Most diodes are made of silicon.

A silicon atom has four electrons in its outer orbit. Silicon atoms together form a crystal structure, leaving no "free electrons" moving around. Hence, pure silicon is an insulator.

Let's replace a few silicon atoms with a atoms that have just three electrons in the outer orbit. Each atom forms a "hole" in the silicon lattice. It has now become attractive for electrons, just as if it were positively charged. This material is called a P type semiconductor.

Of course we can also replace some silicon atoms with atoms that have five outer electrons. This material is called N type semiconductor, since it repells electrons.

What will happen if we join some P and N material together?

The P type material attracts electrons while the N side repells them. So at the junctions some electrons from the N side will fill the holes on the P side, creating a depletion zone:

In this zone are no free holes and no free electrons: this zone is an insulator.

Let's see what happens if we apply a voltage across the PN material. First, we connect the positive to the P side and the negative to the N side:

The negative lead of the voltage source pushes the electrons in the N material through the depletion zone, filling up the holes in the P type metarial. The P side is now negatively charged, and the electrons will flow from the P side to the positive lead of the voltage source. The material has become a conductor!

In our next experiment, we will reverse the voltage:

The negative lead fills holes in the P side and the positive lead attracts free electrons in the N side. The result is that the depletion zone will become larger. In other words, the PN material is now an insulator!

So the PN material forms a device that conducts current in only one direction: a diode.

If you connect a diode reverse biased to a voltage source and increase the voltage, you'll notice that at a certain voltage, current starts to flow. The voltage at which this happens is called the breakdown voltage or zener voltage. As long as the current is kept within certain limits, breakdown will not damage the diode.

The zener voltage depends on the amount of impurities (non-silicon atoms) in the lattice: the more impurities, the lower the voltage will be.

We already saw that the depletion zone is an insulator. The remaining part of the P and N material does conduct electrical current. And what do we call a device that consists of two conductors with an insulator in between? A capacitor!

We also saw that the depletion zone increases when you increase the reverse voltage. This means that the capacity depends on the voltage.

Although all diodes show this behaviour, there are diodes specially designed for this purpose. These are called varicaps. They are used in radio and TV tuners. The frequency can thus be controlled by the reverse voltage across the diode.