Discussion
What is the correct sequence of the following step in the fabrication of a monolithic, bipolar junction transistor ?
1. Emitter diffusion
2. Base diffusion
3. Buried layer formation
4. Epi-layer formation
Select the correct sequence using the codes given below :
Answer: C
The correct sequence is 3, 4, 2, 1: buried layer formation, epitaxial layer growth, base diffusion, emitter diffusion. The n+ buried layer is made first on the p substrate, the n epitaxial layer is grown over it, then the p base is diffused into that layer, and last the n+ emitter is diffused into the base.
The emitter must go after the base because it lies inside it. The buried layer must go before the epi-layer because it lies beneath it. Every other order breaks one of these rules.
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