This work presents the field effect transistor (FET) based biosensors with different sensing membranes to sense neutral biomolecules. The stability and the chemical immunity are the major parameters to enhance the sensing capabilities of the biosensor device which are achieved by stacking the metal oxides with the conventional SiO \(_{2}\) thin layer as a sensing membrane. However, for sensing the neutral biomolecules, the nanocavity has been created by etching out the stacked HfO \(_{2}\) /SiO \(_{2}\) and Al \(_{2}\) O \(_{3}\) /SiO \(_{2}\) thin layer underneath the gate oxide. Thus, to sense the neutral biomolecule the electric parameter such as dielectric constant (K) has been considered. To assess the sensing potential of both devices, we also compared the sensitivity, and noise characteristics of the stacked thin layers of HfO \(_{2}\) /SiO \(_{2}\) and Al \(_{2}\) O \(_{3}\) /SiO \(_{2}\) . From the comparison, the stacked HfO \(_{2}\) /SiO \(_{2}\) thin layer shows the better sensitivity and noise characteristics.