Bipolar transistor with extrinsic base region and methods of fabrication

A method of forming an integrated circuit (IC) structure, the method comprising:

• forming a precursor structure on a substrate, wherein the precursor structure includes:
• a crystalline base layer,
• a first semiconductor layer positioned on the crystalline base layer, and
• a second semiconductor layer positioned on the first semiconductor layer;
• removing portions of the substrate and the precursor structure to expose two underlying portions of the substrate beneath the precursor structure;
• forming a dielectric spacer on the precursor structure;
• selectively removing the first semiconductor layer, the second semiconductor layer, and the exposed two portions of the substrate to undercut two exposed extrinsic base regions of the crystalline base layer, wherein the selectively removing includes:
• removing the substrate before removing the dielectric spacer to form an isolation trench;
• removing the dielectric spacer,
• forming an oxide within the isolation trench up to the second semiconductor layer of the precursor structure, and
• selectively removing a portion of the second semiconductor layer;
• and
• growing the two exposed extrinsic base regions to have a greater thickness than an intrinsic base region of the crystalline base layer positioned between the substrate and the first semiconductor layer and between the two exposed extrinsic base regions.