William Shockley, one of the inventors of the transistor, once explained transistor-amplifiers this way:
>*”If you take a bale of hay and tie it to the tail of a mule and then strike a match and set the bale of hay on fire, and if you then compare the energy expended shortly thereafter by the mule with the energy expended by yourself in the striking of the match, you will understand the concept of amplification.”*
which seems to describe the concept but not the process.
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So … “CMOS” field-effect transistors — the ones which make up 99.99% of modern computers — don’t *on their own* amplify current. They behave more like tiny on/off switches (I.e., transimpedance amplifiers) they do current-to-current or voltage-to-voltage amplifiers. They can, however, be *combined* in many different ways (aka, “circuits”) which can amplify voltage and/or current.
However … Another type of common transistor is a “bipolar junction transistor” which can, on its own, amplify current. It is a three terminal device, and a small current injected into the Base of a BJT will result in a much larger current flowing into the Collector, through the transistor, and out the Emitter.
The underlying semiconductor physics about why Base current is “multiplied” by a BJT in this way is complicated. But i like to think of it that Base current — which can be though if as a stream of tiny charge particles — get absorbed into a place in the semiconductor that wants to be empty of these types of charges. The semiconductor then “overreacts” to the presence of these charges, and works to “flush them out” in a way that we perceive as current multiplication.
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