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Old Jul 31st 2018, 11:12 AM   #1
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Question The current through a pn juction

In the I-V characteristics of diodes under forward bias , I have seen that the current starts increasing from zero only after reaching the barrier potential , say 0.7 for silicon. As far I understand , the formation of barrier potential is somewhat like this : when the diode is just formed, due to the carrier concentration differences, diffusion of carries starts to take place. But this creates a depletion region and an opposing electric field. The opposing electric go on increasing, and a potential is generated which increased from 0 to 0.7 for silicon, as time elapsed. At 0.7 V,the diffusion current and drift current due to this 0.7 v gets into equilibrium and net current become zero.

So my question is this : if I apply a 0.1 V as forward bias, won't it reduce the barrier and make it 0.6 V? As the 0.1 V bias continue to exist, I think the voltage barrier should now remain at 0.6 V. At this 0.6 V diffusion component of current should overcome the drift current and a net current flow should be there. This net current should go on increasing continuously on increasing the forward bias, rather than abruptly start increasing at 0.7 V only (no current till 0.7 V). Why isn't it happening

Last edited by osalselaka; Jul 31st 2018 at 10:41 PM. Reason: Changing the topic
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Old Aug 1st 2018, 03:32 PM   #2
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Originally Posted by osalselaka View Post
In the I-V characteristics of diodes under forward bias , I have seen that the current starts increasing from zero only after reaching the barrier potential , say 0.7 for silicon. As far I understand , the formation of barrier potential is somewhat like this : when the diode is just formed, due to the carrier concentration differences, diffusion of carries starts to take place. But this creates a depletion region and an opposing electric field. The opposing electric go on increasing, and a potential is generated which increased from 0 to 0.7 for silicon, as time elapsed. At 0.7 V,the diffusion current and drift current due to this 0.7 v gets into equilibrium and net current become zero.

So my question is this : if I apply a 0.1 V as forward bias, won't it reduce the barrier and make it 0.6 V? As the 0.1 V bias continue to exist, I think the voltage barrier should now remain at 0.6 V. At this 0.6 V diffusion component of current should overcome the drift current and a net current flow should be there. This net current should go on increasing continuously on increasing the forward bias, rather than abruptly start increasing at 0.7 V only (no current till 0.7 V). Why isn't it happening
You can't apply a forward bias during fabrication of the junction, it doesn't work like that.
And even if you could, the voltage barrier is a function of the materials themselves, not the fabrication technique or the application of external voltages during fabrication. You would not change it that way.

Can you tell me if you understand the following.

Breathable waterproof fabrics for clothing?

Osmosis?

Holes in relation to electrical conduction?

Semiconductor junction potentials arise in the same way as osmotic pressures, but the mechanism is more complicated.

Do you want to go through these in more detail?
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Last edited by studiot; Aug 1st 2018 at 03:35 PM.
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Old Aug 1st 2018, 07:55 PM   #3
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Semiconductor junction potentials arise in the same way as osmotic pressures, but the mechanism is more complicated.

Do you want to go through these in more detail?
In our local syllabus we have no much details on them. I went through the internet but I could't have satisfied details.
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Old Aug 2nd 2018, 05:20 AM   #4
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I am not sure I have understood your post,
I interpreted it as you have incorporated the diode into a circuit which maintains a 0.1V bias at its "zero" setting, thus the effective voltage barrier is now 0.6V above this zero bias.

Studiot has interpreted it differently (you apply the bias during fabrication, so it is then permanently imprinted on the diode).

Perhaps some clarification of your position might result in a clearer response.
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