The depletion layer
Dope one side of a silicon or germanium crystal with a group III element (p-type) and the other with a group V element (n-type). Where they meet there is a p-n junction, which behaves quite differently from an ordinary conductor.
Figure to be added
As soon as the junction forms, free electrons from the n side diffuse across to the p side and holes from the p side diffuse to the n side. Electrons fill holes, leaving a region near the junction with no carriers.
Depletion layer
That carrier-free region is the depletion layer. Its p side has gained electrons and is negative; its n side has gained positive charge. The potential difference across it repels carriers and stops the diffusion: it is a potential barrier.
| Junction | Potential barrier |
|---|---|
| Silicon | ≈ 0.7 V |
| Germanium | ≈ 0.3 V |