There are a number of different electronic devices which tend to be called diodes. Although they're made differently they all have three things in common. As an example we will use a typical diode called a pn-junction. This allows us to explain behaviour of diodes. Remember, however, that there are other sorts of diodes which are built differently but show the same general behaviour.

We create a pn-junction by joining together two pieces of semiconductor, one doped n-type, the other p-type. This causes a depletion zone to form around the junction (the join) between the two materials. This zone controls the behaviour of the diode.

Characteristic curves
Graph showing how the current through a diode varies with the applied voltage .

Follow this link
to see in detail how a diode works.

The animation shows the general behaviour of a pn-junction.

When we apply a potential difference between the two wires in one direction we tend to pull the free electrons and holes away from the junction. This makes it even harder for them to cross the depletion zone.

When we apply the voltage the other way around we push electrons and holes towards the junction, helping to give them extra energy and giving them a chance to cross the junction.

The polarity of applied voltage which causes charge to flow through the diode is called Forward Bias.
The polarity of applied voltage which can't produce any current is called Reverse Bias.

Although we've imagined 'bringing together' two pieces of semiconductor, one n-type and the other p-type, real diodes are normally made by taking a single piece of pure material and doping two adjacent regions differently. This means that we do not actually have to 'glue together' two bits of silicon. However, some other forms of diode are created by depositing one material onto another e.g. Schottky diodes are made by placing some metal in contact with a semiconductor. In general, whenever we join two different, very pure, materials we're likely to make some sort of diode.

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