A system and method are disclosed for providing a fully self aligned bipolar transistor using modified cavity formation to optimize selective epitaxial growth. A collector of a transistor is formed and at least two layers of silicon oxide are formed above the collector and covered with a polysilicon external raised base. Then an emitter window is etched through the polysilicon external raised base down to the top layer of silicon oxide. A wet etch process is performed to form a cavity in the at least two layers of silicon oxide. Different wet etch rates of the silicon layers with respect to the wet etch process cause the cavity to be formed with a shape that optimizes selective epitaxial growth in the cavity. Polysilicon rich corners and a monocrystalline silicon base are then formed within the cavity.