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-Bilttiker 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. (c) 2005 Elsevier B.V. All rights reserved.