Tutorial 4 Extension - Diode Construction and Action

Diodes are made from semi-conductor materials like silicon and germanium.

Conduction in semi conductors is through two different mechanisms:

• Electron movement.  Electrons are negative charge carriers.  They move in an n-type semiconductor like electrons do in a wire.

Graphic by Prof. Marcello Carlà, Wikimedia Commons

• Holes.  These are positive charge carriers.  A hole is where there is a deficiency in electrons.  The principal charge carriers in a p-type semiconductor are holes.  When a voltage is applied, the hole moves about.  The atoms do not move, but the deficiency transfers from atom to atom.  The hole has a positive charge of +1 e (1.6 × 10-19 C).

Graphic by Prof. Marcello Carlà, Wikimedia Commons

When there is no voltage applied the electrons and holes drift about randomly.  When a p.d. is applied, the holes drift to the negative and the electrons go to the positive.  Where there are equal numbers of electrons and holes, i.e. in pure semi conductor materials, the semi conduction is called intrinsic.  The current is very small, although if the material is heated, the conductivity increases.  N-type materials are made by adding small amounts of impurities (doping), like phosphorus.  In P-type materials, aluminium is the doping material.

How do diodes conduct?

Semi-conductors conduct electricity in a different way to metal wires.  Electrons get enough energy to jump into a conduction band.  How does this work?  Look at this model:

Fingers wants to escape from prison. But there is a big wall keeping him on the Inside.  He can jump, but not very high.

In the quantum world, Fingers is in a probability cloud. The vast majority of time, he is on the Inside, but there is a tiny probability that he is on the Outside, so he can make his escape.

In a semi-conductor, there is a forbidden gap, like the prison wall that stops Fingers from getting out.

However, in the world of quantum physics, there is a small probability that electrons can gain the energy to jump the forbidden gap.  The electrons end up in the conduction band.  This is like Fingers having a tiny probability that he gains the energy to leap the prison wall to the outside.

If we apply the forward bias voltage, there is a much greater probability that electrons can jump into the conduction band.

In real crystals there are defects. These might be impurities or missing atoms.  They can cause extra levels to be present in the forbidden gap. These are the defect levels. Electrons can rest there some time.  So doping semi-conductors can improve their conduction characteristics.  It’s like Fingers having a platform to rest on half way up the prison wall.

Semi-conductor diodes are small, reliable, and robust.  The silicon diode is particularly suited to high current applications.  They can have high working temperatures, and can withstand reverse-biased voltages of several hundred volts.

However, if abused, diodes will fail.  Like all semi-conductors they can go into thermal runaway if the current is too heavy.  In semi-conductors the resistance falls with higher temperatures.  Therefore the current gets bigger still, and the diode gets hotter, and so on… Also a high reverse voltage will destroy a diode completely.

The diagram shows the construction of a diode.

The whole component is sealed to prevent the entry of moisture and light that could alter the crystal properties, which would in turn alter the functioning of diode.

If you are studying this at university, you will go on to diode theory which is quite complex and is beyond the scope of these notes.