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Diode types

Standard PN junction diode

Light emitting diode (LED)

PIN diode

Schottky barrier diode

Varactor or varicap diode

Zener diode

Light Emitting Diode, LED

- the basics of information about light emitting diode or LED including LED circuits and the use of discrete LED components.

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 starting to take over from incandescent lamps to provide lighting now that high power versions are becoming available. Even car stop lamps are now using them. LEDs can be seen in many forms. Nor only are discrete LEDs available, but they are also integrated into many larger panels as

LED colours

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:

Red   Orange   Amber   Yellow   Green   Blue   White

Of these colours the blue and white LEDs 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.

LED parameters

Apart from the LED colour, there are several other specifications and parameters that can be seen in the specification sheets.

• Angle:   This is the width of the beam of light produced by the LED and measured in degrees



• Intensity:   The output or brightness of the LED is also a key performance parameter. The actual light output is measured in mcd or milli-candela for a particular current.



• Wavelength: The wavelength refers to the wavelength of the light emitted from the LED. It is measured in nanometres (nm) and obviously the wavelength will govern the actual colour. In most cases it is sufficient just to refer to the colour.



• Current Consumption:   The amount of current to be consumed by the LED is an important parameter for the circuit design. While the level of current is normally stated in the data sheets, for ordinary indicator LED lamps used in many electronics projects a figure of 20 mA is taken as standard, especially if no figure is mentioned.

LED connections

Like any diode LEDs must be fitted into a circuit with the correct orientation. To operate, a LED must have current flowing through it, and this can only happen if it is forward biased. In the reverse direction no current will flow. Also if a LED has a high reverse voltage placed across it, then it can easily blow.

The two connections for a LED are denoted by a for the anode which is positive (+), and K for the cathode which is negative (-).

For the small indicator LED lamps that are often used in electronics equipment and electronics projects the cathode (k) usually has a shorter lead and there may a slight flat on the body of cylindrical LEDs. Sometimes it is possible to see inside the LED and again the cathode normally has the larger electrode.

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.

LED 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 light emitting 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.

LED 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 emitting diode with current limit resistor

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