Diode Types

A diode is a two-terminal semiconductor device that allows current to flow easily in one direction while blocking it in the other. This one-way behaviour comes from a PN junction, the boundary between a region of semiconductor with an excess of electrons and one with a shortage. When the junction is forward-biased, current passes; when it is reverse-biased, it is blocked until a high breakdown voltage is reached. This property makes the ordinary rectifier diode the basis for converting alternating current to direct current.

Many specialised diodes exploit particular aspects of junction behaviour. A Zener diode is designed to break down at a precise reverse voltage and is used as a voltage reference or regulator. A Schottky diode, formed from a metal-semiconductor junction rather than a PN junction, has a low forward voltage drop and switches very quickly. A varicap, or varactor, diode is operated in reverse so that its junction acts as a voltage-controlled capacitor, useful for electronic tuning. Older germanium diodes have a lower forward voltage than silicon and were common in signal detection.

Some diodes display unusual physics. A tunnel diode, also called an Esaki diode, exhibits a region of negative resistance caused by quantum-mechanical tunnelling and can operate at very high frequencies. A Gunn diode generates microwave oscillations from a bulk semiconductor effect rather than from a junction. Light-emitting diodes (LEDs) and laser diodes emit light when forward current passes, and photodiodes do the reverse, producing current in response to light.

Frequently asked questions

What is a PN junction?

It is the boundary between p-type and n-type semiconductor regions. This junction conducts current in one direction and blocks it in the other, which gives the diode its one-way behaviour.

What is a Zener diode used for?

A Zener diode is built to break down at a precise reverse voltage. Operated in that region, it holds a steady voltage and is used as a voltage reference or simple regulator.

What is special about a Schottky diode?

A Schottky diode uses a metal-semiconductor junction, giving it a low forward voltage drop and very fast switching, which suits high-frequency and low-loss applications.