Electronics > Beginners
PN junctions
codingwithethanol:
Hi, I've prepared a image detailing my query, but long story short, I would like to know if it would be a feasible idea to manufacture different sized pieces of pure p or n semiconductors for educational purposes. Said pieces could be enclosed in IC-like packages and breadboarded to create transistors, diodes, thyristors, etc. Would this work, and if not, why?
iMo:
No. The PN junction is created only when the P and N areas are in "direct contact" (with no impurities in between).
codingwithethanol:
But wouldnt a short length of wire create a path of neglible resistance? Shouldnt there still be a force experienced by electrons in a chunk of n type, since there is positive charge just across the wire?
edit: Ok I thought about it and I suppose the electrons in the wire itself would pose an issue, couldnt this still be solved via clever engineering?
T3sl4co1l:
This was something I didn't get, way back in the early days. You see diagrams with squares pushed together, and no one explains what that abstraction is really derived from; because most of them don't even know. Worse still, sometimes you see (internal diagrams of) ICs with back-to-back diodes, being used as diodes, and sometimes as BJTs, and vice versa; so which is it?!
Well, as it turns out, all the magic happens by pushing blocks together, yes, BUT -- the magic only happens over a very short distance, less than 10 micrometers or thereabouts.
So when you see an NPN stack structure, they're really showing a very highly magnified core sample through the device -- what's really made is a stack of pancakes, very thin and relatively wide. They're also curled up at the edges, because it's a planar process, everything applied to the top of a crystal. Or, uh... to take the pancake analogy a little too far, it's more like a large block of cake, with a sequence of different syrups absorbed into the surface, giving different characteristics at different depths.
More specifically, junctions have to do with quantum energy bands and electron/hole flows; it only works within a material that is 99.999%+ the same stuff (i.e., high purity silicon in the typical case), with only extremely nuanced changes distinguishing those layers. If you stick in a piece of metal, well metals have completely different electronic structures (they're literally flooded with free electrons, so holes are recombined instantly on contact, and there's an unlimited supply of free electrons). So the semiconductor physics stops at the door, so to speak; you can't put two blocks of semiconductor on either side of any piece of metal, and maintain those weird semiconducting current flows between the two, the metal just destroys that behavior inbetween.
So, in short, that's why you can't wire two diodes together and make a transistor -- or, to such extent as you can even call it that, it's one with hFE < 10^-10, as far as you'll be able to measure. It's a bit disingenuous to describe a transistor as "two diodes stuck together", and this is why it's not quite that simple.
Hope this helps. :)
Tim
ledtester:
You might search youtube for "jerri ellsworth homemade transistor" to see how she did it and what's involved.
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