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Why only consider excessive min. carriers diffusion current?

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leohart

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Why the excessive majority carriers diffusion current is ignored?This problem confused me for long,but no explicit answer is given.

For example,an n+/p diode under illumination,to calculate the photo current,we only have to calculate:1)the current generated in depleted region 2)the excessive hole generated in n+ region and diffuse into depleted region and get collected 3)the excessive electron generated in p region and diffuse into depelted region and get collected

For 2 and 3,they are both excessive minority carriers diffsion current,why the excessive majority hole(in p) and electron(in n+) cannot diffuse into depleted region and get collected?Photon generates both hole and electron!

It seems in most time the excessive minority carrier plays a more important role in device,can anyone give me a insightful why about it?

Added after 24 minutes:

One more thing on the above photo diode example:

Seems we are always assuming the any excessive carriers born in depleted region or walk into depleted region become a photocurrent.But why this is it?
The depleted region is just inside the device,for a current,the carriers have to be transfered outside the depletion to the ohmic contact,how does this transmission through the outside neutral region happen????

On the other side,the excessive minority diffusion currents also exist in neutral region outside the depleted region,can this diffusion coexist without disturbing the total current running to the outside world?
 

rajanarender_suram

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for my understanding majority carriers conduct through drift current not by diffusion.
 

leohart

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Re: Why only consider excessive min. carriers diffusion curr

rajanarender_suram said:
for my understanding majority carriers conduct through drift current not by diffusion.
That's the classical answer in textbook that I was used to believe...
But in the above photodiode example,the incident photon creates same generated excessive hole and electron profile...For the definition of diffusion current,only the diffirence of concertration matters,so there will be same amount of hole current and electron current,why only one is counted...
 

ch1k0

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Re: Why only consider excessive min. carriers diffusion curr

e-h pairs photon produced in depleted region separate because electric field. no minority or majority there, all are carriers. p+ has excess holes, diffuse to N side. N has excess electgrons, diffuse to p+ side. when carrier diffuses, depleted region created. equilibrium when both diffusion currents equalize, I think.

majority carriers current is because drift, external electric field is reason.
 

leohart

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Re: Why only consider excessive min. carriers diffusion curr

ch1k0 said:
e-h pairs photon produced in depleted region separate because electric field. no minority or majority there, all are carriers. p+ has excess holes, diffuse to N side. N has excess electgrons, diffuse to p+ side. when carrier diffuses, depleted region created. equilibrium when both diffusion currents equalize, I think.

majority carriers current is because drift, external electric field is reason.

Hi,Are you talking about formation of PN junction?It is not excess here...Just concertration diffirence...Excess means more than equilibrium concentration,like photogeneration...

My first question is my we igonre excessive majority's diffusion?Hope get my question clear...

Thanks
 

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