BIPOLAR TRANSISTOR AND METHOD OF MAKING A BIPOLAR TRANSISTOR

A method of making a bipolar transistor comprises forming an extrinsic base layer (104) over an oxide layer on a substrate. After an emitter window is opened in the extrinsic base layer, a sidewall spacer (140, 142) is formed on the sidewall of the emitter window. After forming the sidewall spacer, the oxide layer may be etched away to expose the substrate and to form a cavity extending beneath the extrinsic base layer. Subsequently, an undoped monocrystalline launcher layer (150) and a monocrystalline emitter (130) are formed in the emitter window whereby a peripheral parts of the monocrystalline layers (130, 150) fills the cavity. An anneal is then performed to form an emitter diffusion region (162) in the base layer (112) and a base link region (136) from dopants of the extrinsic base (104) that diffuse into the base layer (112) and into the peripheral parts of the monocrystalline layers (130, 150).