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"But for a variety of reasons, PNP transistors have an entirely different construction." - Can you expand on what some of those reasons are? And why is the emit
by puzzlingcaptcha 5y ago
"But for a variety of reasons, PNP transistors have an entirely different construction." - Can you expand on what some of those reasons are? And why is the emitter in a PNP circular?
- kens 5y agoTheoretically you could make a PNP transistor by reversing the doping of an NPN transistor. The main problem is that boron diffuses rapidly, making it hard to fabricate a buried P-layer. Boron also has less solubility than phosphorus, making it hard to dope the emitter. Also, holes have only 1/3 the mobility of electrons, so PNP and NPN aren't symmetrical. To deal with these issues, PNP transistors are usually built with lateral construction (i.e. horizontally). The ring structure ensures that almost all of the carriers injected by the emitter are intercepted by the collector. (This is based on The Art of Analog Layout, p280. I don't know all this doping stuff myself.)
- puzzlingcaptcha 5y agoThanks! I also have to say that you have a very accessible writing style for what is a rather convoluted topic and that the interactive schematic tool is a great aid.