**8. Conclusion**

increasing and convex step profiles demonstrate poor sensitivity. The reason for this is the proximity of the highest step with the source-channel tunnel junction. In case of decreasing and concave step profiles, the higher steps are present near to the tunnel junction as shown in **Figure 1b**, **d** respectively. As the value of *k* increases, the gate-channel coupling increases in the region of higher steps closer to the tunnel junction. So, the response of the biosensor is better than that of the increasing and convex step profiles where the higher steps are located

There are a number of important simulated and modeled works reported on dielectric-modulated TFET and FET. This section presents a map of the sensitivities of such biosensors pro-

Although the status map of **Figure 7** mentions the maximum or best sensitivities of each work, yet the architectural specifications under which the biosensors have been reported vary from one to another, and hence, drawing comparisons among them through **Figure 7** is not justified. However, there are a few conclusions that can be derived from the status map. Dielectric-modulated TFETs are more sensitive to the presence of biomolecules than MOSFETs due to the difference in their current transport mechanisms. In MOSFETs, sensitivities reduce at lesser channel lengths. The CG TFET, with a channel length of 40 nm, shows significant sensitivity; a fully filled nanogap in CG TFET for *k* = 10 has sensitivity closer to that of DM PNPN TFET for *k* = 10 possessing a channel length of 250 nm and nanogap length of 75 nm. However, the partially filled nanogaps (decreasing and concave step profiles) have

**Figure 7.** Sensitivities of FET-based biosensors of reported works and those of CG TFET. The sensitivities are extracted from the published works, and due to the possible tolerances in extraction, the vertical axis is named as 'approximate sensitivity'. The various biosensors are referred by using serial numbers from 1 to 17, the details of which are listed in

away from the source-channel tunnel junction.

30 Design, Simulation and Construction of Field Effect Transistors

posed till date along with sensitivity of the proposed CG TFET.

lesser sensitivities than the fully filled case as explained in Section 7.2.

**7.3. Status map of biosensors**

**Table 1**.

This chapter has presented an overview on Tunnel Field Effect Transistors (TFETs) as dielectric-modulated biosensors. Tunnel Field Effect Transistors have emerged as one of the most significant devices for low power applications due to their ability to withstand the effects of scaling. With the interests gathering around FET-based biosensors, research on TFETs as biosensors has recently brought new focus. This chapter has discussed the various aspects of dielectric-modulated TFET as biosensor with emphasis on the design and development through simulation analyses. Practical implications of the biosensors are presented. A Circular Gate TFET as a dielectric-modulated biosensor is presented and analyzed at lesser channel length. The CG TFET is observed to offer an impressive sensitivity as compared to other biosensors. The different challenges in implementing a TFET-based dielectric-modulated biosensor are varied, ranging from the problems of steric hindrance, fabrication issues and uncertainty of probe placement. Simulation and modeling may enable one to predict the various effects. Appropriate physics-based models are necessary to validate the results on TCAD tool.
