Abstract <p>Electrodynamic modeling of the interaction of microwaves with a nonlinear distributed semiconductor inclusion (a Gunn diode of planar geometry) in a strip-slot structure has been carried out by solving a nonlinear 3D diffraction problem (Maxwell’s equations together with the equation of motion of charge carriers in a semiconductor) using a computational algorithm developed by the cross-sectional method. Using the method of nonlinear autonomous blocks with Floquet channels, the results of electrodynamic calculations of the amplitudes of reflected waves (at the input sections of a planar Gunn diode) of the fundamental type of the first and three higher harmonics depending on the longitudinal size of the semiconductor inclusion in the strip-slot line have been obtained.</p>