When p-type and n-type materials are joined:

1. Electrons from the n-side diffuse across the junction to the p-side, and holes from the p-side diffuse to the n-side.
2. Upon crossing, they recombine. This leaves behind uncovered ionized donor atoms ($+$) on the n-side and ionized acceptor atoms ($-$) on the p-side.
3. This region of immobile ions is called the depletion region. It creates a built-in electric field that eventually stops further diffusion.