Sensitivity Analysis of Junction Free Electrostatically Doped Tunnel-FET Based Biosensor

The electrostatic doping technique has a remarkable ability to reduce random dopant fluctuations (RDFs), fabrication complexity and high thermal budget requirement in the fabrication process of nano-scale devices. In this paper, for the first time it has been propose and simulated a junction-free electrostatically doped tunnel field-effect transistor (JF-ED-TFET) based biosensor for label-free biosensing applications. The dielectric modulation concept has been used to sense biomolecules using a nano-cavity incorporated within the gate oxide layer near to the source end. The sensing response of the JF-ED-TFET biosensor has been analyzed in terms of the electric field, energy band and transfer characteristic and sensitivity in terms of ON-current, ION/IOFF ratio and subthreshold swing. The sensitivity of the biosensor has been investigated based on practical challenges such as different filling factors and step-profiles generated from the steric hindrance. The effect of temperate and nano-cavity dimension variations on device performance has been also analyzed. In this work, various types of biomolecules such as Streptavidin (k = 2.1), Ferro-cytochrome c (k = 4.7), keratin (k = 8) and Gelatin (k = 12) has been considered for the performance investigation.

limitations of TFET some techniques has been implemented such as hetero material, stack gate with high k material, vertical TFET, compound material, low bandgap source material, halo doping, packet doping [19][20][21]. The random dopant fluctuations (RDFs), complex fabrication process and high thermal budget requirement are critical issues of doped devices [22]. The new concepts of charged plasma and electrostatic doping are being employed to overcome these restrictions, and these techniques are lowering the fabrication cost and complexity of the nanoscale devices [23][24][25].
In this paper, a junction-free electrically doped tunnel field-effect transistor (JF-ED-TFET) based biosensor is proposed for label-free identification of biomolecules. To achieve high sensitivity, the nano-cavity is created near to source-channel interface junction of the device. For the performance investigation, the target biomolecules are considered neutral and charged with different dielectric constants (k) and charge densities (ρ) (positive and negative). The neutral biomolecules can be sense based on their own dielectric constant and charged biomolecules can be detected based on their charge densities as well as dielectric constants. For sensitivity analysis, we have considered the air (k = 1) as reference biomolecules and compared with other biomolecules of different dielectric constants (k > 1) and charge densities (ρ). The sensitivity of the JF-ED-TFET biosensor is proportional to drain current (I DS ) and drain current (I DS ) is directly proportional to changes of the biomolecules dielectric constants (k) as well as charge densities (ρ).

2-D Structure and Design Parameters of Proposed Biosensor
The schematic view of the proposed JF-ED-TFET biosensor is shown in Fig. 1 and designing parameters are given in Table 1. The polarity concept (electrically doping) has been applied to convert the device from n-n-n (device regiondrain, channel and source) to device n + -i-p + (TFET) [26]. Similar work function of polarity gates and control gate of 4.5 eV has been considered. For the drain/source contact Nickel Silicide (NiSi) with barrier potential of 0.45 eV [26] has been used. Spacer gap at drain-channel junction (D gap ) has been considered 8 nm for reducing the ambipolar conduction and source-channel junction (S gap ) has been considered to the 2 nm to increase the drain current. Based on physical dimensions of biomolecules we have considered the nano-cavity height as 5.5 nm [27,28]. During simulation, various kinds of biomolecules such as Strepatvidin (k = 2.1), Ferro-cytochrome c (k = 4.7), Keratin (k = 8) and Gelatin (k = 12) with different dielectric constant as well as charge densities has been used to investigate the sensing performance of JF-ED-TFET biosensor.

Propose device Modulation and Calibration
The Silvaco ATLAS TCAD device simulator tool, version V5.0.10 R [29] is used for simulation of the JF-ED-TFET biosensor. The JF-ED-TFET biosensor works based on the band to band tunneling (BTBT) hence, to investigate the generation of carriers a non-local BTBT model has been activated at each mesh point of the tunneling zone [30]. Universal Schottky Tunneling (UST) model is considered for NiSi drain/source contact. The SRH (Shockley-Read-Hall) and Auger models are considered for concentrationdependent carrier recombination. For the account of carrier mobility, Fermi-Dirac statistic and field-dependent mobility models are used. The Wentzel-Kramers-Brillo-uin method has been employed for numerical tunneling. TAT model is also incorporated for process-dependent issues in simulation. For result accuracy at device interface layers and at the tunneling region a very dense meshing has been created.
For device calibration, the transfer characteristic of proposed device has been calibrated with conventional ntype TFET [31] and the simulated result, displayed in Fig. 2, is nearly identical to the result described in [31].

Impact of Biomolecules Properties on Device Characteristics
The variations of JF-ED-TFET biosensor characteristics due to immobilization of biomolecules in the nanocavity region with different dielectric constants and charge densities have been studied in this section. For the performance investigation of the JF-ED-TFET biosensor, the biomolecule dielectric constants (k) of 1, 2.1, 4.7, 8, and 12, as well as the charge densities (ρ) of ± 1×10 11 , ± 5 ×10 11 , and ±1×10 12 C/cm 2 , have been considered.

Effect on Electric Field
The internal electric field variations of JF-ED-TFET biosensor with neutral biomolecules is shown in Fig. 3(a). It has been observed that when increasing the dielectric constant of biomolecules, the electric field at the tunneling junction increases. The high electric field at the tunneling junction, minimizes the tunneling width hence the drain current of the device gets increased. For dielectric constant (k) = 12, the peak electric field of 3.4×10 6 V/cm can be obtained at the source-channel junction. The electric field increase (decrease) at tunneling junction with an increment

Effect on Energy Band
The energy band profile of proposed biosensor with various dielectric constants (k) and charge densities (ρ) are shown in Fig. 4(a)-(c). It can be seen, when the dielectric constant of neutral biomolecules increase, the band-gap is decreases at tunneling junction hence tunneling probability of charge carriers increase as shown in Fig. 4(a). With dielectric constant (k) = 12, the band gap is very low as compared to dielectric constant (k) = 1, hence the tunneling probability of charge carriers is very high at (k) = 12. Figure 4(b) and (c) are shows the impact on band bending at the tunneling junction, in presence of different charge densities at the Si-SiO 2 interface. It can be seen with positive charge density, band-gap decreases and tunneling probability increases, while in case of negative charge density band-gap increases.

Impact on Drain Current
The I D -V GS characteristic of JF-PE-TFET biosensor with various dielectric constants (k) of neutral biomolecules (ρ = 0) is shown in Fig. 5(a). It has been observed that by increasing the dielectric constant, the drain current of the device increases and the threshold voltage (V T h ) decreases. Figure 5(b) shows the I D -V GS plot with charged biomolecules at fixed dielectric constant (k) = 8, and it has been observed that by increasing the negative charge density the drain current decreases but with positive charge density, it increases. This phenomenon can be understood with the voltage balance equation of MOSFET [32] given as: where In Eq. 1 V G , ψ S , φ MS , N bio , q, k and t ox are representing gate voltage, surface potential, contact potential, biomolecules charge per unit area, electron charge, dielectric constant and oxide thickness respectively. The gate voltage is constant at applied voltage, but surface potential gets decreased (increased) due to the increase in negative (positive) charge density from Eq. 1. The gate voltage of the device decreases (increases), hence the drain current decreases (increases). The JF-ED-TFET biosensor output characteristic I D -V DS has been plotted at constant V GS with various dielectric constants (k) and charge densities (ρ) are shown in Fig. 5(c)-(d) and it has been noticed that it shows similar behavior as depicted in Fig. 5(a)-(b).

Sensitivity Analysis
Sensitivity is a very important parameter of any type of sensor and high sensitivity is desirable. The JF-ED-TFET biosensor sensitivity has been analyzed in terms of drain current (S I DS ), subthreshold swing (S SS ) and I ON /I OF F ratio. The drain current sensitivity (S I DS ) is defined as [33]: Here, I bio DS and I air DS are the drain currents when nano-cavity filled with biomolecules (k > 1) and air(k = 1). Figure 6(a)-(b) are shows the JF-ED-TFET biosensor current sensitivity (S I DS ) along V GS with different dielectric constants (k) and charge densities (ρ). It is observed that with increasing the dielectric constants (k) of biomolecules, the sensitivity of device increases and similarly with positive charge densities due to increasing the I ON current of device. But with negative charge densities the ON-current decreases hence the sensitivity of the device decreases. Figure 6 Table 2. The sensitivity of biosensor increase with an increase in the dielectric constant as well as positive charge density due to increment of drain current but in case of negative charge densities the sensitivity of biosensor decrease due to decrement of drain current.
The sensitivity of the JF-ED-TFET biosensor can be analyzed in terms of I ON /I OF F ratio and calculated as:  Fig. 7(c). The TFET, become a futuristic device for low power applications because TFET offers low subthreshold swing (<60 mV/decade) and low OFF-current. The JF-ED-TFET biosensor offers a low subthreshold swing (SS) of 27.2 mV/decade hence it efficiently work at low voltage and detect the biomolecules within a limited time. The Subthreshold swing (SS) [34] and SS sensitivity of the JF-ED-TFET biosensor are evaluated [33] as:

S I ON /I OF F = (I ON /I OF F ) bio − (I ON /I OF F ) air (I ON /I OF F ) air
and The SS sensitivity variation of proposed biosensor with different dielectric constants (k) and charges density (ρ) is shown in Fig. 7(a)-(c) respectively. It can be observed that the SS sensitivity increase by increasing the dielectric constants (k) and positive charge density (ρ), where as SS The Transconductance-to-current ratio (g m /I DS ) is a sensing metric [35] for better sensitivity and selectivity of neutral biomolecules. The |g m /I DS | of JF-ED-TFET biosensor has been plotted against gate voltage with different dielectric constant shown in Fig. 8(a). From figure, it has been observed that with increment in the dielectric constant, the device offers a higher |g m /I DS | value at lower drain current. The increment in |g m /I DS | value and |g m /I DS | sensitivity with increment in the dielectric constant of biomolecules is shown in Fig. 8(b). The g m /I DS value is obtain as [35]: variation. It can be seen that when temperature increases, the OFF-state current increases but ON-current slightly changes because the ON-current depends on BTBT tunneling rather than the temperature variation. The I ON sensitivity of JF-ED-TFET biosensor decreases because the drain current with empty cavity (k = 1) also increases by increasing the temperature, is shown in Fig. 9(b). The I ON /I OF F sensitivity of JF-ED-TFET biosensor is shown in Fig. 9(c). The I ON /I OF F sensitivity decreases with increasing temperature because, the OFF-state current increases by increasing the temperature. The values of subthreshold swing increases with increasing the temperature, is shown in Fig. 9(d).

Impact of Temperature Variation on Sensitivity
The sensitivity values obtained from proposed JF-ED-TFET biosensor with different dielectric constants have been compared with various previously reported works are given in Table 3 and it has been observed that the JF-ED-TFET biosensor obtained high sensitivity of 1.12×10 11 with neutral biomolecules at dielectric constant k = 12 (varying V DS =0.0 to 1.0 V and constant V GS = 1.2 V).

Considering Non-Ideal Issues
When the biomolecules are conjugated into the cavity, it may be that the cavity is not fully filled with biomolecules. Hence, it is necessary to investigate the performance of biosensors with different fill factors (FF). The fill factor (FF) is defined as follows: Here A bio cavity is area occupied by the target biomolecules and A total cavity is total area of cavity [36]. In this work, we have discussed four possible FF as 25%, 50%, 75% and 100% as depicted in Fig. 10(a). The I ON sensitivity along dielectric constants at different FF, is shown in Fig. 10(b) and it has been observed that, when fill factor (FF) increase, the JF-ED-TFET biosensor I ON sensitivity increase. Figure 10(c)-(d) are shows the SS sensitivity and I ON /I OF F ratio with varying dielectric constant and FF, here it has been observed that the sensitivity of the JF-ED-TFET biosensor improved by increasing the FF as well as the dielectric constant of biomolecules.
The JF-ED-TFET biosensor has been simulated in presence of steric hindrance to understanding the practical challenge of biosensor uses. Here we have considered four varying step profiles as decreasing, increasing, concave and convex with filling factor of ≈ 58%. This arrangement has been illustrated in Fig. 11(a). The I ON sensitivity along with different dielectric constant and step profile is depicted in Fig. 11(b) and it has been observed, the high sensitivity obtained with increasing and concave step profile because in these two-step profiles got highest proximity of target biomolecules at the source-channel interface, hence tunneling barrier decrease with these step profile [37]. Similarly the SS sensitivity and the I ON /I OF F ratio are gets high with increasing and concave step profile as compared to decreasing and convex step profile. The SS sensitivity and I ON /I OF F ratio with defferent sep profiles are shown in Fig. 11(c)-(d).

Effect of Nano-Cavity Geometry Variations
The effect of nano-cavity dimension variations at the performance of biosensors has been investigated by simulation with two major constraints as cavity length (L cavity ) and cavity thickness (T cavity ). The I DS -V GS characteristic of JF-ED-TFET biosensor with neutral biomolecule (k = 8) is shown in Fig. 12(a). It can be seen that very less impact of L cavity variations over the drain current because the proposed device works based on the BTBT tunneling phenomenon. From Fig. 12(b), it has been observed that the I ON sensitivity increases with increasing the cavity length because drain current of the device decreases with an empty cavity (k = 1) by an increment in the cavity length. The effect of variation in cavity thickness (T cavty ) on the I ON sensitivity of the JF-ED-TFET biosensor is illustrated in Fig. 13(b) and it has been observe that, by increasing the cavity thickness (T cavity ), the sensitivity of the device decreases, as shown in Fig. 13(a), due to decreasing the effective gate capacitance of the device.

Conclusion
In this paper, the sensing performance of nano-cavity embedded dielectric modulated JF-ED-TFET biosensor has been investigated for label-free detection of biomolecules.
The results show that the JF-ED-TFET biosensor can be used for the intuitive examination of charged or neutral biomolecules with different dielectric constants. The electrostatic doping concept offers less fabrication complexity, low cost and reliable device against RDFs. The effect of neutral and charged biomolecules on the electrical parameters of JF-ED-TFET biosensor as electric field, energy band, transfer characteristic, subthreshold swing (SS) and I ON /I OF F ratio have been studied. The simulated results as peak drain current (I DS ) of 8.55×10 − 6 A/μm, Electric field of 3.70×10 6 V/cm, steeper subthreshold swing (SS) of 27.2 mV/decade (< theoretical limit as 60 mV/decade), I ON /I OF F ratio of 2.81×10 11 and g m /I DS value of 67 (> theoretical limit 38.4) are obtained from this proposed model. The JF-ED-TFET biosensor offers high drain current sensitivity of 1.12×10 11 and I ON /I OF F sensitivity of 5.74x10 7 , SS sensitivity is 0.65 and g m /I DS sensitivity of 1.8 for neutral biomolecules with a dielectric constant 'k' = 12. In order to validate the realistic approach of the JF-ED-TFET biosensor, the irregular arrangement of biomolecules filled (step profile) in the cavity and different fill factors have been considered in the simulation. The device design parameters have been optimized for high sensitivity, hence the proposed device JF-PE-TFET becomes highly desirable device in the field of biosensing applications.