Why is there a depletion region in np-junction?

valjok

Ok, initially some electrons and holes recombine producing the region. They draw it like a rectangular charge distribution.



But why does it persist? P-holes are mobile in left and are in contact with negatively -charged minus-bar. They will move into the depletion region! So will electrons from the left. Moreover, as particles enter the depletion region, they neutralize the oppositely charged bar at the region border making it easier for the particles in the opposite region to come even closer!

Ok, p-holes will not enter the n-doped part and electrons will not escape it because as these mobile particles move into the "depletion region",
1. their host region is charged oppositely (e.g., p-doped left becomes more negative as holes move closer to the center) and pulls the particles back while
2. opposite region remains charged the same and keeps the particles away (e.g., n-doped right is more positive and does not let p-holes to enter it)

I see how this will keep the particles from further recombining. But, I do not see any reason for the charge distribution to have the rectangular shape? It means that the concentration of mobile particles is constant (0?) there. What prevents the particles from entering the depletion region? The electrical force that stops the particles from moving to the center increases gradually. It is almost non-existent at the junction of p-region with minus-bar. Meantime, the force that pushes the particles is the diffusion. It is infinite as gradient if there are particles in one side and no in another! The concentration must fade out gradually for sure!