Cox also increases as shown in equation (7) [16] which in turns increases the ON current (Ion) and OFF current (Ioff). OFF current leakage is more for higher dielectric material values which increases the sensitivity. Therefore, sensitivity is figured for different gate dielectric materials SiOS (3.9), SiTN (7.5), AlS OT (9.5), HfOS (21). It is discovered that the highest sensitivity with respect to Ioff is observed for HfOS with most elevated dielectric values followed by Al2O3, Si3N4, and SiO2.

OFF Current Sensitivity, IIoff

6.8

(a)

SYZ Y[\] ^_

6.6

6.55 6.5

6.45 6.4

EP

6.65

TE D

6.75 6.7

6.75 OFF Current Sensitivity, SIoff

AC C

SiO2 Si3N4 Al2O3 Gate Oxide Materials

(7)

% ( `a/`cd)

7.75 7.7 7.65 7.6 7.55 7.5 7.45 7.4 7.35 7.3 7.25

(b)

OFF Current Sensitivity, SIoff

Cox =

HfO2

SiO2

Si3N4 Al2O3 HfO2 Gate Oxide Materials

(c)

6.7 6.65 6.6 6.55 6.5 6.45 6.4 6.35 SiO2

Si3N4 Al2O3 HfO2 Gate Oxide Materials

Fig. 15. Variation of dielectric materials of sensitivity for ammonia gas sensor for 50meV change in work function for (a) Cobalt, (b) Molybdenum and (c) Ruthenium

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14

CONCLUSION

VII. REFERENCES

7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

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Veloso, A., Altamirano-Sánchez, E., Brus, S., Chan, B.T., Cupak, M., Dehan, M., Delvaux, C., Devriendt, K., Eneman, G., Ercken, M. and Huynh-Bao, T., 2016. Vertical nanowire FET integration and device aspects. ECS Transactions, 72(4), pp.31-42. Suresh, A., Wellenius, P. and Muth, J.F., 2007, December. High performance transparent thin film transistors based on indium gallium zinc oxide as the channel material. In Electron Devices Meeting, 2007. IEDM 2007. IEEE International (pp. 587-590). IEEE. Ionescu, A.M. and Riel, H., 2011. Tunnel field-effect transistors as energy-efficient electronic switches. Nature, 479(7373), p.329. Singh, S. and Raman, A., 2018. Gate-All-Around Charge Plasma-Based Dual Material Gate-Stack Nanowire FET for Enhanced Analog Performance. IEEE Transactions on Electron Devices, (99), pp.1-7. De Marchi, M., Sacchetto, D., Frache, S., Zhang, J., Gaillardon, P.E., Leblebici, Y. and De Micheli, G., 2012, December. Polarity control in double-gate, gate-all-around vertically stacked silicon nanowire FETs. In Electron Devices Meeting (IEDM), 2012 IEEE International (pp. 8-4). IEEE. Cecil, K. and Singh, J., 2015. Electrostatically doped heterojunction TFET with enhanced driving capabilities for low power applications. arXiv preprint arXiv:1512.06232. Lahgere, A., Sahu, C., and Singh, J., 2015. PVT-aware design of dopingless dynamically configurable tunnel FET. IEEE Transactions on Electron Devices, 62(8), pp.2404-2409. Anand, S., Amin, S.I. and Sarin, R.K., 2016. Performance analysis of charge plasma-based dual electrode tunnel FET. Journal of Semiconductors, 37(5), p.054003. Hueting, R.J., Rajasekharan, B., Salm, C. and Schmitz, J., 2008. The charge plasma PN diode. IEEE Electron Device Lett, 29(12), pp.1367-1368. Timmer, B., Olthuis, W., and Van Den Berg, A., 2005. Ammonia sensors and their applications—a review. Sensors and Actuators B: Chemical, 107(2), pp.666-677. Chen, X., Wong, C.K., Yuan, C.A. and Zhang, G., 2013. Nanowire-based gas sensors. Sensors and Actuators B: Chemical, 177, pp.178-195. Wöllenstein, J., Burgmair, M., Plescher, G., Sulima, T., Hildenbrand, J., Böttner, H. and Eisele, I., 2003. Cobalt oxide based gas sensors on silicon substrate for operation at low temperatures. Sensors and Actuators B: Chemical, 93(1-3), pp.442-448. Fine, G.F., Cavanagh, L.M., Afonja, A. and Binions, R., 2010. Metal oxide semi-conductor gas sensors in environmental monitoring. Sensors, 10(6), pp.5469-5502. Manual, A.U., 2010. Device simulation software, Silvaco Int. Santa Clara, CA, Version, 5(0). Amin, S.I. and Sarin, R.K., 2015. Charge-plasma based dual-material and gate-stacked architecture of junctionless transistor for enhanced analog performance. Superlattices and Microstructures, 88, pp.582-590. Rajasekharan, B., Hueting, R.J., Salm, C., van Hemert, T., Wolters, R.A. and Schmitz, J., 2010. Fabrication and characterization of the charge-plasma diode. IEEE electron device letters, 31(6), p.528. Sahu, C. and Singh, J., 2014. Charge-plasma based process variation immune junctionless transistor. IEEE Electron Device Letters, 35(3), pp.411-413. Ming-Hung Han, Chun-Yen Chang, Yi-Ruei Jhan, Jia-Jiun Wu,, Hung-Bin Chen, Ya-Chi Cheng, Yung-Chun Wu,2013. Characteristics of p-type Junctionless Gate all around Nanowire transistor and Sensitivity analysis. IEEE Electron Device Letters, 34(2), pp. 157-159. Singh, N.K., Raman, A., Singh, S. and Kumar, N., 2017. A novel high mobility In1-xGaxAs cylindrical-gate-nanowire FET for gas sensing application with enhanced sensitivity. Superlattices and Microstructures, 111, pp.518-528. Oh, S.H., Monroe, D. and Hergenrother, J.M., 2000. Analytic description of short-channel effects in fully-depleted double-gate and cylindrical, surrounding-gate MOSFETs. IEEE electron device letters, 21(9), pp.445-447. Dan, Y., Evoy, S. and Johnson, A.T., 2008. Chemical gas sensors based on nanowires. arXiv preprint arXiv:0804.4828. Pradhan, K.P., Kumar, M.R., Mohapatra, S.K. and Sahu, P.K., 2015. Analytical modeling of threshold voltage for Cylindrical Gate All Around (CGAA) MOSFET using center potential. Ain Shams Engineering Journal, 6(4), pp.1171-1177. Gautam, R., Saxena, M., Gupta, R.S., and Gupta, M., 2013. Gate-all-around nanowire MOSFET with catalytic metal gate for gas sensing applications. IEEE transactions on nanotechnology, 12(6), pp.939-944. Butzen, P.F. and Ribas, R.P., 2006. Leakage current in sub-micrometer CMOS gates. Universidade Federal do Rio Grande do Sul, pp.1-28.

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1.

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The design and performance of IGZO VNWFET are analyzed. IGZO VNWFET for ammonia gas shows higher OFF current sensitivity for Molybdenum and ON current sensitivity for Cobalt. Cobalt is cheaper metal; cobalt as catalytic gate metal is a better option to make economical Ammonia gas sensor. Among the dimensional parameters of the device, with increasing channel length or decreasing radius the sensitivity of gas sensors increases. Use of different gate dielectric materials is also analyzed. Simulations shows, HfO2 with highest dielectric constant gives the highest sensitivity. Therefore, IGZO VNWFET with catalytic gate metal is the promising structure for gas sensing applications. IGZO VNWFET based ammonia gas sensor can be used for many applications like environmental, automotive, chemical industry and medical diagnostics, and can help in calculating the ammonia concentration in these fields.

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Highlights:

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 

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

Designing of novel integration of Electrostatic-Charge plasma technique to realize Cuboidal Nanowire FET. Use of artificial material for structure design, i.e., Indium Gallium Zinc Oxide (IGZO) to enhance ON current and device stability at a higher temperature. Application of the proposed device for catalytic gate based ammonia gas sensing. Comparison analysis of proposed device with different catalytic gate metal to optimize the sensing parameters.

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