Vertical Scaling Considerations for Polysilicon-Emitter Bipolar Transistors

Until the early 1980s, the industrial standard of high-speed bipolar processes was characterized by implanted base and arsenic implanted, metal-contacted emitter devices. One of the essential achievements leading to the “renaissance” of bipolar technology [3.1] that has occurred since then is the use of polycrystalline silicon (poly-Si) as diffusion source and contact material for the emitter. Besides facilitating the second important achievement, self-alignment between emitter and base contact, the poly-Si emitter has the following major advantages: i.) Without compromising current gain, extremely shallow emitter junction depths can be formed, leading to a strong reduction in emitter charge storage. ii.) Due to the shallow emitter junction, extremely narrow base regions can be realized with sufficient process control. This, of course, serves to reduce base transit time. iii.) The polysilicon layer interposed between the active emitter region and the metal contact enhances device yield considerably due to its gettering capability and its blocking action against metal sintering and spiking .

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