Why is a depletion region formed

Ion formation in the diode

This answer describes what happens on the P side of the PN junction. The same concepts can easily be applied to the N-side, with the signs of mobile and static electricity changing accordingly.

PN Junction Schematics:

Forward Bias:

Under forward bias conditions (P-side positively biased relative to N-side), electrons are injected into the depletion region from the N-side. They are flushed over the depletion area by the electric field and injected into the quasi-neutral area of ​​the P-side:

Due to this injection, the electron concentration near the edge of the depletion region on the P side is increased dramatically. In the quasi-neutral region there is no net electric field that could influence these electrons any more, but they diffuse further away from the depletion region due to concentration gradients.

While they diffuse through the quasi-neutral P-side, excess electrons have the opportunity to "hit" a hole and perform a recombination. The hole is just an empty electron state of an atom or ion - yes, recombination can either ionize the acceptor atom or neutralize an ionized silicon atom. We say "electron recombines with hole", but in reality the electron is only trapped in the free space in the lattice.

Anyway, it seems that this process results in a loss of cellular carriers and your question is, "If cellular carriers are lost, how is the power going to keep the power going?" I'm right?

The answer:

You forget the conservation of charge: If the electrons that are injected into the P-region recombined with the holes initially present there, the negative charge on the P-side would increase net. This mechanism cannot maintain a steady state current flowing through the diode.

What happens is that for every hole that recombines with excess electron on the P-side, there is an extra hole fed by the P-side metal contact. This means that there is a diffusion flow of electrons supplied from the N-side, but there is also a flow of holes that compensates for the recombination supplied by the P-side contact (I say "P-side Contact ", but there is a power supply that is responsible for the power supply.

The current densities are:

In the diagrams above you can see that the sum of electron current and hole current is constant throughout the diode. This means that holes that are lost during recombination are compensated for by the current from the P-side metal contact - the net charge density remains the same in the quasi-neutral P-area (neutral) and the amount of free charge carriers remains the same.

In summary:

In steady-state forward bias:

  • The current density is constant throughout the device
  • The charge neutrality remains in quasi-neutral regions