AbstractA comprehensive analysis procedure is presented to investigate mode propagation in a class of semiconductor‐based transmission lines suitable for application in microwave or optoelectronic circuits. The method of lines (MoL) has been used to investigate single or multiconductor planar transmission line structures printed on a combination of insulating and semiconducting substrate. Homogeneous as well as inhomogeneous doping areas in the semiconductor are included in the theoretical formulation. Numerical results are presented for III–V semiconductor travelling wave electro‐optic modulators in double‐rib, multilayer strip waveguide configuration, microslabTMtransmission lines with partial and full strip cover and slow‐wave MIS microstrip/coplanar transmission lines on thick and thin film semiconductor substrate with gradually inhomogeneous dop