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Following this question, Visual example of NPN transistor, is the following a correct understanding of a PNP transistor?

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To be honest, I have no idea why the top of the transistor (where the current flows into) is called the Collector for an NPN transistor and an Emitter for a PNP transistor. Why isn't it just called the same thing, if it's "where the conventional current flows in" ? That part seems incredibly confusing to me (aside from understanding the actual current flow in the two transistors).

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  • \$\begingroup\$ Transistors were first created by physicists and they talked in terms of actual electron flow, not the conventional current that is typically used by engineers. And not everyone at that time adopted the "conventional current" notion. That's why the terminals are named the way they are. \$\endgroup\$ – Elliot Alderson Jan 18 '20 at 14:19
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Yes, it's correct. In a real design you'd want more base current to ensure the transistor is fully turned on, maybe 5% of the collector current.

The collector and emitter are different (usually) on transistors. It's possible to make a transistor that is symmetrical where they are the same, but few available BJTs are made that way. JFETs, on the other hand, are symmetrical.

One difference is that the emitter-base breakdown voltage is typically much less than the collector-base breakdown voltage (when the respective junctions are reverse biased). That's a side effect of optimizing the gain, which is much higher when the transistor is used the right way 'round. On a 5V circuit, you could swap the collector and emitter on most transistors and the above circuit would sort-of work, but you'd see the LED illuminate much more dimly when the switch is closed because the gain (reverse beta) would be much less than 100.

For example, the gain of a 2N4401 might be 250 in the forward direction (under specified conditions of gain and voltage drop) but maybe only 5 or 10 in the reverse direction, so it's not a very good transistor at all. The C-B breakdown might be 60V (guaranteed to be better) but the E-B breakdown more like 6 or 9V typically. The reverse typical figures are not given on the datasheet (they do tell you 5V is okay for the E-B) because most people don't care about the characteristics in that mode.

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