Semiconductor device structures and related fabrication methods are provided. An exemplary semiconductor device structure includes a collector region of semiconductor material having a first conductivity type, a base region of semiconductor material within the collector region, the base region having a second conductivity type opposite the first conductivity type, and a doped region of semiconductor material having the second conductivity type, wherein the doped region is electrically connected to the base region and the collector region resides between the base region and the doped region. In exemplary embodiments, the dopant concentration of the doped region is greater than a dopant concentration of the collector region to deplete the collector region as the electrical potential of the base region exceeds that of the collector region.