eli5: What is a reverse emitter inside a transistor?
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Take an NPN transistor as an example. This consists of a region of P-doped silicon surrounded by 2 regions of N-doped silicon. One N region is termed the collector and one the emitter.
However they are both N regions connected directly to the base. This means that the transistor is a reversible device! You can connect the collect and emitter in reverse and the transistor should still work.
So why isn’t this done much? The answer is that the CB junction and EB junction need different optimisation of properties and when the transistor is made the construction of the CB and EB junctions is done very differently.
Importantly, the CB diode needs a high reverse breakdown voltage to ensure that the transistor does not breakdown in normal use. However, the EB junction has a major impact on gain, so it is optimised for gain and reverse breakdown isn’t a significant consideration (typically a maximum of 5V is permitted).
When operated in reverse mode, the transistor will work, but it will usually have very low gain and is at risk of destruction if the blocking voltage is more than about 5-7V. The other issue is a low saturation current in reverse active mode.
Tgere have been a few products which have used reverse connected transistor to achieve an unusually low gain and or low dynamic range, for example for generating audio distortion.
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