Formation of a p-n junction

A p-n junction underlies the diode, the transistor and many other devices. It is made by adding a precise small amount of pentavalent impurity to part of a thin p-type Si wafer, turning that part n-type.

Two processes shape the junction: diffusion and drift.

Diffusion: holes are concentrated on the p-side and electrons on the n-side, so holes spread p → n and electrons spread n → p. This gives the diffusion current.

Each electron leaving the n-side leaves an immobile ionised donor (positive) behind; each hole leaving the p-side leaves an immobile ionised acceptor (negative). A positive space-charge layer builds on the n-side and a negative one on the p-side.

Together these layers form the depletion region, emptied of free carriers. It is about one-tenth of a micrometre thick.

The space charges set up an electric field from the n-side (positive) to the p-side (negative). It pushes electrons from p to n and holes from n to p; this field-driven motion is drift, and the drift current runs opposite to the diffusion current.

At first diffusion dominates. As the space charge grows, so do the field and the drift current, until drift equals diffusion. In equilibrium there is no net current across the junction.

The n-side has lost electrons and the p-side gained them, so the n-side is positive relative to the p-side. This barrier potential opposes further flow of electrons from n to p.

Example 14.3: two separate slabs pressed together cannot make a junction. Even the flattest surface is far rougher than the interatomic spacing (about 2 to 3 Å), so there is no continuous atomic contact and carriers meet a break.