Disclosed are a transistor (e.g., bipolar junction transistor (BJT) or a heterojunction bipolar transistor (HBT)) and a method of forming the transistor with a narrow in-substrate collector region for reduced base-collector junction capacitance. The transistor has, within a substrate, a collector region positioned laterally adjacent to a trench isolation region. A relatively thin seed layer covers the trench isolation region and collector region. This seed layer has a monocrystalline center, which is aligned above and wider than the collector region (e.g., due to a solid phase epitaxy regrowth process), and a polycrystalline outer section. An intrinsic base layer is epitaxially deposited on the seed layer such that it similarly has a monocrystalline center section that is aligned above and wider than the collector region. An extrinsic base layer is the intrinsic base layer and has a monocrystalline extrinsic base-to-intrinsic base link-up region that is offset vertically from the collector region.