how bjt amplifier works
zahrein
Strictly speaking, IC design engineer is not required to know this. It's the device engineer who needs to know the solid-state device physics.
You may want to use a 3-ply NPN schematic to visualise, but technically it doesn't looks this way. In reality it is a N-type substrate, in it a P-well by diffusion or implantation, than a highly-doped N+ in the P-well. You can imagine 3 'U's or bowls such that the smallest bowl in the middle bowl and the middle bowl in the largest bowl.
Assuming an NPN Transistor,
1. In BJT, Emitter is heavily doped, i.e. N+. Base is p-type and narrow. Collector is moderately doped N.
2. With N+ so heavily-doped in Emitter, this prevents back-injection from Base and also provides a steep gradient (i.e. potential energy) for the electrons (majority carriers) in Emitter to become kinetically energetic to flow into Base, when Emitter-Base Junction is Forward Biased. Electrons in Base are now called minority carriers where the holes are the majority carriers.
3. The Base is delibrately made narrow to serve one purpose, i.e. to provide limited replacement holes when Emitter-Base Junction is forward-biased. Some holes must be present in the Base for Base current to enter (i.e. when electrons leave from the Base), in order to attract electrons from the Emitter. This principal concept comes from a PN junction diode called Drift when a diode is biased by applying an external source.
4. Since Collector is tied to the highest potential than Base, it creates a sizable electric field to attract most of the electrons from Base into Collector, and electrons leave from Collector, i.e. permitting the flow of Collector, which is β times than Base current.
BJT is called Bipolar instead of a Unipolar like MOSFET or JFET.
As you can see, both holes and electrons are active carriers. Electrons and Holes are opposite poles (i.e. having opposite polarities), thus calling any transistor with such distinction a Bipolar Junction Transistor.