Impedance spectroscopy is used to characterize the structure of biomaterial layers on the gate surface of ISFET devices, and to elucidate antigen-antibody binding interactions on the gate interface. The method is based on the recording of the transfer function of the system as a function of frequency and on the extraction of the respective iota (1) and iota (2) values of the systems. From the experimental values, the film thicknesses of the respective protein layers are estimated. The method is applied to elucidate the structures of a glucose oxidase, GOx, multilayer assembly, and of a di-biotin-cross-linked avidin multilayer system. The method is also used to sense the dinitrophenyl antibody by following the formation of the antigen-antibody complex on the gate surface.