Device structures for a bipolar junction transistor. The device structure includes a collector region, an intrinsic base formed on the collector region, an emitter coupled with the intrinsic base and separated from the collector by the intrinsic base, and an isolation region extending through the intrinsic base to the collector region. The isolation region is formed with a first section having first sidewalls that extend through the intrinsic base and a second section with second sidewalls that extend into the collector region. The second sidewalls are inclined relative to the first sidewalls. The isolation region is positioned in a trench that is formed with first and second etching process in which the latter etches different crystallographic directions of a single-crystal semiconductor material at different etch rates.