1. The p-n Junction in Equilibrium
The p-n junction is the transition region where n-doped and p-doped semiconductor crystals meet. In this region there are no free charge carriers, because the free electrons of the n-conductor and the free holes of the p-doped crystal recombine with one another near the contact surface of the two crystals, i.e. the electrons fill the vacant holes. This movement of charge carriers (diffusion) occurs as a result of a concentration gradient: since there are only a few electrons in the p-crystal and only a few holes in the n-crystal, the majority charge carriers (electrons in the n-region, holes in the p-region) migrate into the oppositely doped semiconductor crystal. The crystal lattice at the interface must not be interrupted; simply "pressing together" a p-doped and an n-doped silicon crystal does not result in a functional p-n junction.
Due to the migrated free charge carriers, the regions near the interface become positively charged (n-crystal) or negatively charged (p-crystal). The more charge carriers recombine, the larger this depletion or space-charge region (SCR) becomes, and with it the voltage difference between the n-crystal and the p-crystal. At a certain level of this potential difference, the recombination of holes and electrons comes to a stop, since the charge carriers can no longer overcome the electric field. In silicon, this limit is approximately 0.7 V (see band model of a p-n junction).
p-n junction with no applied voltage
A p-n junction corresponds to an electrical component that, when a voltage is applied, conducts current in one direction (forward direction) and blocks it in the other direction (reverse direction): a diode.