Design of Si0.5Ge0.5 Sourced Nano-cantilever Pressure Sensor Based on Charge Plasma and Gate Stacked Nanowire Tunnel Field Effect Transistor

In this paper, Si0.5Ge0.5 source with charge plasma and gate stacked Nanowire Tunnel Field Effect Transistor (CP-GS-NWTFET) based pressure sensor is proposed. The pressure sensor is one of the essential components in sensing and actuating applications. Nanocavity is created beneath the gate electrode for possible bending due to the pressure applied. The presented sensor is based on the capacitive gate coupling principle, owing to which the tunneling current is changed. The applied pressure on the nanocantilever (hanging gate electrode towards the cavity) bends the nanocantilever which changes the electrical characteristics of the device. Various bending of the nanocantilever beam used are 0.5, 1, 1.5, and 2 nm. Several device parameters including electric field, potential, carrier concentration, energy band diagram, Ion/Ioff ratio, subthreshold slope, etc. are evaluated as performance parameters of the presented device. The sensitivity is defined as the change in the current ratio per nm of bending with respect to a structure having no bending. The study reveals that the presented pressor sensor is more sensitive for lower bendings. The sensitivity for 0.5 nm and 2 nm of bending is 2.74 × 1008 /nm and 4.73 × 1007 /nm respectively. Simulation unearths a remarkable connection between hypothetical and practical accepts of formation. The total length of the proposed device, CP-GS-NWTFET is 92 nm.


Introduction
In the twenty-first century, MEMS becomes a trademarked technology for numerous applications, especially in sensors and actuators.Various types of MEMS-based sensors have been developed after the discovery of MEMS devices [1].Planar MOSFETs based on MEMS technology have been used to develop many pressure sensors.Nicolas André et al. [2] have presented an airflow sensor, in which the flow of the air can be detected by the defection of the microcantilever which consumes very minor static power.Terunobu Akiyama et al. [3] developed a force sensor that is based on pressure-detecting MOS devices.In this, a cantilevers diaphragm of length 400 μm to 950 μm, spring constant 1 N/m, and having a resonant frequency in the range 6.2-35 kHz was efficiently used as a part of scanning microscopy both in steady and dynamic mode.The semiconductor industry needs to switch from single-gate MOSFET to multigate MOSFET for effective downscaling of devices.In which nanowire devices show superior electrostatic control over the channel.The downscaling of MOSFETs in the nanoscale region has faced many problems due to short-channel effects (SCEs) [4,5].The nanowire has been evolving as a capable candidate for the next generation of sensors [6].Hui Pan et al. [7] have established the rise in the bandgap of the nanowire with the increase in the ratio of surface to volume.The nanowire has a superior Debye length in comparison to other devices and is easily integrated into microelectronic materials [8].Nowadays, nanocantilevers present a major technological improvement that has given way to the nanoelectromechanical system (NEMS).NEMS is a potential candidate for evolving highly sensitive, and fast multiple sensors with high efficiency [9].Gagan Kumar et al. [10] have conferred for the first time about the pressure sensor based on the MEMS nanocantilever beam as a gate electrode on charge plasma-based double-gate TFET.The pressure sensor based on MEMS technology needs a higher ON current, but charge plasma-based double-gate TFET offers poor ON current and parametric variations in transistor performance due to poor electrostatic control of the channel [11].This problem can be overcome by a charge plasma and gate stackedbased nanowire field effect transistor (JLNWFET), which offers excellent ON-state current, and improved subthreshold slope (SS) [12].This paper reveals a pressure sensor with a circular nanocantilever structure as the gate electrodes on the proposed nanowire device is cylindrical in shape.Due to lower ON-state current, a gate stacking of SiO 2 and HfO 2 of thickness 1 and 2 nm is used respectively to increase the gate capacitance which in turn improves the drain current [13].Various bending combinations of all cantilever diaphragms, measure the magnitude of pressure by showing different electrical characteristics.The change in electrical characteristics of all cantilever diaphragms delivers its direction detection capability, which can be used in sensors such as gyroscopes and accelerometers [14,15].This article presents the design and analysis of the cantilever-based pressure sensor using the charge plasma technique.The pressure sensor has major applications in medical, automotive industrial, consumer, and building devices, where stable and accurate pressure measurements are required.
This paper is divided into 4 sections.The structure of the device, simulation methodology, and simulation tool have been discussed in Section 2. Section 3 presents the device characteristics, the impact of diaphragm bending on electrical performances, and sensitivity calculation.Finally, the conclusions of the presented paper have been discussed in Section 4.

Device Design and Simulation Parameters
For the design of the CP-GS-NWTFET based pressure sensor, the p + source and the n + drain is created by using the charge plasma technique.The condition of generating the electron/hole charge plasma is that the work function of drain/source should satisfy [ϕ m < χ si + (E g ∕2)] and [ϕ m > χ si + (E g ∕2)] , respectively [16].Here, χ si is electron affinity of Si equals 4.17 eV and E g is the bandgap of silicon.At thermal equilibrium, metal work functions of 5.4 eV and 4.7 eV are used at the source and drain electrode   Gate Voltage, V gs (V)   Reference [18] Simulated Fig. 2 Calibration of the proposed CP-GS-NWTFET based pressure sensor with published fabricated data from [18] 1 3 to induce the "p + " and "n + " regions respectively.The source and drain work function is used to satisfy the above conditions for generating hole and electron plasma.The three-dimensional and two-dimensional views of the proposed CP-GS-NWTFET structure are shown in Fig. 1a and b respectively.Gate stacking of SiO 2 and HfO 2 is used to increase the gate capacitance, which in turn increases the drive current [17].In the CP-GS-NWTFET based pressure sensor gate stack of SiO 2 and HfO 2 with individual oxide thicknesses of 1 nm and 2 nm, the combined thickness of t sio 2 + t hfo 2 = 2 is used.The material used at the source region is Si 1-x Ge x with a germanium molar concentration of 0.5.The use of low bandgap material (Si 1-x Ge x ) at the source side reduces the tunneling path which in turn increases the ON-state current.The device dimensions used for the design of the proposed sensor are listed in Table 1.The SiO 2 dielectric is deposited under the source and drain electrode to avoid the formation of silicide between the source/drain electrode and silicon material of the source/ drain region.The source, drain, and channel length in the proposed sensor are 30 nm each.The source is considered as intrinsic Si 1-x Ge x whereas the channel and drain region is considered as intrinsic silicon material.Spacers at the source side as well as the drain side are used with a spacer length of 10 nm and spacer width of 3 nm.The calibration of the presented work with the published fabricated data from [18] has been done.Calibration of proposed CP-GS-NWTFET

Results and Discussions
To implement the pressure sensor in CP-GS-NWTFET, various bending is considered e.g., 0.5, 1, 1.5, and 2 nm.These bendings are the results of the pressure applied to the cantilever beam structure.For this, a cantilever beam of length 15 nm is considered after the removal of HfO 2 under half portion of the gate electrode.These bendings are implemented in the design manually.As the different diaphragm bending is implemented, the change in I ON , I OFF , I ON /I OFF , and V th is observed.So, the change in these electrical parameters can sense the bending of the diaphragm.Further, sensitivity can be calculated by considering any of these electrical parameters.In this work, the sensitivity is calculated as the change in the current ratio with the change in the diaphragm bending (Fig. 3 and Eq. 1).

Device Characteristics of the Sensor
Figure 4 shows the energy band diagram for various diaphragm bending along the device length.It can be observed from the graph that with an increase in bending the energy of the conduction band as well as the valance band is increased to a higher degree.This increase in energy of the valance band and conduction band combinedly increases the tunneling rate.Higher bending of the diaphragm increases the induced carrier concentration, which shifts the fermi level downwards followed by energy bands.Figure 5 shows the electron and hole concentration along the device length for various diaphragm bending in the ON state.The electron and hole concentration are calculated at V ds = 1 V & V gs = 1.5 V, which shows that higher bending of gate metal, induces reduced hole concentration and electron concentration.However, the law of mass action will be followed at every diaphragm bending.From the plot, it can be observed that both electron and hole concentration decrease with the device length, especially in the drain region.Figure 6 shows the electric field variation along the device length for without bending, with bending of 1 and 2 nm of the diaphragm.The electric field is the differential of electric potential over the device length [19].The electric field first increases, reaches a peak and then decreases.The peak electric field is observed for without any bending.From the graph, it can be observed that the peak value of the electric field at the source-channel junction decreases with the diaphragm bending.Figure 7 shows the variation of electric potential along the device length for numerous gates bending structures.It can be seen from the plot that electric potential increases with the device length.It can also be observed that by increasing the bending of the gate over the channel, the electric potential shows lesser values for higher bending.

Impact of Sensor's Bending on Transconductance
In the proposed CP-GS-NWTFET based pressure sensor, various bending of 0.5, 1, 1.5, and 2 nm are considered.The sensor is simulated in SILVACO TCAD for these bending and different I ON , I OFF , I ON /I OFF and V th are observed.The sensing mechanism lies in observing these changes in the electrical parameters for a bending.Figure 8 shows the plot of the comparison of transfer characteristics for the CP-GS-NWTFET based pressure sensor for no bending, 0.5 nm bending, 1 nm bending, 1.5 nm bending, and 2 nm bending.From the plot, it can be observed that with bending, the ON-state current is decreased.For sensitivity calculations, I ON /I OFF is used in this work.
Figure 9 shows the variation of transconductance with the gate to source voltage for no bending, 0.5 nm bending, 1 nm bending, 1.5 nm bending, and 2 nm bending.Transconductance is defined as the first-order derivative of drain current.Transconductance shows the ability of an amplifier to amplify the signal.For a good amplifier, the value of transconductance should be high.From the plot, it can be observed that transconductance first increases, reaches a peak and starts decreasing.It can also be concluded that with the increase in diaphragm bending the peak value of transconductance decreases.Figure 10 shows the comparison of second-order transconductance, g m2 , and third-order transconductance g m3 for the proposed CP-GS-NWTFET based pressure sensor for no bending, 0.5 nm bending, 1 nm bending, 1.5 nm bending, and 2 nm bending.The second and third-order transconductance shows the ability to produce noise in the drain characteristics.For a better drain characteristic, this second and third-order transconductance should be minimized.

Impact of Sensor's Bending on Electrical Performance
Figure 11 shows the impact of bending on I ON , I OFF , I ON /I OFF , subthreshold slope, and threshold voltage for the CP-GS-NWTFET based pressure sensor.These electrical parameters are calculated for no bending, 0.5 nm bending, 1 nm bending, 1.5 nm bending, and 2 nm bending.In Fig. 11a the I ON is plotted with the diaphragm bending.The plot shows that with bending, I ON first decreases and then increases.For better device characteristics I ON should be high.In Fig. 11b the I OFF is plotted with the diaphragm bending.The plot shows that with bending, I OFF increases.For a better device, the leakage in the device should be minimum.In Fig. 11c the I ON /I OFF is plotted with the diaphragm bending.The plot shows that with bending, I ON /I OFF decreases first and then increases.For better switching characteristics current ratio should be high.In Fig. 11d the subthreshold slope (ss) is plotted with the diaphragm bending.The plot shows that with bending, the subthreshold slope first increases and then decreases.For a better device, the subthreshold slope in the device should be minimum (< 60 mV/decade).In Fig. 11e the threshold voltage is plotted with the diaphragm bending.
The plot shows that with bending, threshold voltage first decreases and then increases.For a better switching of the device, the threshold voltage should be minimum.

Impact of Sensor's Bending on Sensitivity
When pressure is applied to the cantilever diaphragm of the proposed GS-NWTFET based pressure sensor, the electrical properties changes.The I ON , I OFF , I ON /I OFF , subthreshold slope, and threshold voltage have changed with the bending.The author has considered the I ON /I OFF ratio as the sensitivity parameter for sensitivity calculation.The sensitivity is defined as the change in the current ratio by the change in deflection of the GS-NWTFET based pressure sensor as shown in Eq. ( 1) [20].where, Δ(I ON ∕I OFF ) is the change in current ratio with the bending, and Δ is the length of the bending.Figure 12 shows the variation of sensitivity with respect to diaphragm bending.The plot shows that sensitivity decreases with an increase in bending.
Figure 13 shows the change in threshold voltage, I OFF , I ON , and subthreshold slope with bending in the proposed CP-GS-NWTFET based pressure sensor.Figure 13a shows the change in threshold voltage with the diaphragm bending.From the plot, it can be concluded that the change in threshold voltage decreases with the diaphragm bending.Figure 13b shows the change in OFF-state current with the diaphragm bending.From the plot, it can be concluded that the change in OFF-state current increases with the diaphragm bending.Figure 13c shows the change in ON-state current with the diaphragm bending.
From the plot, it can be concluded that the change in ONstate current decreases with the diaphragm bending.Figure 13d shows the change in subthreshold slope with the diaphragm bending.From the plot, it can be concluded that the change in subthreshold slope decreases with the diaphragm bending.Figure 14 shows the contour plot of total current density in the proposed CP-GS-NWTFET based pressure sensor for bending of 0 nm, 1 nm, and 2 nm.The comparison of device characteristics and sensitivity of the proposed sensor with the existing devices are shown in Table 2. From the Table 2, we can conclude that proposed sensor has highest current ratio as well as sensitivity.Therefore, the presented sensor can be a better candidate for pressure sensor among existing devices.

Conclusion
In this work, a Dopingless and gate stack nanowire TFETbased pressure sensor is designed.The III-V group material Si 1-x Ge x is used at the source side to increase the tunneling rate, which finally increases the drain current.For various bending, the device characteristic, and linearity parameters like g m2 and g m3 have been investigated.With an increase in bending, I ON , I ON /I OFF , and threshold voltage first decreases and then increase.I OFF increases as the bending increases.The subthreshold slope first increases the decreases with the diaphragm bending increases.Further sensitivity analysis has been done.For sensitivity calculations, I ON /I OFF have been considered.The sensitivity for 0.5 nm and 2 nm of bending is 2.74 × 10 08 /nm and 4.73 × 10 07 /nm respectively.Therefore, the proposed CP-GS-NWTFET based pressure sensor can be a potential sensor for detecting the pressure applied on the cantilever gate diaphragm on nanowire TFET.

Fig. 1 a
Fig. 1 a Three dimensional and b two-dimensional structure of the proposed CP-GS-NWTFET based pressure sensor

Fig. 3 2 Fig. 4 Fig. 5
Fig. 3 Structures of the proposed CP-GS-NWTFET based pressure sensor with a no bending b 0.5 nm bending c 1 nm bending d 1.5 nm bending e 2 nm bending

Fig. 6 Fig. 7 Fig. 8 Fig. 9
Fig.6 Comparison of electric field in the proposed CP-GS-NWT-FET based pressure sensor for no bending, 1 nm bending and 2 nm bending

Fig. 10 Fig. 11
Fig.10 Comparison of a g m2 and b g m3 for CP-GS-NWTFET based pressure sensor for no bending, 0.5 nm bending, 1 nm bending, 1.5 nm bending and 2 nm bending

Fig. 12 Fig. 13
Fig.12 Variation of the sensitivity with the bendings in the proposed CP-GS-NWTFET based pressure sensor

Fig. 14
Fig.14 The contour plot of total current density in the proposed CP-GS-NWTFET based pressure sensor for bending of a 0 nm b 1 nm, and c 2 nm ON ∕I OFF ) Δ Author Contributions Navaneet Kumar Singh have done solely whole work of the manuscript.

Table 1
Device parameters used for the design of CP-GS-NWTFET based pressure sensor

Table 2
Comparisons of device characteristics and sensitivity within