Methods of forming bipolar junction transistors having preferred base electrode extensions include the steps of forming a base electrode of second conductivity type (e.g., P-type) on a face of a substrate. A conductive base electrode extension layer is then formed in contact with a sidewall of the base electrode. The base electrode extension layer may be doped or undoped. An electrically insulating base electrode spacer is then formed on the conductive base electrode extension layer, opposite the sidewall of the base electrode. The conductive base electrode extension layer is then etched to define a L-shaped base electrode extension, using the base electrode spacer as an etching mask. Dopants of second conductivity type are then diffused from the base electrode, through the base electrode extension and into the substrate to define an extrinsic base region therein. This diffusion step can be performed in a carefully controlled manner to limit the extent to which the extrinsic base region dopants adversely effect the electrical characteristics of surrounding regions or contribute to parasitic capacitance. Dopants of second conductivity type are also preferably implanted into the substrate to define an intrinsic base region therein. This dopant implant step is preferably performed using the base electrode spacer as an implant mask. Accordingly, the base electrode spacer is used advantageously as an etching mask and as a dopant implantation mask. An emitter region of first conductivity type is also preferably formed in the intrinsic base region.