A bipolar junction transistor (BJT) may include a substrate defining a collector region therein, and a silicon-germanium (SiGe) base over the collector region. A superlattice may be embedded within the SiGe base, the superlattice having a plurality of stacked groups of layers, with each group of layers having a plurality of stacked base semiconductor monolayers defining a base semiconductor portion, and at least one carbon monolayer constrained within a crystal lattice of adjacent base semiconductor portions. The BJT may also include an emitter on the SiGe base. The embedded superlattice may suppress boron diffusion within the SiGe base during thermal processing. The BJT may further include additional superlattices between the collector region and the SiGe base, and/or between the SiGe base and the emitter.