Diffusion current is a current in a semiconductor caused by the diffusion of charge carriers (holes and/or electrons). ... Diffusion current can be in the same or opposite direction of a drift current.

Explanation:

Overview of Bipolar Junction Transistor (BJT)

A bipolar junction transistor (BJT) is a three-layer semiconductor device consisting of two p-n junctions. The BJT has three terminals, namely emitter, base, and collector. The emitter is heavily doped, and the collector is lightly doped compared to the emitter. The base region is very thin and lightly doped.

Working of NPN BJT

In an NPN transistor, the emitter is made up of n-type material, and the base is made up of p-type material. The collector is made up of n-type material. When a small current is applied to the base terminal, it allows the flow of majority carriers across the base region. The majority carriers in the emitter are electrons, and they flow towards the collector through the base region.

Mainstream of Current in Base Region

The base region of an NPN transistor is very thin and lightly doped. When a small current is applied to the base terminal, it allows the flow of majority carriers across the base region. The majority carriers in the emitter are electrons, and they flow towards the collector through the base region. However, some of these electrons recombine with the majority carriers in the base region (holes). This recombination is due to the fact that the base region is lightly doped, and hence, it has a low number of holes.

The main stream of current in the base region is due to the diffusion of holes. The electrons that flow towards the collector through the base region constitute only a small fraction of the total current that flows through the base region. The majority of the current in the base region is due to the diffusion of holes.

Conclusion

In conclusion, the main stream of current in the base region of an NPN bipolar transistor is due to the diffusion of holes. When a small current is applied to the base terminal, it allows the flow of majority carriers across the base region. The majority carriers in the emitter are electrons, and they flow towards the collector through the base region. However, some of these electrons recombine with the majority carriers in the base region (holes). The main stream of current in the base region is due to the diffusion of holes.

B. bcuz base has more concentration of holes and collector & emitter has low concentration of holes.. so the main purpose of current in the base will be diffusion of holes bcuz diffusion occurs due to difference of concentration and goes from higher concentration to lower concentration...