Diode Technology and Applications

The diode is one of the most basic semiconductor devices, and understanding how it behaves underlies a great deal of circuit design. Its action comes from the PN junction, where p-type and n-type semiconductor meet. Under forward bias the junction conducts once a small threshold voltage is exceeded; under reverse bias it conducts only a tiny leakage current until the reverse voltage becomes large enough to cause breakdown. These characteristics are summarised in a diode’s current-voltage curve.

Several ratings describe a diode and decide where it can be used. The forward voltage drop is the voltage lost across the diode when conducting, around a few tenths of a volt for germanium and somewhat higher for silicon. The maximum reverse voltage, or peak inverse voltage, is the reverse voltage the diode can withstand without breaking down. The maximum forward current and the switching speed matter when a diode must carry heavy current or operate at high frequency. Choosing a diode means matching these ratings to the demands of the circuit.

Diodes are applied in many ways. Rectifier diodes convert alternating current to direct current in power supplies. Small-signal diodes detect and shape radio and audio signals. Clamping and clipping circuits use diodes to limit voltage swings and protect sensitive parts, while a diode placed across an inductor or relay coil safely absorbs the voltage spike produced when the current is switched off. Application notes from manufacturers give worked examples of these standard uses.

Frequently asked questions

What is forward voltage drop?

It is the voltage lost across a diode when it is conducting, about a few tenths of a volt for germanium and somewhat higher for silicon. It represents power lost in the diode.

What is peak inverse voltage?

It is the largest reverse voltage a diode can withstand without breaking down. Exceeding it can cause the diode to conduct in reverse and may damage it.

Why is a diode placed across a relay coil?

When the current through a coil is switched off, the collapsing magnetic field produces a high voltage spike. A diode across the coil safely conducts this away, protecting the switching device.