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Why electron from N side is not atracted towards positive immobile ion of depletion layer?enter image description here

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  • \$\begingroup\$ There's something wrong with your picture. Can you upload a new one? \$\endgroup\$ – KingDuken Jul 18 '17 at 20:25
  • \$\begingroup\$ Attraction accelerates with E field and in some e.g. MOSFET devices, ionization occurs when the electron jumps over the insulation just below or above the breakdown threshold depending on dV/dt and create an Avalanche effect. Structure for heat dissipation must be designed to absorb this heat if/ when this occurs. Just as in HV in air ionizes and then avalanche effect when Electrons leap in clusters across the conductors which can lead to either repeating charge/discharged (Corona) or continuous current often with heavy follow-on current leading to damage (short cct) depends on source. \$\endgroup\$ – Sunnyskyguy EE75 Jul 18 '17 at 20:39
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The reason you can treat the PN junction as if there is no attraction between the electrons of the quasi static region and the immobile holes in the depletion region is because of some of the approximations that are being used in the simplified model.

The most relevant approximation in this case is that, because everything is very close together, the electric field is determined only on the amount of charge on either side of a given plane in the bulk semiconductor. As there are an equal number of holes and electrons to the left of the N type QSR, you approximate there as being no electric field.

To convince yourself that this approximation holds, think back to basic electricity and magnetism and the infinite plane of charge. If you recall, the electric field of this seemingly absurd example is uniform regardless of distance, but it can be applied to analyzing capacitors because of a similar approximation.

Assuming you are working with bulk materials (as apposed to graphene and CNTs) (and you are if you are working with depletion regions), you have an inherently 3d structure with two large blocks of charge that can be seen to hold a similar role to the plates in a capacitor (only with a volume based charge density rather than surface charge density) in the previous example. On either side of those two blocks of charge, there will be in reality very minor fields, but can very reasonably be approximated as 0.

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