Two regimes of defect generation have been found in MOVPE GaAs/Ge layers upon changing the V/III ratio between 1.3 and 11.8. For low V/III ratio the layers contained misfit dislocations along with stacking faults that had been generated by dissociation of the misfit dislocations. The stacking fault density increased with decreasing V/III ratio. This might be explained by an enhanced mobility of the dissociated partials due the reduced unintentional doping of the layer caused by reduced Ge outdiffusion from the substrate when V/III is small. The secon regime corresponds to high V/III ratios and is characterized by the absence of misfit dislocations and the presence of a high density of planar defects. This means that breakdown of the 2D layer-by-layer growth occurred and 3D island growth prevailed.