The p-n Junction & Diodes

Join p-type and n-type silicon in one crystal and you get a one-way valve for current. That simple junction is a diode, and with light or a carefully chosen breakdown it becomes a light sensor, an LED or a voltage regulator.

Making the junction#

A p-n junction is a single crystal doped with donors on one side and acceptors on the other. One way to make it is to dope a whole wafer n-type, then diffuse a stronger dose of acceptors in from one face to turn that layer p-type. Metal contacts are added to both sides.

The depletion layer#

Imagine the junction just formed. The p side is full of holes, the n side full of conduction electrons. Each kind spreads from where it is crowded to where it is scarce: holes diffuse into the n side, electrons into the p side, and near the junction they meet and cancel. The heavy ions cannot move, so they are left behind uncovered: negative acceptor ions on the p side, positive donor ions on the n side.

These fixed charges create a field pointing from the n side to the p side. It pushes any hole near the junction back to the p side and any electron back to the n side, so a thin region around the junction is left with no mobile carriers at all: the depletion layer. The n side ends up at a higher potential than the p side: a potential barrier of a few tenths of a volt.

Two currents that balance

With no battery connected the two are equal, and no net charge crosses the junction.

Forward and reverse bias#

Forward bias: the battery’s positive terminal goes to the p side. That raises the p side’s potential, lowering the barrier and narrowing the depletion layer. Many more carriers can diffuse across, while the drift current (set by how fast pairs are made) barely changes. The net current from p to n grows very rapidly with voltage, not in proportion to it: a junction does not obey Ohm’s law.

Reverse bias: the positive terminal goes to the n side. The barrier grows and the depletion layer widens, so diffusion almost stops. What remains is the small drift current, typically microamperes, flowing from n to p. The junction now behaves like a very large resistance.

Dynamic resistance

Because the i–V graph is curved, a single “resistance” doesn’t describe it. Instead we use the response to a small change:

Diodes that use light#

Photodiode: if light with photon energy hc/λ at least equal to the gap falls on the junction, it breaks bonds and creates extra electron–hole pairs. In reverse bias, where the current is just the drift of such pairs, the current rises in proportion to the light intensity: a light sensor.

Light-emitting diode (LED): in forward bias, conduction electrons fall into holes near the junction, and the energy released can come out as a photon. Silicon and germanium give infrared; compounds such as gallium arsenide, gallium phosphide and indium phosphide give visible light (380–780 nm). LEDs are now everywhere, from indicator lamps to room lighting.

Breakdown and the Zener diode#

Increase a reverse bias far enough and, at a particular breakdown voltage, the reverse current suddenly shoots up. Two mechanisms can cause it:

• Avalanche breakdown: minority carriers accelerated by the field gain enough energy to knock valence electrons free; each new pair is accelerated in turn.
• Zener breakdown: in a very thin, heavily doped junction the field is strong enough to tear electrons out of their bonds directly.

Diodes built to work in breakdown (avalanche or Zener diodes) have a useful property: once broken down, the voltage across them stays almost constant whatever the current. Placed in reverse across a load, with a resistor in series, one keeps the load voltage steady even when the supply or the load changes.