The p-n junction diode

A p-n junction is made by doping one part of a single crystal p-type and the next part n-type. (Pressing two separate slabs together does not work: the surfaces are too rough at the atomic scale.)

What happens at the junction:

• Diffusion: electrons from the n side spread into the p side, and holes from the p side spread into the n side, because there are far fewer of them over there.
• They meet and cancel near the junction, leaving a thin region with no free carriers: the depletion layer. Fixed positive ions sit on its n side and fixed negative ions on its p side.
• These ions create a barrier potential (about 0.7 V for silicon) that stops further diffusion.

A junction with two terminals is a diode. It lets current through one way only.

• Forward bias (p side to +, n side to –): the battery lowers the barrier and the depletion layer narrows. Once the voltage passes about 0.7 V (silicon), the current rises steeply.
• Reverse bias (p side to –): the barrier rises and the depletion layer widens. Only a tiny current (microamps) flows, from minority carriers.

Dynamic resistance: since the graph is curved, we use r = ΔV / ΔI from a small piece of it. Example: 0.60 V gives 5 mA and 0.65 V gives 15 mA. r = 0.05 / 0.010 = 5 Ω. Forward resistance is small; reverse resistance is millions of ohms.

Rectifier: the main use of a diode, turning ac into dc.

• Half-wave: one diode passes only the positive halves of the ac. The output pulses at the same frequency as the input (50 Hz), and half the input is wasted.
• Full-wave: two diodes (with a centre-tapped transformer) use both halves, flipping the negative halves up. The output pulses at double the frequency (100 Hz).

The pulsing output is smoothed into steady dc by a capacitor connected across the output (a filter). Every phone charger contains a rectifier and a filter.