Heterojunction bipolar transistors with multiple emitter fingers and undercut extrinsic base regions

A method of forming a heterojunction bipolar transistor, the method comprising:

• forming a plurality of trench isolation regions surrounding a plurality of active regions that each include a collector;
• forming a base layer that includes a plurality of first sections containing single-crystal semiconductor material that are respectively arranged over the active regions and a second plurality of sections containing polycrystalline semiconductor material that are respectively arranged over the trench isolation regions;
• removing a first semiconductor layer of each of the second sections of the base layer selective to a second semiconductor layer of each of the second sections of the base layer to define a plurality of cavities that are arranged in a vertical direction between the second semiconductor layer and the trench isolation regions;
• and
• forming a plurality of emitters respectively arranged on the first sections of the base layer.