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Maybe someone else can chime in, but two questions:

* How do you know if the base silicon is N-doped or P-doped? (does it matter?)

* Why are the layouts of NPN and PNP so different? I’ve seen many die photos and you can (usually) easily tell which transistor is which.

* Lastly, are there any guides on learning how to decipher a die shot into a “schematic”? How do I know which colors mean what? (they vary between IC)

(if it’s not obvious, I’m still learning about this stuff)




The book "The Art of Analog Design" discusses the layouts of components in detail. It says (p280) that theoretically you could make a PNP transistor like an NPN transistor in reverse, but the characteristics of the dopants don't work well for this. Boron diffuses rapidly so it doesn't work well for a buried P layer. And boron only has 1/3 of the solid solubility of phosphorus. The other issue is that the mobility of holes is only 1/3 that of electrons in silicon, so there's an inherent disadvantage for PNP.

I'm kind of guessing that the substrate silicon is P-type since that seems to be more common. As for the colors, each IC is different. There are charts relating colors to thickness, but that doesn't really help me.


N type and p type have different resistance so to get a balanced transistor you need different area proportions of each. If memory serves that is.


Holes have lower mobilty than electrons, so P-doped areas must be larger for the same equivalent resistance.




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