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Why is the direction of the electric field from N to P? Is it a standard convention? Why are the directions of the drift current Is and diffusion current Id in their respective directions? In my book, the reason isn't mentioned and I'm very curious.

Another question would be their electric potentials. I don't understand the electric potentials of the two regions, if someone can help explain.

A few more follow up questions that raised in my mind while I was reading my book. It quoted:

d

Can someone explain this to me? Why does this voltage drop occur? And what are the factors it's dependent on? Also Voltage drop is also referred to as the barrier voltage here right?

Source: Microelectronic Circuits by Sedra Smith(7th Edition by Oxford University Press), Page number 179

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In a PN junction in thermal equilibrium with no applied voltage (as in your diagram), holes will diffuse out of the P-type region into the N-type region and electrons will diffuse the other way. This is due to concentration gradients. This creates an area in the N-type region with "extra" positive charges and an area in the P-type region with extra negative charges. These extra charges create an electric field that, by convention, "points" from the area of positive charge to the area of negative charge. This field will act to pull charge carriers back to where they came from, holes back to the P-type side, electrons back to the N-type side, which effect is called "drift". Eventually drift will balance diffusion and there is no net flow of charge across the junction.

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  • \$\begingroup\$ Is there a reason for the phenomenon of "drift" to happen? Why does the electric field oppose the diffusion of holes and electrons? \$\endgroup\$
    – SubbSE
    Commented Apr 18, 2023 at 20:01
  • \$\begingroup\$ And is the reason for the diffusion current ID's direction, as mentioned in the figure also a convention? from p to n side? \$\endgroup\$
    – SubbSE
    Commented Apr 18, 2023 at 20:02
  • \$\begingroup\$ Holes diffuse from P to N because there are more holes on the P side than the N side. Electrons diffuse from N to P because there are more electrons on the N side than the P side. Diffusion adds positive charge to the N side and removes it from the P side; it adds negative charge to the P side and removes it from the N side. These displaced/separated charges create an electric field, just like any separated charges. Free charge carriers will move under the influence of this field. \$\endgroup\$
    – vir
    Commented Apr 18, 2023 at 20:07
  • \$\begingroup\$ Okay so from what I read further, the electric field opposes the normal diffusion of holes and free electrons to N and P region respectively. AND the drift current is the result of the movement of the minority charge carriers that are generated due to thermal generation! They get affected by the electric field and are swept across to the other region, hence giving rise to the drift current. The Drift you are talking about is most likely mentioned here as the voltage drop or the barrier voltage V0, which the holes and electrons need to pass in order to give rise to the diffusion current \$\endgroup\$
    – SubbSE
    Commented Apr 18, 2023 at 20:40

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