What is recombination current????

[kaps_nit]

Im facing problems in realizing the meaning of recombination current.....

As we start with the conductiviy property we find that electrons with higher energy moves to conduction band(e- gains energy from thermal or optical excitation). And only conduction band electron offers conductivity in general. Valance band electrons are not responsible for conductivity.

We also know that when recombination takes place, e- from conduction band comes down to valance band and recombines with holes and thereby give off energy as photons.

I referred to Millman Halkias, there also it is clearly written, that in recombination process e- moves to valance band and recombines with a hole and hence a mobile EHP disappears. And the condition for that is, these two elements should have the same momentum with opposite sign.

Now, take the case of transistors, say an npn transistor, here in the active region e- flows from emitter to base region and few of them gets recombined with holes, and leaves the base region as recombination current.

Now how to justify this apparent contradiction, as in this case e- in the valance band contributing in the conduction??????

 

[sankudey]

If I have understood ur statement properly....than there should not be any contradiction....my view is following.....

for npn bjt: from emitter heavy density electron flows....drains towards collector....while going some of them get recombined with lightly dopped base regin p+ (holes)....this gives rise to some current....very less in magnitude....u are right....this is the base current....and the other part....the succesful electrons reaching at collector gives rise to collector current and much more in magnitude.....from this thing only beta (current gain) parameter comes......

pls note that there are some other mechanisms which contribute to base currents...


sankudey

 

[kaps_nit]

What you are saying is known to all of us. You discussed about the dividing process of emitter current into base current and collector current.
But my question is only with the base current: this is the current produced due to recombination of e- and holes in base region:

I referred to Millman Halkias, there also it is clearly written, that in recombination process e- moves to valance band and recombines with a hole and hence a mobile EHP disappears. And the condition for that is, these two elements should have the same momentum with opposite sign.

how would you define this when the e- has already lost its momentum, hence i suppose the velocity.

 

[arunkumar446]

due to recombination, the e- go to the valnce band by filling the hole with which it recombined. to replenish the holes again in the base of the n-p-n, the valane electrons move towards the base - it effectively is holes coming in to the base to recombine - this is what the current is accounted for.

 

[ostchina]

Would you upload a schematic?

 

[Sumit Pandey]

Im facing problems in realizing the meaning of recombination current.....

As we start with the conductiviy property we find that electrons with higher energy moves to conduction band(e- gains energy from thermal or optical excitation). And only conduction band electron offers conductivity in general. Valance band electrons are not responsible for conductivity.

We also know that when recombination takes place, e- from conduction band comes down to valance band and recombines with holes and thereby give off energy as photons.

I referred to Millman Halkias, there also it is clearly written, that in recombination process e- moves to valance band and recombines with a hole and hence a mobile EHP disappears. And the condition for that is, these two elements should have the same momentum with opposite sign.

Now, take the case of transistors, say an npn transistor, here in the active region e- flows from emitter to base region and few of them gets recombined with holes, and leaves the base region as recombination current.

Now how to justify this apparent contradiction, as in this case e- in the valance band contributing in the conduction??????

Yeah, That term is a bit confusing. It is actually the current due to the holes (in pnp) in base region which contribute to recobmination in the base region with the electrons provided by the base contact or in other words current due to the flow of electrons from base terminal which are provided in the base for the purpose of recombination with the holes supplied by the emitter region.
So it is basically current due to carriers which will contribute to recombination in base region.
I hope, I have cleared my point. Here is a link for better understanding **broken link removed**.
sorry for my bad english it is not my native language.
And I advice you to analyse BJT using pnp transistor. Its easier to see the currents that way.