The light emitting diode

Light emitting diodes (LEDs) are very widely used in today's electronics equipment. In fact over 20 billion LEDs are manufactured each year and this number is rising. With new forms of light emitting diodes being developed that produce white light (white LEDs) and blue light (blue LEDs) they are likely to find even more uses, and the production of these diodes is likely to increase still further.

LEDs are used in a wide variety of applications. One of their first applications was as small indicator lamps. They were also used in alphanumeric displays, although in this particular application they have now been superseded by other forms of display. With recent developments light emitting diodes are being used instead of incandescent lamps for illumination. In these and many other applications. LEDs are in widespread use and are expected to remain so for many years to come.

Invention
Despite the fact that the light emitting diode was first introduced commercially in the 1960s, the effect had been noticed many years ago. A British engineer named H J Round working for Marconi was undertaking some experiments using crystal detectors and he noted that one of them emitted light when a current was passed through it. He published his findings in 1907 in a magazine of the day named Electrical World.

The idea lay dormant for some years before it was observed again by O.V. Losov in 1922. Unfortunately Losev lived in Leningrad and he was killed during the Second World War. He had published a total of four patents between 1927 and 1942, but all this work was lost as records were destroyed in Leningrad.

The idea for the light emitting diode resurfaced in 1951. This time work was to be more successful, although it took some years to reach completion. This time the research was undertaken by a team lead by K Lehovec. The work took many years and involved a number of companies and researchers. Even Shockley became involved. Then in the late 1960s the first diodes became available commercially.

Operation
The LED is a specialised form of PN junction that uses a compound junction. The semiconductor material used for the junction must be a compound semiconductor. The commonly used semiconductor materials including silicon and germanium are simple elements and junction made from these materials do not emit light. Instead compound semiconductors including gallium arsenide, gallium phosphide and indium phosphide are compound semiconductors and junctions made from these materials do emit light.

These compound semiconductors are classified by the valence bands their constituents occupy. For gallium arsenide, gallium has a valency of three and arsenic a valency of five and this is what is termed a group III-V semiconductor and there are a number of other semiconductors that fit this category. It is also possible to have semiconductors that are formed from group III-V materials.

The diode emits light when it is forward biased. When a voltage is applied across the junction to make it forward biased, current flows as in the case of any PN junction. Holes from the p-type region and electrons from the n-type region enter the junction and recombine like a normal diode to enable the current to flow. When this occurs energy is released, some of which is in the form of light photons.

It is found that the majority of the light is produced from the area of the junction nearer to the P-type region. As a result the design of the diodes is made such that this area is kept as close to the surface of the device as possible to ensure that the minimum amount of light is absorbed in the structure.

To produce light which can be seen the junction must be optimised and the correct materials must be chosen. Pure gallium arsenide releases energy in the infra read portion of the spectrum. To bring the light emission into the visible red end of the spectrum aluminium is added to the semiconductor to give aluminium gallium arsenide (AlGaAs). Phosphorus can also be added to give red light. For other colours other materials are used. For example galium phoshide gives green light and aluminium indium gallium phosphide is used for yellow and orange light. Most LEDs are based on gallium semiconductors.

Circuit design
In an electronics circuit a light emitting diode behaves very much like any other diode. As they are often used to indicate the presence of a voltage at a particular point, often being used as a supply rail indicator. When used in this fashion there must be a current limiting resistor placed in the circuit. This should be calculated to give the required level of current. For many devices a current of around 20 mA is suitable, although it is often possible to run them at a lower current. If less current is drawn the device will obviously be dimmer. When calculating the amount of current drawn the voltage across the LED itself may need to be taken into consideration. The voltage across a LED in its forward biased condition is just over a volt, although the exact voltage is dependent upon the diode, and in particular its colour. Typically a red one has a forward voltage of just under 2 volts, and around 2.5 volts for green or yellow.

Light emtting diode with current limit resistor

Great care must be taken not to allow a reverse bias to be applied to the diode. Usually they only have a reverse breakdown of a very few volts. If breakdown occurs then the LED is destroyed. To prevent this happening, an ordinary silicon diode can be placed across the LED in the reverse direction to prevent any reverse bias being applied.

Summary
Although LEDs will continue to be very widely used as small indicator lamps, the number of applications they can find is increasing as the technology improves. New very high luminance diodes are now available. These are even being used as a form of illumination, an application which they were previously not able to fulfil because of their low light output. New colours are being introduced. White and blue LEDs, which were previously very difficult to manufacture are now available. IN view of the ongoing technology development, and their convenience of use, these dievices will remain in the electronics catalogues for many years to come.