Design and Performance Analysis of Ultrathin Nanowire FET Ammonia Gas Sensor

In this work, an ultrathin 3 nm nanowire field-effect transistor (NWFET) based ammonia gas sensor is designed, and its sensitivity is analyzed at room temperature. The designed NWFET for gas sensing is observed to have a higher ratio of ION to IOFF than 109, lower DIBL and better gate controlling due to a higher surface to volume ratio. The gas-sensing performance analysis has been done for three different catalysts, iridium (Ir), ruthenium (Ru), and palladium (Pd), by gradually increasing the work function by a difference of 50 meV. The device showed higher OFF current sensitivity compared to ON current sensitivity. The power consumption and threshold voltage are observed to be least for palladium catalytic gate electrodes making palladium the most favorable catalytic for ammonia gas for the designed gas sensor.


Introduction
Various types of the gas sensor have been designed, studied, and fabricated based on different sensing mechanisms and sensing material, and many new designs and materials are being studied presently [1,2]. The demand for small-sized and integrated circuit compatible gas sensors has made the researcher keener towards organic and nanomaterial FET type gas sensors [3,4]. These gas sensors play a vital role in the safety of chemical industries, factories by monitoring any leakages [5]. Among various gases, ammonia (NH 3 ) gas is the crucial base material for different chemical products and is used in food packaging industries, agriculture, medical diagnostics labs, and fertilizer industries [6]. However, ammonia is a colorless gas with a suffocating smell and, if accidentally leaked, could be hazardous to living beings [7].
The semiconductor nanowires FET are observed to have higher surface-area-to-volume ratio, lower resistance, and higher aspect ratio, making it ideal for designing gas sensors [8]. The sensing mechanism of the designed device is based on the deposition/absorption of gas molecules to the surface of the catalytic gate electrode [9,10]. The deposition/absorption of gas molecules on metal electrodes disturb its composition and lead to the change in work function of the metal electrode, which affects the analog parameters of transistors. In the proposed gas sensor, the effect of change in metal function on threshold voltage has been studied on transfer characteristics. The relation of change in work function (Δϕ m ) to the partial pressure (P) of detectable gas is shown in Eq. (1) [11].
In this paper, we have discussed the device design and simulation model and parameters of the proposed device in section (2), followed by results and performance analysis in section (3) and conclusion in section (4).

Device Design and Simulation Parameters
The three-dimensional schematic structure of the proposed nanowire FET is shown in Fig. 1a. The diameter of the cross-section is chosen as 3 nm, where the inner core has n-p-n configuration on silicon nanowire of diameter 2 nm, which is coated with silicon dioxide all-around of 1 nm thickness [12,13]. A thin layer of the catalytic metal electrode (Ir/Ru/Pd) is coated on the oxide layer, all around the channel. The vertical length of the proposed NWFET is 50 nm with a channel length of 30 nm, and drain and source region length is 10 nm each. The source and drain are doped with an N-type uniform doping profile of 1 × 10 18 /cm 3 and 1 × 10 20 /cm 3, whereas the channel is doped with a P-type doping profile of 1 × 10 15 /cm 3 . The two-dimensional structure of the designed silicon NWFET has been used for current simulation, which is shown in Fig. 1b. The various design parameters of the device are mentioned in Table 1.
For simulation, the Silvaco TCAD Atlas tool has been used. The Shockley-Read-Hall model and band-to-band tunneling have been used for generation-recombination of electron-hole pairs, and self-consistent coupled Schrödinger Poisson model has been used to derive electron density. The non-equilibrium green function model (NEGF model) solves these coupled equations through a stable and oscillation-less iteration [14,15]. The obtained solution of electron density is incorporated in the driftdiffusion model to get current and current density [16,17].

Device Performance and Characteristics Discussion
At room temperature, the transfer characteristic (I D -V GS ) of the proposed device is shown in Fig. 2a for V DS = 1 V has been obtained for the design parameter given in Table 1. The ON-current (I ON ) at V GS = 1.5 V is noticed as 6.70 × 10 −7 (A/μm) and 8.67 × 10 −7 (A/μm), whereas the OFF-current (I OFF ) is observed as 5.67 × 10 −16 (A/μm). The device is observed to have a lower DIBL and high I ON to I OFF ratio in the order of 10 9 . The sub-threshold swing of 57.80 mV/dec and threshold voltage 0.589 V has been observed. The output characteristics (I D -V DS ) for V GS = 0.6 V and V GS = 0 V (inset) has shown in Fig. 2b. From the inset graph in Fig. 2b, It can be observed that even though the V DS is increased to 1 v, the device remains in OFF state only, and OFF current is 1 × 10 −14 A/μm, which shows higher controllability of gate voltage on the device.

Performance Analysis of Nanowire FET Gas Sensor for Three Different Metal Catalysts
The three different metals i.e. Iridium (Ir), Ruthenium (Ru) and Palladium (Pd) are selected as the catalyst metal electrodes due their higher sensitivity towards ammonia gas. In Table 2 the work function of the above-mentioned metal catalysts is listed. Considering the exposure of constant flow of ammonia gas at constant pressure on the proposed gas sensor, change in its drain current characteristics is studied by increasing the metal work function with a small value of 50 meV gradually. In the obtained transfer (I D -V GS ) characteristics for each change in work function, change in I OFF to I ON ratio and threshold voltage can be observed. The sensing mechanism of the proposed gas sensor is based on the change in metal work function due to the absorption or deposition of ammonia on the surface of catalytic gate metal. As the ammonia gas settle Source doping concentration n-type1 x 10 15 /cm 3 7 Drain doping concentration n-type1 x 10 20 / cm 3 8 Channel doping concentration p-type 1 x 10 15 cm 3 down on the surface of these catalyst, dissociate into nitrogen gas, hydrogen gas and hydrogen ion [18][19][20] as shown in Fig. 3. These hydrogen ions penetrate into the gate metal, which changes the structural composition of metal catalysts resulting in a change metal work function [21,22]. Increasing the concentration of detectable gas will increase the work function more. The change in metal work function affects analog parameters, such as OFF-current (I OFF ), ON-current (I ON ), transconductance and threshold voltage, etc. Figure 4a-c, transfer characteristics of the proposed device for palladium, ruthenium, and iridium shown respectively is obtained by step-wise linearly incrementing work function from 50 to 200 meV with step size of 50 meV. In the transfer characteristics of the three metal catalysts, it can be observed that the drain current curve shifts downwards with increasing work function. The palladium metal which has lowest work function among three has higher ON-Current and lower OFF-Current compared to that of iridium and ruthenium.
Since the threshold voltage depends directly on metal work function as shown in Eqs. (2) and (3) [11], hence increasing work function increases the threshold voltage which is shown in Fig. 5. So, increasing the concentration of detectable gas on metal surfaces will increase the device's threshold voltage, which could be used as a sensing parameter for the proposed gas sensor. As the change in OFF and ON-state current in the device is very significant, hence drain current response have considered as sensing response parameter of the device and the current sensitivity and sensitivity response for mentioned three catalytic metals are studied. Table 3 shows the different threshold voltage observed by varying ΔΦ M for all three metal catalysts.

Sensitivity Analysis
Sensitivity of proposed nanowire FET gas sensor for ammonia is more in OFF state at V GS = 0 V than ON state as in the off  state higher work function induces more minority charges in the channel [11]. The increase in work function of device leads to formation of depletion layer at interface of an oxide layer and channel even in absence of gate bias which requires more positive gate voltage to switch-ON the device [23,24].
The change in OFF current is referred as OFF-state current sensitivity, which can be obtained from the ratio of OFF current when no-gas/air exposed to device to OFF current when the detectable gas is exposed to device, which is mathematically expressed by following Eq. (4) [11].
Similarly, the change in ON current is referred as ON-state current sensitivity which can be obtained from the Eq. (5) expressed below [11].
The sensitivity curve for both ON and OFF state with respect to change in work function for iridium, palladium and ruthenium catalytic metal is shown in the Fig. 6. Tables 4, 5 and 6 shows ON and OFF current, it's ratio and sensitivity in OFF and ON state for iridium (Ir), ruthenium (Ru) and palladium (Pd) gate metal respectively. Here, we can observe that the palladium gate metal NWFET gas sensor has higher OFF current sensitivity compared to the iridium and ruthenium gate metal NWFET gas sensors. The outcome forms the sensitivity calculation from the proposed NWFET-based NH3 gas sensor with Ir, Ru and Pd gate metal for different gas molecule concentration equivalent work function have been shown in Tables 4, 5 and 6 respectively. From these Tables 4, 5 and 6 we can conclude that maximum ON-state current sensitivity (S ON ) is reported to be −0.385, 0.061 and 0.182 form maximum gas concentration equivalent to 200 meV. Sensing Response of the proposed device as function of gate voltage for varying metal work function can obtained from the ratio of drain current in presence of ammonia to drain current in absence of ammonia [6] can be calculated from the expression by Eq. (6).
Since increase in concentration of detectable gas (ammonia) is considered as higher change in metal work function and the sensing response for higher change in work function i.e. 200 meV is maximum. It can be very clearly observed from the Fig. 7a that the NWFET gas sensor with Palladium gate metal electrode is highly sensitivity in OFF state and least sensitivity in ON state. In Fig. 7b and c, It can be observed that proposed device shows more sensitivity response when the gate voltage is equal to or higher than the threshold voltage i.e. VGS =1.08 V and VGS = 1.28 V for both iridium and ruthenium metal electrode respectively compared to OFFstate (V GS = 0 V).

Conclusion
The proposed nanowire FET gas sensor has higher OFF current sensitivity for palladium metal electrode as compared to other two metal electrodes as sensing response for palladium increases by 90.2% by increasing work function (Δϕm) from 50 to 200 meV where for iridium and ruthenium metal electrode sensing response increases by 78.5% and 31.81%   respectively. The threshold voltage for palladium metal electrode gas sensor is always below 0.9 V whereas for ruthenium and iridium metal electrode gas sensor, it is always more than 1 V; hence the palladium metal electrode gas sensor would be a wise choice for VLSI circuit operating at lower voltages. The designed gas sensor has higher gate controllability, even the drain voltage of 1 V won't be able to switch ON the gas sensor which reduces the chances of getting false alert in case of voltage spikes in applied voltages. The designed device has higher I ON /I OFF ratio in order of 10 9 which checks the leakage current in device and reduces the chances of false alert due to  leakage current. The current sensitivity increases with increase in concentration of ammonia gas on the surface of device, where the threshold voltage increases with increase in concentration of exposed ammonia gas, hence the threshold voltage of device can also be considered as one of sensing parameters. Compared to ruthenium and iridium metal, palladium metal seems to be favorable option for catalyst metal for detection of ammonia for the designed nanowire FET gas sensor.