A composite metal-polymer electrode is designed to investigate electric-field-induced changes of the dielectric function of gate insulators in metal-oxide-semiconductor structures using infrared spectroscopy. We studied structures based on TiO2 dielectric insulator on doped silicon, a combination commonly used in field-effect transistors. It is shown that the voltage-induced changes of the dielectric constant in TiO2 originate from a modification of the lattice vibration modes of this compound. (c) 2005 American Institute of Physics.