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On page 354 of the 6th edition of Microelectronic Circuits by Sedra & Smith under Current flow, the following statement is made;

Current will consist of two components: electrons injected from the emitter into the base, and holes injected from the base into the emitter.

According to wikipedia,

an electron hole is a conceptual and mathematical opposite of an electron.

Moreover,

If an electron is excited into a higher state it leaves a hole in its old state.

This is where it get confusing. Does the injection of holes from the base into the emitter imply a loss of electrons, i.e electrons injected from the emitter into the base or something else altogether?

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You're correct. A hole is a place where an electron could be, but is not. This has to do with the quantized nature of energy/charge. When it comes to charge flow, there are no positive charges, just lack of negative charges. So the term, hole, is a much more convenient way of saying that.

So, anytime you read "injection of holes" you know it means "ejection of electrons". You'll likely find though, that when discussing it or thinking much about it, it's much easier to think of holes as being the things that push electrons out of the way. Hence the creation of the concept.

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