[SOLVED] PN Junction - Do immobile ions attract charge carriers?

Hi all,
I have a doubt in PN Junction under equilibrium condition.
When the space charge region is formed with immobile ions, lets say, +ve ions in the n-region. Do these ions attract electrons (majority carriers) in the n-region? If not, why?

Regards,
Raghu

Raghuram TR said:

Hi all,
I have a doubt in PN Junction under equilibrium condition.
When the space charge region is formed with immobile ions, lets say, +ve ions in the n-region. Do these ions attract electrons (majority carriers) in the n-region? If not, why?

Regards,
Raghu

Click to expand...

You are asking why the positive ions located at the PN junction in the N-region don't attract the remaining available free electrons in the N-region? It is because the repulsion from the negative ions just across the PN junction in the P-region repel any N-type material electrons in the vicinity of the junction from coming too close to the junction to combine with the positive ions there.

Ratch

Hi Ratch,

Just to make it very clear, so you mean the "Negative immobile ions" in P region is Strong enough to repel the electrons in the N region from getting attracted to the "Positive immobile ions" in the N region? And so the barrier formed under equilibrium will not stop/reduce this repulsive force of the Negative ions?

Raghu

Raghuram TR said:

Hi Ratch,

Just to make it very clear, so you mean the "Negative immobile ions" in P region is Strong enough to repel the electrons in the N region from getting attracted to the "Positive immobile ions" in the N region? And so the barrier formed under equilibrium will not stop/reduce this repulsive force of the Negative ions?

Raghu

Click to expand...

Pure crystalline silcon has about 5E22 atoms/cubic cm. A typical doping concentration is 10E14 atoms/cubic cm. That means that the N-dopant atoms with their extra electrons are very sparse. The extra electrons on the dopant atoms are very weakly bound, and are mostly free to wander around the N-material at room temperature. Why would the free electrons prefer the positive ions at the junction boundary when there are other positive ions throughout the N-material that have lost their electrons by absorbing thermal energy? The electrons have more incentive to cross the junction barrier to combine with the holes, which they do until the barrier voltage increases enough to prevent that from happening any more. In other words, the positive ions at the barrier are no more attractive to electrons than any other positive ions scattered throughout the N-material. And any attraction caused by the concentration of positive charge at the junction barrier is neutralized by the repulsion from the opposite side of the barrier. The barrier is less than the thickness of a human hair.

Ratch

The space charge layer contains an electric field that originates on the exposed immobile positive ions on the n-side and terminates on the exposed immobile negative ions on the p-side. This field is strongest at the junction between the p and n sides and weakest at the edges of the space charge layer on either side. Outside of the space charge layer, this electric field does not exist where there is charge neutrality. Any free electrons or holes that approach the edge of the space charge layer experience the effect of the electric field existing there, which is directed in such a way as to repel the majority carriers and maintain the overall equilibrium state of the junction.

Now am clear about this.
Thank you Ratch and Ibkay.