I was looking over the PN junction stuff again as a refresher today on the site, and something came up that has bothered me since my college days. Why don't all the available free carriers recombine? I understand how the depletion region is formed, but what causes it to stop before drawing in all the excess E-H pairs? Does anyone have some insight here?

-Jimmy

 

I believe it is because atoms within the depletion region, although with "happy" numbers of electrons, become electrically charged when they gain or lose electrons. This eventually prevents more electrons from entering the P-type silicon (which takes on a negative electrical charge) and and electron "holes" from entering the N-type silicon (which takes on a positive electrical charge).

Anyone with more experience wanna verify that?

 

A lot of the available free carriers do recombine. The time-constant for recombination (it's a probabilistic process) is called the recombination rate.

However, thermal energy is breaking bonds and creating new free carriers all the time. When you calculate the number of free carriers you are calculating a mean, the instantaneous value is shifting around a bit.

When you lower the temperature you are reducing the average number of free carriers produced.

To Austin: the space charge you mention is what sets the physical size of the pn junction. It isn't what generates free carriers, though.

 

Why don't all the available free carriers recombine?

The system is not at a zero energy state. Thermal/electrical/light/whatever else energy is enough to prevent 100% of E-H pairs from recombining...

I understand how the depletion region is formed, but what causes it to stop before drawing in all the excess E-H pairs?

The immobile lattice imperfections and the energy barrier created from them prevent that from happening...

 

I was looking over the PN junction stuff again as a refresher today on the site, and something came up that has bothered me since my college days. Why don't all the available free carriers recombine? I understand how the depletion region is formed, but what causes it to stop before drawing in all the excess E-H pairs? Does anyone have some insight here?
-Jimmy

Recombination stops when the is an equilibrium between the "diffusion pressure" (tendency to recombine) and the electric field caused by the diffusion process (which acts against the diffusion direction). As a result, there is a diffusion voltage across the pn junction of about 0.6 volts.