We deposit phospholipid monolayers on highly doped p-GaAs electrodes that are precoated with methyl-mercaptobiphenyl monolayers and operate such a biofunctional electrolyte-insulator-semiconductor (EIS) setup as an analogue of a metal-oxide-semiconductor setup. Electrochemical impedance spectra measured over a wide frequency range demonstrate that the presence of a lipid monolayer remarkably slows down the diffusion of ions so that the membrane-functionalized GaAs can be subjected to electrochemical investigations for more than 3 days with no sign of degradation. The biofunctional EIS setup enables us to translate changes in the surface charge density Q and bias potentials U-bias into the change in the interface capacitance C-p. Since C-p is governed by the capacitance of semiconductor space charge region C-SC, the linear relationships obtained for 1/Cp2 vs Q and 1C(P)(2) VS U-bias, suggests that C-p can be used to detect the surface charges with a high sensitivity (1 charge per 18 nm(2)). Furthermore, the kinetics of phospholipids degradation by phospholipase A(2) can also be monitored by a significant decrease in diffusion coefficients through the membrane by a factor of 104. Thus, the operation of GaAs membrane composites established here allows for electrochemical sensing of surface potential and barrier capability of biological membranes in a quantitative manner.