in a P-N junction diode, in an equilibrium state, a depletion layer forms in the surface of contact of N type and P type. But the problem is, this layer is formed by diffusion of electron to a lower electron density zone. Why does it always have to form right in the contactt surface? when electron comes from N side to P side, why does it havve to stay in the contact surface area? It should diffuse further away from the N type.