**2. A brief survey**

The compactness, compatibility in fabrication and label-free detection have made FET-based biosensors one of the promising area of interests. Generally, there are two methods to detect the presence of biomolecules: gating effect and dielectric modulation. The gating effect uses 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 modulation can assist in sensing charged and neutral biomolecules.

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 dielectricmodulated TFET [40].
