Here we can see a Bipolar Junction Transistor (BJT), which has a common base configuration. My instructor recently explained to me its working principle, but I didn't understand his explanation completely as I had some issues with his explanation. His explanation was as follows:
The emitter is very heavily doped, and the base is very, very thin and very lightly doped. The collector is moderately doped.
We have two voltage sources, one having a significantly greater voltage than the other. The DC source with the higher voltage, A, is connected in reverse bias to the base-collector diode, and the DC source with the lower voltage, B, is connected in forward bias with the emitter-base diode. Upon being connected in reverse-bias, the depletion zone of the base-collector diode widens.
Now, as the emitter-base diode is connected in forward-bias with B, electrons will start to flow from the negative terminal of B with an aim to move to the positive terminal of B. However, upon reaching the base, electrons find very less holes compared to their number. So, a very small number of electrons recombine with the holes in the base and generate a small current $I_B$ flowing towards the base ($i$). The rest of the electrons, which are most of them (about 95%), being attracted by the positive donor ions of the other side of the depletion zone of base-collector diode, scoot over to the collector and move towards the positive terminal of A ($ii$). Thus, a much greater current than $I_B$, $I_C$ flows from the positive terminal of A to the negative terminal of A.
- My first issue is with ($i$): current doesn't move by means of recombination of electrons and holes! Current moves by means of flow of free electrons or holes. Why did my instructor mention this? I'm very confused.
- My second issue is with ($ii$): electrons feel attracted to the positive donor ions on the other side of the depletion zone. According to my instructor, that's why the electrons scoot over to the collector and move towards the positive terminal of A, but my question is, the electrons should feel equally repelled by the uncovered negative acceptor ions in the base. So, why do the electrons move to the positive terminal of A?