**1. Introduction**

The dependence of economics of the semiconductor industry on Moore's Law led to the downscaling of device dimensions in metal oxide semiconductor field effect transistors (MOSFETs) in order to accommodate more transistors in the same chip area. As the device dimensions were scaled down to the nanometer regime, degradation in the performance of MOSFETs in the form of short channel effects (SCEs) was observed. The inversion charge sharing by the source and drain regions in short channel MOSFETs led to problems of drain induced barrier lowering (DIBL), threshold voltage roll-off, mobility degradation and high field saturation [1–3]. These problems hindered the progress of MOSFETs towards low power applications which required reduced supply voltage and targets of low off currents. Since

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

then, the semiconductor industry has been on the lookout for novel devices which can effectively address the issues of scaling and depict performance which is superior to MOSFETs.

the gate dielectric material with receptors on its surface to immobilize the biomolecules [42]. Dielectric modulation, on the other hand, employs the effect of change in dielectric constant in a portion of the gate dielectric on the drain current and the associated electrical parameters [37–41]. The gating effect is effective for detecting charged biomolecules, while dielectric

Dielectric-Modulated TFETs as Label-Free Biosensors http://dx.doi.org/10.5772/intechopen.76000 19

Im et al. proposed a dielectric-modulated (DM) FET-based biosensor [36] after the ion sensitive FETs proposed in 1970s [43]. Sarkar and Banerjee presented a nanowire TFET in [42] demonstrating the gating effect for positively charged biomolecules. It is convenient to assume that the embedded nanogap is completely filled with biomolecules. However, Kim et al. reported on partially filled nanogaps in practical cases due to steric hindrance, and proposed a parameter, fill factor, defined as the percentage of the nanogap occupied by biomolecules [44]. Narang et al. provided similar simulation analyses for partially filled nanogaps for DM FET and PNPN TFET [11]. Narang et al. further presented a Poisson equation based analytical model to account for the effect of dielectric modulation in TFETs [38]. Partially filled nanogaps decrease the response of the biosensor, as the effective dielectric constant varies with position within the nanogap. Abdi and Kumar proposed the concept of deriving the sensitivity through ambipolar current in TFET [45]. Ahangari presented reports of a dual material gate nanowire junctionless TFET as a biosensor [46]. As the nanogap length increased, the sensitivity improved considerably. Kanungo et al. reported that a short gate dielectric-modulated TFET biosensor showed improved sensitivity than a full gate dielectric-

modulation can assist in sensing charged and neutral biomolecules.

**3. Dielectric modulation in TFETs: concept and geometry**

depicted in **Figure 1** [3]. This contributes to the drain current.

*<sup>T</sup>*(*E*) <sup>=</sup> *exp*(<sup>−</sup>

WKB approximation, its tunneling probability is calculated as [47].

A conventional homojunction TFET is a gated reverse-biased p-i-n structure [3]. As opposed to the thermionic emission in MOSFETs, the mechanism of transport in TFETs is band-to-band tunneling. In an n-TFET, when positive gate bias increases, the energy bands get suppressed as a result of which the width between the p + source valence band, and i-channel conduction bands reduces, thus facilitating the tunneling of electrons from the former to the latter as

The tunnel barrier at the source-channel junction is modeled as a triangular barrier, and using

where m\* is the effective mass, *EG* is the energy band gap at the source-channel tunnel junction, is the energy overlap of the bands at tunnel junction, *λ* is the screening tunneling length, q is the electronic charge, and ℏ is the reduced Planck's constant. The screening length *λ* is

4*λ* √

\_\_\_\_\_\_\_\_ 2 *m*<sup>∗</sup> *EG* 3/2 \_\_\_\_\_\_\_\_\_\_

<sup>3</sup>*q*ℏ( <sup>+</sup> *EG*)) (1)

modulated TFET [40].

**3.1. Principle of operation**

defined as [47].

Over the past few decades, industries and researchers have proposed a number of devices as prominent alternatives to MOSFETs for low power applications. Most of these devices possess principles of operation which are different from MOSFETs. The International Technology Roadmap for Semiconductors in its document 'Beyond CMOS' published in 2015 reported the emerging devices based on structure or materials and charge/non-charge entity [4]. This include a number of devices like nanowire FET [5–7], carbon nanotube FET [8–10], graphene FET [11–13], TFET [14–16], spin FET [17–19] and negative gate capacitance FET [20, 21]. Of these devices, TFETs have gained concentrated focus for low power applications due to their fundamental fabrication methodologies being similar to MOSFETs, and their ability to achieve sub-60 mV/dec subthreshold swing and lower off currents than MOSFETs. TFETs operate by interband tunneling mechanism unlike thermionic emission in MOSFETs due to which the high energy tails of the Fermi distribution of carriers while moving from source to drain get curtailed, resulting in low subthreshold swings and off currents. Different architectures of TFETs have been proposed till date to improve their performance and increase the on currents. [22, 23], nanowire TFET [24, 25], heterojunction TFET [26, 27], III-V TFET [28], triple material gate TFET [29, 30], cylindrical TFET [31] and SOI TFET [32] are some of the widely used structures.

TFETs have found their uses in a wide range of low power applications like digital circuits and memory applications [33–35]. However, recently, the emergence of FET-based biosensors has projected TFETs as biosensors based on dielectric modulation in which the dielectric constant along with the charge of the biomolecules in the gate dielectric region affect the drain current [36]. The sensitivity of the biosensor in presence of biomolecules is defined with respect to a reference value. A number of geometries of TFETs has been proposed as dielectric-modulated biosensors, and the analyses of their sensitivities having dependence on device parameters have been reported [37–41].

Section 2 of this chapter presents a brief report on the existing works on FET-based biosensors. In Section 3, the principle of dielectric modulation in TFETs and a reference architecture for TFETs as biosensors are discussed. Section 4 mentions the different physics-based models to be considered while simulating a TFET on a Technology Computer Aided Design (TCAD) tool. The different sensitivities are defined in Section 5. Section 6 mentions the various nonideal conditions that may possibly exist in case of FET-based biosensors. A circular gate TFET is analyzed as a dielectric-modulated biosensor through TCAD simulation in Section 7. Section 8 concludes the chapter and comments on future scope.
