Dislocations act in semiconductors as combined positron traps, consisting of a shallow precursor state, which is related to regular parts of the dislocation line, and bound vacancy-like defects. The motion of dislocations during plastic deformation generates a high number of point defects. The clustering of vacancies to stable voids is regarded as a primary process of the jog dragging mechanism. Vacancy densities were quantitatively determined for different deformation conditions of GaAs.