The defect substructure produced by a double diffusion of boron and phosphorus into a silicon wafer has been studied by transmission electron microscopy. Specimens were examined at all stages in the diffusion and at all important levels in the finaln‐p‐ntransistor wafer for both {111}‐ and {110}‐oriented wafers. Similar specimens in the {112} orientation were examined at the emitter surface only.Diffusion‐induced dislocations and precipitates were observed at the emitter surface in all orientations. No defects were found at other levels in the doubly diffused wafers except for {110} foils in which long dislocations capable of glide on the inclined {111} planes were observed at the emitter base junction.The precipitates were in the form of thin platelets which produced displacement fringes parallel to their intersection with the surface (g·Rcontrast) and parallel moire´ fringe contrast. In addition they were strongly attacked by both HF and HNO3solutions. Diffraction patterns from the precipitates were indexed as from a base‐centered orthorhombic structure witha=6.3,b=3.8, andc=6.75 Å. The orientation relationship with the silicon matrix was [111] Si ‖ [011]p. The thin platelets appeared to be a partially coherent precipitate with an associated misfit vector of ∼⅓[111] such that the platelets compressively stressed the matrix. This compressive stress acts to partially relieve the diffusion‐induced tensile stress normal to the habit plane, and so the operating planes are those which are most perpendicular to the diffusion front so as to have the greatest possible resolved compressive stress in the diffusion front.