Current flow in NPN transistor

[anhnha]

Here is an image show how currents flow in NPN transistor from Microelectronic Circuits by Sedra Smith:

Can you explain more about injected holes (iB1)? I can't imagine how holes can flow into emitter (N type)
Is it possible to for holes to flow from base to emitter?

 

[anhnha]

I think my main question is how holes diffuse from P type to N type semiconductor.
Can anyone help me explain it (how holes diffuse) in terms of electrons?
I can't imagine how holes can exist in N type by diffusion.

 

[CWatters]

It's been a few years so I'm a bit rusty but..

The base is +ve with respect to the emitter so you would expect holes to flow from base to emitter and electrons the other way. The number of holes injected into the emitter is a lot less than the number of electrons injected the other way due to the doping.

Due to the number of free electrons in the n-type emitter a hole injected there is not likely to go far before it meets an electron and is destroyed.

Since the base is short most of the electrons make it from the emitter to the collector and since only a few holes make it from base to emitter Ic>>Ib and the transitor has current gain.

 

[anhnha]

Hi,
I am confused about this:
When we say that holes are injected from base to emitter it means that electrons in valence band (not electrons in conduction band) move from emitter to base.
Is this right?

And this is another question:
When an electron in conduction band from emitter falls into hole in base, it become electron in valence band. I wonder where the electron will go.
- the electron now is in valence band and it moves in valence band to positive lead of battery VBE
- the the electron now is in valence band and it moves in valence band to depletion region in CB junction and here it gets energy from VCB exits valence band and go into depletion region.

The first possibility is what I understand from the book but I wonder if the second is possible.

 

[Nickie]

I can provide some clarification on the concept of injected holes in an NPN transistor. In this type of transistor, the base region is made of a P-type semiconductor material, while the emitter and collector regions are made of N-type semiconductor material.

When a voltage is applied to the base-emitter junction, it creates a depletion region where there are no free charge carriers. However, when the voltage is increased beyond a certain threshold, it causes the injection of minority carriers (holes in the case of an NPN transistor) from the base into the emitter region.

These injected holes can then flow into the emitter region, which is N-type, as they are attracted to the region with an opposite charge. This flow of holes from the base to the emitter is what allows for the amplification of current in an NPN transistor.

So, to answer your question, yes, it is possible for holes to flow from the base to the emitter in an NPN transistor through the process of minority carrier injection. This is a fundamental principle in understanding the operation of transistors and is essential in the design and analysis of electronic circuits.

I hope this explanation helps to clarify the concept of injected holes in an NPN transistor. If you have any further questions, please don't hesitate to ask.