The mystery of diodes unravelled

Light-emitting diodes (LEDs for short) are increasingly becoming part of our everyday environment.

We see them as the small status indicator lights on many electric devices and on the instrument consoles of cars.

They also appear on more and more things such as direction indicators and high-level brake lights on cars, and in torches where their low power consumption provides long battery life.

Clearly they produce light in a way that's different from an ordinary tungsten filament bulb or a fluorescent tube. How do they do it, and what are their advantages and disadvantages?

As the name implies, a LED is a special kind of diode. A diode is made by adding carefully controlled amounts of different impurities to a piece of special material, let's say silicon, the most widely used, so as to create two distinct regions inside the material.

On the one side you want a so-called n region, in which there must be a plentiful supply of free electrons, negative charge carriers, drifting around randomly, not tied to any particular atoms in the lattice. On the other side is the p region, in which the p-type impurities create gaps ("holes") in the crystal lattice into which drifting electrons can fall very nicely if conditions are right.

The regions are separated by the p-n junction. If an external electrical voltage, from a battery or power supply, is now connected across the piece of "doped" silicon in such a way that the positive side of the voltage is connected to the p region, and the negative side to the n region, the drifting electrons in the n region will be chased towards the p-n junction.

If they are chased hard enough, they will cross the junction, losing some speed in the process so that they are just ready to slot into the available holes in the p region, and then jump from hole to hole on their way to the positive external voltage connection.

At the same time the negative external connection feeds electrons into the n region to take the place of the drifters that were chased across the junction.

The nett effect is a flow of electrons through the diode, in other words, the diode becomes a conductor of electrical current, just like copper or aluminium. All of this depends on the external voltage being connected the right way round; if its polarity is reversed, positive to n region and negative to p region, the drifting electrons in the n region are pulled away from the p-n junction, and therefore, they cannot carry current through the diode.

A diode is therefore your basic semi-conductor device - a conductor in one direction, but not in the opposite direction.

Every time a drifting electron, having been chased across the p-n junction, slots into a waiting hole, it gives up a bit of energy in the form of heat and or light.

In the silicon diode described above it is mainly heat. But if more exotic materials are used instead of silicon, light is produced, its colour depending on the mix of impurities added during the doping process.

So, in a nutshell, the light from an LED comes from millions of electrons slotting into holes all the time while the current is flowing.

The advantages of LED's include the following:

l They are resistant to physical shock and vibration;

l They light up very quickly - important in car brake lights;

l Unlike fluorescents, they are not damaged by frequent on or off cycling;

l They have a longer useful life even than fluorescent lights;

l They fail by dimming over time, not abruptly like filaments;

l They can be very small which makes them ideal for fitting on printed circuit boards.

Some disadvantages of LED's are:

l Their initial purchase price, in Rands a unit of light output, is more than other types of lighting, though their total cost of ownership is far lower than filament bulbs and is threatening fluorescents.

l Their emissions are highly directional and unless this is corrected by a covering lens, the light appears dim when viewed from an off-axis position.

Since an LED will only produce light if the supply voltage is connected the right way round, it is important, when fitting or replacing LED's, to make sure which of the two wires coming out of it must be connected to positive and which to negative.

Various identifying markings are used, but one can always test the LED to see which connection will make it light up.