1
$\begingroup$

I was reading about diffusion current in pn junction with forward bias. However, I got stuck at understanding how diffusion current increases as bias voltage increases.

I searched for books and got the answer below. However, I still don't have a good understanding of it. Under forward bias, energy barrier is reduced, why " and more electrons can thus participate in the diffusion towards the p-type region"?
Also, the diffusion current only depends on gradient of carrier concentration not their absolute values. So the answer doesn't make much sense to me. Can anyone explain it?

UPDATE:
Sorry I didn't express myself clearly. I understand how pn junction is created, how energy barrier changes under forward and reverse bias.
Here is what I am confused: under forward bias, the energy barrier is reduced, and as the text said this make it easier for electrons and holes to diffuse, more electrons can diffuse through the barrier. However, how does this results in increasing diffusion current?
Diffusion current density depends only on gradient of carrier concentrations not on there absolute values. So more electrons can diffuse through the barrier doesn't mean that diffusion current increases. What really matters is the gradient. So how does the forward bias helps increase diffusion current (by increasing carrier concentration gradient? how?)? Does lower energy barrier increase gradient of carrier concentration?

Source: Technology of Quantum Devices by Manijeh Razeghi here.

enter image description here enter image description here

Improve this question
$\endgroup$

3 Answers 3

Reset to default
1
$\begingroup$

The applied voltage and the voltage inside the pn junction are opposite to each other in direction, so the energy barrier gets reduced so more electrons can cross the depletion zone and thus increasing the current.

Improve this answer
$\endgroup$
1
  • $\begingroup$ I understand how energy barrier is reduced. However, how does this relate to diffusion current? Why reduced barrier helps increase diffusion current? Also, diffusion current only cares about carrier concentration gradient not absolute value, so why more electrons matter? $\endgroup$
    – emnha
    Commented Jan 19, 2017 at 19:00
1
$\begingroup$

enter image description here

Before the depletion zone is created electrons move form n-type semiconductor to the p-type semiconductor due to the concentration difference of electrons between these two.

Now as the electrons move from n-type to p-type, they leave +ve charge behind which causes a electric field to emerge and restricts the further flow of electrons (electrons move in opposite direction of electric field, which points from +ve charge to -ve charge).enter image description here

After electric field cancels out the requirement to equalize the concentration of electrons, the depletion zone is created.

Now when the pn junction is forward biased as shown is the 1st figure the electric field generated by the battery is in opposite direction to that of the field in the depletion zone. This reduces the electric field strength in the junction.

Eventually the electric field, being reduced, cannot stop the transfer of electrons due to the concentration difference as strongly as it did before forward biasing.

So this increases the current flow.

I hope this explanation succeeds to solve your problem.

Improve this answer
$\endgroup$
1
  • $\begingroup$ Thanks for the answer. Sorry I didn't express it well enough. I have just added the update section in my original post above. Could you have a look and help me answer the questions? $\endgroup$
    – emnha
    Commented Jan 20, 2017 at 3:49
1
$\begingroup$

the correct reason is in forward bias p-n junction positive holes and electrons diffuses to each other forming a depletion layer containing immovable ions which when operated in forward bias the external electric field has direction opposite to the barrier electric field and hence decreasing the size of depletion layer therefore majority charge carriers can easily diffuse to each other and diffusion current is more.....

Improve this answer
$\endgroup$