Light Emitting Diode, LED: LED types, symbol, etc

The use of light emitting diodes, LEDs is huge and their use is growing as the technology advances and more LED types are introduced.


Light Emitting Diode Tutorial Includes:
Light emitting diode     How does a LED work     How a LED is made     LED datasheet specifications     LED lifetime     LED packages     High power / brightness LEDs     Organic LEDs, OLEDs    

Other diodes: Diode types    


Light emitting diodes, LEDs are very widely used in today's electronics equipment and they are one of the major display technologies in use today.

Light emitting diodes, LEDs are used for many jobs. Not only are they used as panel indicators on everything from televisions, radios and other forms of domestic electronic and industrial equipment, but they are also replacing more traditional technologies for lighting applications. To accommodate all these needs, there are many different types of LED that are available.

With organic LED technologies also being developed and introduced, LED technology is making an even greater impact on today’s technology.

LED history

The story of how the LED was discovered is fascinating and tinged with sadness. It took many years for it to come from the initial observations to final commercial success.

Note on Light Emitting Diode History:

The first reports of diodes emitting light appear to have been made by an English radio engineer named H J Round in 1907. Since then many attempts were made to bring the LED to the world, but fate seemed to prevent this until allied technologies were far more mature.

Read more about Light emitting diode, LED history.

Light emitting diode, LED symbol

The circuit symbol for the LED is relatively straightforward. The LED symbol comprises a diode symbol with two arrows indicating outwards to signify that light emanated from the diode.

 Light emitting diode, LED circuit symbol
Light emitting diode, LED circuit symbol

Sometimes the light emitting diode symbol is shown only as an outline and without the filled in shapes. The outline shape is equally acceptable,

 Light emitting diode, LED circuit symbol with outline shapes
Alternative light emitting diode, LED circuit symbol

Other versions of LED symbols may also be seen. Sometimes the light emitting diode symbol may be enclosed in a circles. This symbol is not as widely used these days but may still be seen on many circuits.

LED types

Since the introduction of the first LEDs, the technology has spawned a huge variety of different types of LED, each with their own properties and applications.

  • Traditional inorganic LEDs:   This type of LED is the traditional form of diode that has been available since the 1960s. It is manufactured from inorganic materials. Some of the more widely used are compound semiconductors such as Aluminium gallium arsenide, Gallium arsenide phosphide, and many more – the colour of the light is often dependent upon the materials used.

    These LEDs are typified by the small LED lamps that are used as panel indicators, although there are very many formats for LEDs of this type. However even within the inorganic LED category, there are many different styles of LED that can be seen and used:
    • Single colour 5 mm, etc - the very traditional LED package
    • Surface mount LEDs
    • Bi-colour and multicolour LEDs - the types of LEDs contain several individual LEDs that are turned on by different voltages, etc.
    • Flashing LEDs - with a small time integrated into the package
    • Alphanumeric LED displays
    •     . . . . . more . . . .
    All these different types of inorganic LED are used in very large quantities
  • High brightness LEDs:   High brightness LEDs, HBLEDs, are a type of inorganic LED that are starting to be used for lighting applications. This type of LED is essentially the same as the basic inorganic LED, but has a much greater light output. To generate the higher light output, this LED type requires to be able to handle much higher current levels and power dissipation. Often these LEDs are mounted such that they can be mounted onto a heatsink to remove the unwanted heat.

    In view of their greater efficiency, this type of LED is being used as a replacement for many more traditional forms of lighting. Domestic lighting along with automotive lamps are now in widespread use. They have advantages in terms of efficiency and environmental factors over incandescent and Compact Fluorescent Lightbulbs, CFLs. The HBLEDs have a greater efficiency level and they also have a longer life, especially when being switched on and off many times. However they do have a finite life, a factor that is sometimes overlooked.
  • Organic LEDs:   Organic LEDs are a development of the basic idea for the light emitting diode. This type of LED uses organic materials as the name indicates.

    The traditional types of light emitting diode utilise traditional inorganic semiconductors with varying dopant levels and they produce light from the defined PN junction - often this is a point of light. The organic type of LED display is based on organic materials which are manufactured in sheets and provide a diffuse area of light. Typically a very thin film of organic material is printed onto a substrate made of glass. A semiconductor circuit is then used to carry the electrical charges to the imprinted pixels, causing them to glow.

With LED technology improving all the time, the efficiency levels of all the different types of LEDs is bound to improve, and their use will increase.

LED colours

Traditional inorganic LEDs are available in a variety of colours. The first LEDs to be produced were red, but since then many other colours have been introduced. Now they are available in the following colours:

Of the colours available the blue and white LED types are more expensive than LEDs in other colours as a result of the higher manufacture costs.

In addition to the LEDs that emit visible light, others are manufactured to emit infra-red. These ones are often used for applications such as television remote controls where no visible light is seen.

The colour of a light emitting diode is determined by the semiconductor material used in the diode. Although the plastic body of the diode may appear to be coloured, this is not what gives the diode its colour.

Multicolour LEDs

Sometimes it can be very useful to have a lamp that has more than one colour, indicating a different colour to indicate a different state. This can be done using LEDs. There are two sorts:

  • Bi-colour LEDs   A bi-colour LED is constructed by having two LEDs in parallel with each other in the same package, but they are wired with one external connection of the package going to the cathode of one diode, and the anode of the other. The other lead is again connected to the anode of the first diode and the cathode of the second. In this way when a voltage is applied one way round, one LED will light, and when it is applied the other way round, the other one will light.
  • Tri-colour LEDs   This type of LED has three leads enabling any combination of LEDs to be light, i.e. the first LED, the second, or both. The most popular form of tri-colour LED uses a red and green diode. This means that when one diode is on, then either red or green is produced. If both are light, then the colours combine to form yellow.

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 devices will remain in the electronics catalogues for many years to come.



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