A most common technique used in the fabrication of optoelectronic devices involves the growth of one or several epitaxial layers of a givenAIIIBVsemiconductor compound over a single crystal substrate. The choice of the epitaxial compound composition (binary, ternary, or even quaternary compounds are used) is usually guided by the desire to achieve a given band gap. As a result, a lattice mismatch is generally introduced between the epitaxial layers and substrate. The lattice mismatch parameterf, which is a function of the film composition, and the temperature of the layer‐substrate structure, is an important parameter for achieving control over the film quality and the device performances. The misfit‐induced defect generation and the role of these defects on the device performances are analyzed. The results presented which cover experiments on Ga1−xAlxAs1−yPy–GaAs double heterojunction structures and GaP homojunction structures are general enough to be applied to other types of mismatched epitaxial structures.The buildup in misfit strain energyEfduring the growth of a mismatched epitaxial layer is released by elastic and/or plastic deformation of the lattice. The introduction of misfit dislocations, one form of the film plastic deformation, takes place in several stages which are functions of the magnitutde ofEfand the structure temperature.1,2Substrate defects such as dislocations, dislocation loops, and precipitates are playing a major role in the first stage of the misfit dislocation formation since they originate from these substrate defects. During the first stage of formation an interesting asymmetrical distribution of misfit dislocations along a 〈110〉 direction is also observed for {100} Ga1−xAlxAs1−yPy–GaAs structures.1,2Use of this asymmetrical behavior is made to obtain defect‐free double heterostructures. The second stage of formation corresponds to the generation of ’’60° type’’ dislocation half loops to accomodate the misfit strain in the other 〈110〉 direction. The third stage results in an increase in the density of interfacial dislocations with Burger’s vectors in the interface plane.In the case of GaP epitaxial layers3with small misfit values, several defect‐formation stages are observed during the film growth. The first one corresponds to a large increase (10–100 times) in the epitaxial‐layer dislocation density over that of the substrate. These new dislocations which originate from substrate dislocation loops near the interface are not efficient in relieving the misfit strain. The other stages are analogous to those described above for the formation of misfit dislocations in Ga1−xAlxAs1−yPy–GaAs structures. The role of dislocations on the device efficiency and degradation behavior1,4is analyzed. Evidence for a stimulated dislocation motion and multiplication process upon minority carrier injection conditions is presented. This dislocation motion is attributed to the recombination‐enhanced mobility of point defects in these materials and to the efficient point‐defect sink behavior of the dislocations.5