**6.1. Steric hindrance**

is necessary [50]. Bandgap narrowing is important for heavily doped geometries [50]. Nonlocal band-to-band tunneling models are essential to create a suitable simulation environment for the device whose principle of operation is interband tunneling [50]. Field dependent

The performance of a TFET or any MOS-based device is reflected through its electrical parameters. The most commonly used parameters for defining sensitivity are the drain current and threshold voltage. Subthreshold swing (SS) may also be considered for defining a sensitivity parameter; however, the measurement of SS is dependent on the orders of the logarithmic scale over which the drain current is measured. This value or the measurement may not be consistent as the drain current changes with the variation in the dielectric constant of the immobilized biomolecules. Sensitivities are usually defined with respect to reference values. The reference value in case of a dielectric-modulated biosensor is considered when the nanogap is devoid of biomolecules, and hence, is assumed to be filled with air with a dielectric constant of 1 without any charges at the oxide-semiconductor interface. The drain current

> \_\_\_\_\_ *D*,*k I <sup>D</sup>*,*<sup>k</sup>*=1|*VGS*

ecules and the nanogap is unfilled. The values must be measured at the same gate voltage so

The threshold voltage has a dependence on the dielectric constant of the gate dielectric and charge at the semiconductor-oxide interface. The shift in threshold voltage with the immobilization of biomolecules with different dielectric constants may be taken up as a sensitivity

There are many threshold voltage extraction methods for MOSFETs, and TFETs. Of them, the Linear Extrapolation (LE) Method is the most widely used [51, 52]. According to this extraction principle, the intercept on the gate voltage axis made by the tangent to the drain current

age. Although this extraction method may result in change in threshold voltage with change in range in gate voltage, however, we have considered a fixed range of gate voltage in all the cases of comparison. So, we have used this method of threshold voltage extraction from

are the values of drain currents when the nanogap is filled with biomol-

are the threshold voltages for the cases when the nanogap is filled with

= *VT*,*k*=1 − *VT*,*<sup>k</sup>* (5)

*<sup>D</sup>*/*<sup>d</sup> VGS* is defined as the threshold volt-

(4)

mobility models may be used [50].

22 Design, Simulation and Construction of Field Effect Transistors

**5. Sensitivity parameters**

where *I D*,*k* and *I D*,*k*=1

where *VT*,*<sup>k</sup>*

and *VT*,*k*=1

simulation transfer characteristics.

based sensitivity is mathematically expressed as [44].

*Sensitivity*, *SI* <sup>=</sup> *<sup>I</sup>*

as to get a justified value of sensitivity.

parameter. It is mathematically expressed as

*Sensitivity*, *SVT*

biomolecules and when the nanogap is completely unfilled.

curve corresponding to the maximum value of *gm* <sup>=</sup> *<sup>d</sup> <sup>I</sup>*

In case of steric hindrance, the biomolecules which get immobilized first prevent the further entry of biomolecules. In fact, there is a hindrance to the biomolecules which are likely to get immobilized in the nanogaps, resulting in partial hybridization. In order to account for this on a TCAD tool for simulation, different patterns of immobilization inside the nanogap are assumed, like increasing, decreasing, concave and convex profiles of placement [39]. As explained in Section 2, this can be designed by defining different heights of gate dielectric material mimicking the biomolecules according to the profile of biomolecules.
