Device structures and fabrication methods for heterojunction bipolar transistors. Trench isolation regions are positioned in a semiconductor substrate to define active regions. A base layer includes first sections that are respectively positioned over the active regions and second sections that are respectively positioned over the trench isolation regions. Emitter fingers are respectively positioned on the first sections of the base layer. The first sections of the base layer include single-crystal semiconductor material, and the second sections of the base layer include polycrystalline semiconductor material. The second sections of the base layer are spaced in a vertical direction from the trench isolation regions to define a first cavity that extends about a perimeter of the base layer and second cavities that are connected to the first cavity.