1. p-n junction at thermal equilibrium
The p-n junction is the transition area between two n- and p-doped semiconductor crystals. In this area there are no free charge carriers, since the free electrons of the n-conductor, and the holes of the p-doped crystal in the vicinity of the interface recombine with each other, which means that the electrons fill the holes. This charge movement (diffusion) is obtained in consequence of a concentration gradient: since there is only a few number of electrons in the p-area and only a few number of holes in the n-region, the majority charge carriers (electrons in the n-crystal, holes in the p-crystal ) move into the contrary doped semiconductor. The crystal lattice at the interface must not be interrupted, a simple "pressing together" of a p-type and a n-doped silicon crystal does not allow a functional p-n junction.
The regions near the interface are loaded due to the loss of free charge carriers (positive charge in the n-crystal, negative charge in the p-crystal). The more charge carriers recombine, the greater the depletion zone and thus the voltage difference of n- and p-crystal. With a certain amount of this potential gap, the recombination of holes and electrons comes to a complete standstill, the charge carriers can no longer overcome the electric field. In silicon this limit is at about 0.7 V.
p-n junction without an external applied voltage
A p-n junction represents an electrical component with the function to allow an electric current in one direction (called the forward biased condition) and to block the current in the opposite direction (the reverse biased condition): a diode.