I am trying to understand why and how a depletion layer forms between a P type and N type semiconductor.
Before putting into contact a P type and N type semiconductor, both are electrically neutral. However, as soon as we put them in contact, "free electrons" from N type semiconductor begin to diffuse into P type semiconductor, forming positive ions in N type and negative ions in P type.
My question is, what causes these "free electrons" to move from N type semiconductor to P type semiconductor? It certainly isn't due to presence of an electric field. Because initially, both P type and N type materials are electrically neutral. Even conversely, the electric field that forms after sufficient number of electrons move from N side to P side prevents the movement of further electrons from N side to P side.
So if it isn't due to an electric field, then why do the electrons move from N type semiconductor to P type? What motivates the electrons to move? What is the force behind this?
Best Answer
It is a thermal effect, the corresponding "force" is a so called stochastic force and not a fundamental force, but rather an effective description of entropic effects. Having a temperature causes the charge carriers to move about randomly, and therefore they tend to move from regions with high concentration to regions with low concentrations.
At first it is just like dissolving sugar in water: Near the surface of the sugar there is a high sugar concentration, so the random thermal motion of the sugar molecules tends to transport the sugar away from the source (as less sugar flows back from the region with lower concentration, than flows to the region with lower concentration). But now something happens that has no analogue in the comparison with sugar: Because the diffusing objects are charge carriers, an electric field forms, which counteracts the motion of the particles, so a steady state is reached.
The equations for the steady state of this process are described at the Wikipedia page on the p-n junction.