where, the Vth (air) and Vth (bio) are the threshold voltages of the DM DT GE-MOSFET (measured with the help of constant current method at ID = 10− 7 A/µm) in the absence of biomolecules in the nanocavity (air-filled) and the presence of biomolecules in the nanocavity, respectively. The maximum change of threshold voltage is nearly 100 mV at K = 10 (keratin filled) and the minimum change is 52 mV at K = 2 (streptavidin filled) shown in Fig. 6b. Furthermore, the relative change in Vth shift values for keratin and streptavidin is ~ 92.3%, which indicates that the proposed device is highly sensitive in case of detecting specific biomolecules. Additionally, the absolute sensitivity profile shown in Fig. 6b depicts that the proposed device is capable of detecting biomolecules at a very low supply voltage and thus it is suitable for low-power sensing application.
3.5 Threshold Voltage Sensitivity (S
Vth) for Charged Biomolecules (Positive and Negative):
In addition, the DM MOSFET based biosensors are capable of providing high sensitivity against the charged biomolecules and the device type (n- or p type MOSFET) needs to be changed to effectively capture the different polarity of charges depending on the ionized biomolecules 24. These charges are generally localized at the surface of the gate dielectric layer and it modulates the surface potential of the channel region.
Figure 7 (a-b) depicts the effect of charged biomolecules on the sensitivity (SVth) of DM DT GE-MOSFET based biosensor. Moreover, the positively charged biomolecules generally increase the surface potential, leading to an increased Vth shift and a consequent decrement in the threshold voltage. Similarly, the negatively charged biomolecules result in an increased threshold voltage and the Vth shift reduces. Therefore, the SVth increases with an increased charge density of positively charged biomolecules at the SiO2 surface in the DM DT GE-MOSFET. The DM DT GE-MOSFET exhibits a significantly high sensitivity (SVth) of 0.67, when the nanocavity is filled with keratin (K = 10) at a positive charge density of 1012 C/cm2 shown in Fig. 7b. However, the SVth of the device degrades in the presence of negatively charged biomolecules inside the nanocavity at a VDS of 0.2 V as shown in Fig. 7a.
3.7 Assessment of linearity and distortion profile of the proposed device:
Furthermore, the linearity and intermodulation distortion (IMD) profiles are one of the pivotal parameters to analyse in case of a MOSFET based biosensor. To achieve higher speed and improved sensitivity in case of a FET based biosensor, less amount of distortion and improved linearity is required. Besides, nonlinearity introduces IMD and generates an undesired distorted signal, which results into the degradation of device performance. Additionally, the parameters used in analysing the biosensor performance in presence of noise are gm3 (higher order transconductance coefficient), third-order voltage intercept point (VIP3), third-order current intercept point (IIP3) and third-order IMD (IMD3) [
25]. To achieve a linear behaviour, the higher order transconductance coefficient or gm3 must be as minimum as possible. As it limits the distortion level of the device and consequently fixes the dc bias point for optimal device performance. Figure 9a shows that gm3 has a peak value of 0.11 A/V
3 and overall value lies in the range [-0.09 0.11] A/V
3. VIP3 signifies that extrapolated voltage and its peak value should be as high as possible to assert less distortion and high linearity [
25]. Variation VIP3 with input V
GS is shown in Fig. 9b. The peak of the VIP3 curve is about 1.4 V. Moreover, the point at which the input signal and third-order distortion signal amplitude converges is noted as IIP3.
Like VIP3, IIP3 should also have a high peak value. Figure 9c depicts the IIP3 profile and has a peak of 0.00125 dB (approx.) at a gate bias of 0.65 V. Furthermore, another pivotal parameter to analyze the device reliability is IMD3, as it indicates the impact of nonlinearity which is a potent issue for linear amplifier. The intermodulation current at which the first- and third-order intermodulation harmonic currents are the same is represented by the IMD3. The IMD3 profile is shown in Fig. 9d, which depicts a peak value of 2.25 db.
3.8 Noise Immunity Analysis of the proposed device:
Figure 10 (a-c) demonstrates the noise characteristics of the proposed device. In addition, the presence of any external element can sometimes lead to noise, which has a detrimental effect on the biosensor performance. Therefore, to investigate the noise immunity of DM DT GE-MOSFET based biosensor, noise parameters like the minimum noise figure, noise conductance and output source impedance etc. has to be analysed in depth [
26]. Moreover, when the biomolecules are trapped and immobilized inside the nanocavity, all the noise FOMs seem to improve at high frequency. This observation can mostly be attributed to the transparent gate material (ITO) used in the DM DT GE-MOSFET architecture as the random motion of free charge carrier (electron) tend to reduce in such materials. Furthermore, in case of a transparent gate material, the electron temperature does not increase significantly with an increase in the concentration of charge carriers (electron). Thus, it results into improved noise immunity profile of the DM DT GE-MOSFET based biosensor in the presence of biomolecules. Here, the minimum noise figure for keratin (K = 10) filled cavity is 1.8 dB at a high frequency of 100 GHz and thus it can be said that the proposed device is noise immune. Figure 10c illustrates another pivotal noise FOM, termed as the optimum source impedance, which is very close to zero at higher frequencies in case of air-filled cavity, but it gradually increases against the increasing dielectric constant of the biomolecule. Therefore, it indicates that the proposed device is capable of exhibiting very low noise in the presence of biomolecules and can be used as an alternative for biosensing applications.