Our simulation methodology is based on a self-consistent method provided by the Site-Binding equations and Gouy-Chapman-Stern model used to describe two measurable quantities as the surface capacitance (CT) and surface potential (Ψo), which depend on orthogonal properties such as the charge, the dielectric constant and the length of the AAs. The charges at the solid-liquid interface are considered from the contributions of immobilized AAs or polypeptides using the sitebinding theory. We have assumed a surface coverage (NS) for single species of AA or small peptides immobilized at the interface between the dielectric barrier of the FET and the electrolyte . Similar to reactive sites on an oxide, each AA or small peptide has different radicals that contribute to the total charge of the molecule depending on the external pH value . The positive and negative surface charge density contributions (σ+ and σ-, respectively) for single AA or a polypeptide can be calculated from the contributions of proton affinities or dissociation constants of each radical (ki);