Electronics > Beginners
PN junctions
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codingwithethanol:
Thanks to everyone in the thread for sharing their knowledge and expertise, I appreciate it.

@MrAl
I might actually try this once i get through my current backlog of unfinished projects, why would rust exhibit this effect?

exe:
I also wondered for years if I can make a bjt from two diodes. For me the key to understand how transitors work is "depletion region". I'm still not sure if I got correctly all the details, but at least now I don't try to build bjts from diodes and also not surprized that Vce can be less than 0.6V. I think the Art of Electronics explicitly mentions that conduction of a bjt is not an action of a (intrinsic?) diode.

Oh, btw, diode drop is not fixed at 0.6, it's rather a function of voltage, current, and other parameters. I think this is another "myth" widely spread.

I'd say explanations I was given at university were not helpful. I had to watch youtube and some forum posts to understand it better.
kosine:
If you want to know more, get "Integrated Electronics: Analog and Digital Circuits and Systems" by Millman & Halkias.

PDFs are available online, but secondhand hard-copies aren't very expensive. A classic text that covers all the theory in great depth, highly recommended by many on this forum.

The origin of the "0.6V" diode drop is covered more fully in one of the later chapters, it's affected by the level of doping of the junction (and temperature), so is just a rule of thumb since many different devices have differing doping level and operating conditions.

From a practical perspective, it's where the diode starts to go linear on its VI curve. For the most part a PN junction can be viewed as a logarithmic resistance, i.e. it's a straight line if you plot V against ln(I). Most diodes and BJTs will operate down to 0.1V or lower, just with exponentially less current. Once you get enough current to drop ~0.6V the PN depletion region has all-but vanished, so no more logarithmic response and it operates more like a normal linear resistor.

Wimberleytech:

--- Quote from: ledtester on July 25, 2019, 10:43:20 pm ---You might search youtube for "jerri ellsworth homemade transistor" to see how she did it and what's involved.

--- End quote ---

Thanks for the jerri info.  Never heard of her.  Watched the video of making a mosfet.  Then watched an interview...very interesting story.

Making a mosfet at home...too funny.
ejeffrey:

--- Quote from: exe on July 26, 2019, 10:36:20 am --- think the Art of Electronics explicitly mentions that conduction of a bjt is not an action of a (intrinsic?) diode.

--- End quote ---

Well, the emitter current is legitimately due to diode behavior on the BE junction.  That is why the shockley diode equation and the ebers-moll model are the same formula.  It is the collector current, and by extension the lack of base current that can't be explained that way.
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