The only existing theory, developed by Nee, for the enhanced intersubband infrared absorption in a metal-insulator-semiconductor system due to dielectric irregularities has been significantly extended. We have rederived the relevant absorption formulas with reduction of some of the approximations used in Nee's paper and obtained more-general results. With these more-transparent formulas we can qualitatively explain some experimental data. Correlations between the experimental and theoretical power absorption are analyzed and discussed. A model is proposed. The calculated absorption spectra of this model are generally broadened and enhanced when including more possible transition momenta.