Quantum theory for ac-admittance

Starting from a mean field calculation for the static capacitance of a MIS-nanostructure with a near back gate [P.N. Racec, E. R. Racec and U. Wulf, Phys. Rev. B 65, 193314, (2002)] we develop an approach to determine its ac-admittance. Mainly because of the interaction with the near back gate the inversion electron layer which forms in the considered MIS-nanostructure assumes on open character which is taken into account in the Landauer–Büttiker formalism. For the Coulomb interaction the Hartree approximation is applied. In quantitative agreement with experiments a characteristic step in the static C–V trace results when the inversion layer is populated from the back gate. We found that this characteristic step is dominated by a particular resonance which we call intermediate resonance. Consistent with our static calculations we determine the density–density correlation function in the random phase approximation to find the ac-admittance. As an example we demonstrate that the lifetime of the static resonance induces a characteristic turnover frequency for the ac-admittance. An equivalent small-signal circuit is proposed and the dependence of its elements (capacitance and resistance) on the working point for low and high frequencies are presented.