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The dynamic behavior of electron‐beam‐pumped semiconductor lasers can change drastically depending on whether transient or permanent waveguiding predominates. Measurements of the far‐field diffraction patterns at lasing threshold from GaAs bulk crystal and GaInAs heteroepitaxial layers of various thicknesses are reported. The refractive‐index step at the interface between the epi‐layer and the substrate due to the changes in material composition results in optical confinement of the laser radiation within the GaInAs layer. A theoretical model for such a laser cavity is introduced to investigate the dependence of the diffraction loss and the transverse distribution of the radiation on the cavity parameters. The effects of the confining surface decrease as it becomes less accessible to the laser radiation. The critical parameters are the thickness of the passive region, the optical losses in that same region, and the refractive‐index difference between active and passive regions. The analysis permits the determination of the specific conditions under which the effects of the back‐reflecting surface become negligible.