Is it possible for electrons to fall into holes and go straight into emitter?

[Osawa_Odessa]

I took this picture from forum aboutcircuits to illustrate current flow in NPN transistor.

Here is what I am confused:
A few at Figure above(a) fall into holes in the base that contributes to base current flow to the (+) battery terminal.
Why these electrons that fall into holes necessary flow into base terminal and contribute to base current flow?
Is it possible that there are some electrons fallen into holes and then go straight into collector?

Re-express my 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 enery 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.

PS: Here is the link to the page:https://www.allaboutcircuits.com/vol_3/chpt_2/8.html

 

[rahdirs]

Re: Is it possible electrons fallen into holes and go straight to emitter?

Here is what I am confused:
A few at Figure above(a) fall into holes in the base that contributes to base current flow to the (+) battery terminal.
Why these electrons that fall into holes necessary flow into base terminal and contribute to base current flow?
Is it possible that there are some electrons fallen into holes and then go straight into emitter?

• There must be some recombination of injected electrons from emitter with holes in the base, even with Wb << Lp.The holes lost to recombination must be resupplied through the base contact.Hence,the holes that are resupplied by the base contact contributes to base current.
• It is not like you think,that electrons that recombine with holes flow into base.Once an electron combines with a hole,a photon is released,if it is direct recombination.There is no charge particle left.Electrons on recombining with hole,there is no charge particle left.There is no such thing as after recombining they go straight to emitter.There is no charge particle left.

 

[christophersid]

Re: Is it possible electrons fallen into holes and go straight to emitter?

But it is possible that holes and electrons generate due to thermal generation.These electrons can go to collector and thus being a part of leakage current

 

[rahdirs]

Re: Is it possible electrons fallen into holes and go straight to emitter?

• But,in his question he was talking about "electrons falling into holes",so he must be talking about Recombination & if there would be any current because of Recombination.But we'll have to wait for him to ask his question clearly.
• Some holes will be injected from p to n in the forward-biased emitter junction, even if the emitter is heavily doped compared to the base. These electrons must also be supplied by the base terminal.
• Some holes are swept into the base at the reverse-biased collector junction due to thermal generation in the collector. This small current reduces Ib by supplying holes to the base.

 

[Osawa_Odessa]

Re: Is it possible electrons fallen into holes and go straight to emitter?

Hi again!

There must be some recombination of injected electrons from emitter with holes in the base, even with Wb << Lp.The holes lost to recombination must be resupplied through the base contact.Hence,the holes that are resupplied by the base contact contributes to base current.

Can you explain more about this? Here is what I think:
When there is an electron from emitter falls into a hole, then at the same time also there is an electron exit base lead and create a new hole at this position.
Therefore, electron recombined can move to the base lead.

It is not like you think,that electrons that recombine with holes flow into base.Once an electron combines with a hole,a photon is released,if it is direct recombination.There is no charge particle left.Electrons on recombining with hole,there is no charge particle left.There is no such thing as after recombining they go straight to emitter.There is no charge particle left.

I am not quite understand it. What is a photon?

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It is not like you think,that electrons that recombine with holes flow into base.Once an electron combines with a hole,a photon is released,if it is direct recombination.There is no charge particle left.Electrons on recombining with hole,there is no charge particle left.There is no such thing as after recombining they go straight to emitter.There is no charge particle left.

I think after recombination, the electron can move through holes in base to base lead.

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But,in his question he was talking about "electrons falling into holes",so he must be talking about Recombination & if there would be any current because of Recombination.But we'll have to wait for him to ask his question clearly.

Yes, I meant that after falling into holes, where do these electrons go?
The ebook say that they will go to base lead but I wonder if they can go to collector after falling into holes in base.

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Hum, I forgot to added the link to the original page: https://www.allaboutcircuits.com/vol_3/chpt_2/8.html

 

[rahdirs]

Re: Is it possible electrons fallen into holes and go straight to emitter?

Can you explain more about this?

Wb is base width & Lp is diffusion length. The base width is kept small so that all the electrons entering the base would go to collector.
If you have large base width,many electrons entering from emitter would recombine with holes in p-region.Hence to minimize re-combinations base width is kept small.

What happens when electron recombines with hole.The hole is a vacant space in valence band,while electrons were in conduction band & electrons were free to move.On direct recombination,conduction electrons and holes decay by conduction electrons falling from the conduction band to empty states (holes) in the valence band.The result of recombination is an electron in valence band which is not useful for conduction purposes.
The electron left in valence band after recombination is only useful to conduction if energy was supplied to it to make a transition to conduction band.

Here is what I think:When there is an electron from emitter falls into a hole, then at the same time also there is an electron exit base lead and create a new hole at this position.Therefore, electron recombined can move to the base lead.

No,a slight mistake.When an electron recombines with hole,hole is lost.The hole lost to recombination must be supplied by base contact,this is equivalent to electron leaving base terminal,but this is a different electron.You may think that because an electron has recombined with hole,another conduction electron has left base terminal.While the electron that has recombined with hole is in valence band.

Those electrons after recombination are in valence band of Si atom.They are now revolving around nucleus of Si atom,they are fixed with Si atom,they are not free to move away until energy is supplied to make them jump into conduction band.

A photon is an elementary particle, the quantum of light and all other forms of electromagnetic radiation, and the force carrier for the electromagnetic force.In simple terms an energy particle.
A photon is released when electron falls from conduction band to valence band.

 

[godfreyl]

Re: Is it possible electrons fallen into holes and go straight to emitter?

Is it possible that there are some electrons fallen into holes and then go straight into emitter?

The ebook say that they will go to base lead but I wonder if they can go to emmiter after falling into holes in base.

In general, the electrons are moving from the emitter towards the collector. Whatever happens in the base region, the electrons are not going to turn around and go back to the emitter.

 

[Osawa_Odessa]

Re: Is it possible electrons fallen into holes and go straight to emitter?

In general, the electrons are moving from the emitter towards the collector. Whatever happens in the base region, the electrons are not going to turn around and go back to the emitter.

Oh, sorry. I meant "collector". I am going to edit it.

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Hi,

Wb is base width & Lp is diffusion length. The base width is kept small so that all the electrons entering the base would go to collector.
If you have large base width,many electrons entering from emitter would recombine with holes in p-region.Hence to minimize re-combinations base width is kept small.

This part is OK.

What happens when electron recombines with hole.The hole is a vacant space in valence band,while electrons were in conduction band & electrons were free to move.On direct recombination,conduction electrons and holes decay by conduction electrons falling from the conduction band to empty states (holes) in the valence band.The result of recombination is an electron in valence band which is not useful for conduction purposes.
The electron left in valence band after recombination is only useful to conduction if energy was supplied to it to make a transition to conduction band.

I am not sure about this.
As I know P type is a good conductor. If I connect a battery, a lightbulb, and a resistor in series the lightbulb will illuminate.
Here is the journey of an electrons in the circuit:
-First an electron left/exit from negative of the battery and go along the wire to P type.
-In P semiconductor it jump from conductor band to valence band and continue go along the p type in valence band.
-Finally, the electron exit P type and go to positive lead of battery.
In the description, I know that this is not true that an electron move from negative lead to positive lead of battery but it is other electron but the result is same.
Then if P type can conduct, to me, it seems that electrons after falling into holes can go to base lead of battery or depletion region in CB junction.

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Hi, I will express my question more clear:
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 enery 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.

 

[rahdirs]

Re: Is it possible electrons fallen into holes and go straight to emitter?

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 enery from VCB exits valence band and go into depletion region.

Let us take an intrinsic Si material.Si atom has 14 electrons around it revolving around nucleus.Si-1s2,2s2,2p6,3s1,3p3(when excited). The electrons in the 3p band are the farthest from nucleus.Hence when energy hv or photon is incident on electron,it gets excited to conduction band,leaving an empty space in 3p band.That empty space is hole & the electron that is in conduction band isn't under the control of any Si atom & hence is useful for conduction.An electron in valence band is under influence of Si nucleus & can't wander & is of no use in conduction.

So,when electron & hole recombine,the electron is in that valence band.Hence it can't take part in conduction.The electron in valence band cannot go to positive lead of battery VBE.You see when electron & hole recombine,there is a loss of a hole in base,hence a hole is to be supplied by base terminal,in other words an electron leaving base terminal.Generally,this & that electron have no relation.

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Regarding your example circuit of a battery,P-region,resistor in series.Assume in a hypothetical case that 1 electron left the negative of battery & entered p-region.The p-region has become has become slightly negative due to addition of electron.But connecting a battery doesn't make materials negatively charged.So,an electron must leave the p-region to positive end of battery.
Even,better explanation is,we know that in p-region,there are conduction electrons at room temperature(26 mev).While electrons enter the p-region,some conduction electrons are attracted to positive terminal of battery & leave p-region to positive terminal of battery.Your notion that electrons move through the p-region while in valence band is false.Electrons while they are in valence band are fixed to Si atom.

 

[dick_freebird]

Re: Is it possible electrons fallen into holes and go straight to emitter?

The recombining carriers are responsible for the "nonideal
base current" and the rolloff of hFE at lower collector
(base) currents. You can get bulk and surface recombination.
The latter is why lateral devices svck so badly.