Formation of the junction

• A p-n junction is made by converting part of a crystal of one type into the opposite type (for example, by doping one end with donors and the other with acceptors).
• The name "pn" refers to the p-region and n-region, not "positive-negative".
• It is not two crystals glued together but one continuous crystal.

At the instant of formation:

1. The p-side has a high concentration of holes, the n-side a high concentration of electrons. By diffusion (from high to low concentration), holes move from p to n and electrons from n to p. The movement produces a diffusion current.
2. Each region was neutral. When an electron leaves the n-side, an equal number of positive donor ions are left uncovered; when a hole leaves the p-side, an equal number of negative acceptor ions are left uncovered.
3. This creates a middle layer with positive ions on the n-side and negative ions on the p-side, with practically no free electrons or holes: the transition (depletion) region. The width of this region is of the order of a micrometre.
4. An electric field is set up in the transition region, directed from the positive ions to the negative ions (from n to p). It opposes the further diffusion of the majority carriers and causes a drift current of minority carriers in the opposite direction to the diffusion current.
5. At equilibrium the diffusion current equals the drift current, so the net current is zero. A potential difference (the barrier potential, about 0.7 V for silicon) is established across the junction.