The Alternating-Direction-Implicit Finite-Difference Time-Domain (ADI-FDTD) method is used to analyze Metal-Insulator-Semiconductor-Metal interconnects by solving Maxwell's equations in the time domain. This analysis shows that the silicon substrate losses and the metal line losses can be modeled with high resolution. Our modeling method is supported by experimental data. We find that semiconductors readily operate in the slow wave mode and skin-effect mode for selected doping densities. The ADI-FDTD method is also applied to study the effect of epitaxial layers in different propagating modes. Simulation indicates that inserting epitaxial layers in highly doped substrates should help to keep the signal integrities and reduce substrate noise.