FET sensor is fully depleted at the lowest pH by setting a constant reference bias and it starts conducting with an increase in pH. The CT is the series combination of the doublelayer capacitance, Cstern and effective capacitance of the oligopeptides , . Figure schematically shows the molecular arrangement at the FET interface with deposited linker and immobilized oligopeptides immersed in an electrolyte. Figure shows the details of the molecular structure of the peptides (IPP & LKP) with Amine and carboxylic sites responsible for positive and negative charges of the oligopeptides. In our simulation, we considered R and D AAs joined either by the carboxylic terminal (yellow) or by both carboxylic and amine (light purple) terminals depending on the position in the sequence. Considering the linker, the presence of APTES shifts the surface potential to more alkaline values due to the higher affinity of amine sites to get protonated resulting in a better sensitivity at lower pH. Our analytical model successfully captures the effect of deposited APTES with and without the presence of silanol sites.