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(LED) light emitting diode

LED tutorial includes:

    •  LED types
    •  LED technology
    •  LED history
    •  LED characteristics
    •  LED specifications & parameters
    •  LED configurations & packages
    •  LED structure & fabrication
    •  OLED basics
    •  OLED technology & operation
    •  PMOLED - Passive Matrix OLED
    •  AMOLED - Active Matrix OLED
    •  High Brightness, HBLED
    •  LED life expectancy

LEDs, or Light emitting diodes 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 They were also used in alphanumeric displays, although in this particular application they have now been superseded by other forms of display.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.

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 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 an LED, 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.

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 on-going technology development, and their convenience of use, these devices will remain in the electronics catalogues for many years to come.

LED history dates back many more years than many people imagine. LED history dates back to the beginnings of radio and electronics.

While LEDs have now been available since the 1960s, the LED history extends many years before this. The LED took many years to develop for a number of reasons - the first discoveries were well ahead of their time, other discoveries were lost. It was only when the technology was sufficiently mature that the LED could be fully developed and marketed. Even after the first devices appeared, LED history was not finished - new developments have been made and LEDs are addressing new markets, never really envisaged before.

Early LED history

The first recorded effects of the light emitting diode effect were noticed back at the beginning of the twentieth century. A British engineer named H J Round working for Marconi was undertaking some experiments using crystal detectors. At the time radio detectors were one of the major limiting factors within the early wireless of radio sets.

The early detectors were often made by using a small piece of material - we would now know them as forms of semiconductor - and placing a small wire onto the surface. These were called "Cat's Whiskers" for obvious reasons. In trying to investigate the effects and improve their performance, Round had passed a current through some of his detectors. He noted that one of them emitted light when a current was passed through it. Although he did not understand the mechanism for the effect, he published his findings in 1907 in a magazine of the day named Electrical World.

LEDs investigated by Losov

The idea lay dormant for some years before it was observed again by a Russian engineer named Oleg Vladimirovich Losov. He was the son of a Russian Imperial Army Office - born into a noble family. This would have counted against him in the post revolution Russia era.

Losov has attended a number of university lectures but never undertook any formal university education, but instead was a technical at the Leningrad Medical Institute.

Losov made some major advances and is a key person in the LED history. He undertook a considerable amount of work investigating light emission from Cat's Whisker style detectors. He observed and investigated the light emission from zinc oxide and silicon carbide crystal rectifiers.

As a result of his observations and investigations, Losov published a number of papers in the technical press of the day between 1924 and 1930. His first paper was entitled: "Luminous carborundum detector and detection crystals" which was published in a Russian journal. Soon he published his findings in other British and German. Losov detailed a variety of aspects of these diodes including the spectra of their light emission as well as many other aspects of their operation. In one article published in the Philosophical Magazine in 1928 he detailed the I-V characteristics of a carborundum diode along with the onset of light emission. This formed part of his work on investigating the nature of the diode emission - recognising it was not a thermal effect, but arising from the semiconductor action.

In further work, Losov investigated the temperature relationships of the effect, cooling the semiconductors down to very low temperatures. He also modulated the LED to see the effects of frequency of any current applied to the diode.

Losov went on to investigate further ideas associated with diode and what would alter be called semiconductor technology. Sadly though, he lived in Leningrad and he was killed during the siege of Leningrad 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.

Semiconductor technology advances

During the Second World War, radar was seen as a major enabler. Accordingly a large amount of development of practical devices was initiated. This utilised much of the materials science work that had been undertaken in the 1920s and 1930s.

As a result of the work new point contact diodes were developed. These were able to provide better performance than thermionc valve / vacuum tube diodes. As a result of the research into semiconductor diodes, the idea for the light emitting diode re-surfaced in 1951. This time work was to be more successful, although it took some years to reach completion. One research team was lead by Kurt Lehovec. He applied for a patent in 1952 for Silicon carbide diodes that emitted light. However this was only the first phase of the work that was needed.

Following the work by Kurt Lehovec, others also started to work on LED technology. The work took many years and involved a number of companies and researchers. Even Shockley became involved.

Although LEDs did not become commercially available for a number of years, several people made some significant discoveries and improvements. Lehovac himself investigated introducing different impurities to change the colour of the light making blue, green/yellow, and pale yellow from different combinations.

Also researchers working at RCA patented a green LED in 1958. All of these LED developments added more to LED technology, furthering the technology within the overall LED history.

Commercial LED history

The first commercially available LEDs started to appear in the late/mid 1960s. These LEDs early LEDs used a semiconductor made using gallium, arsenic and phosphorus - GaAsP. This produced a red light, and although the efficiency of the devices was low (typically around 1 - 10 mcd at 20mA) they started to be widely used as indicators on equipment.

One of the first companies to manufacture LEDs on any scale was Monsanto. Monsanto was actually a company supplying the raw semiconductor materials. They had aimed at working with Hewlett Packard - then a test equipment company - with Monsanto supplying the semiconductor and Hewlett Packard manufacturing the diodes. However the relationship did not work out and Monsanto ended up developing the LEDs themselves. [The name of Monsanto is not seen today. The business was sold in 1979 to General Instrument.]

With the original GaAsP devices being manufactured, the next development saw gallium phosphide devices developed. GaP devices were not widely used because the light they produced was at the far end of the red spectrum where the sensitivity of the human eye is low, and even though they produced a high output, the human perception was of a dim light.

High output LED lamps

As LEDs were developed, the light levels increased to the extent that they could be considered for applications outside simple indicator lamps. By 1987 the Hewlett Packard AlGaAs (aluminium gallium arsenide) diodes being produced were bright enough for the first applications within lighting. The first applications for these diodes was within the automotive industry where red LEDs were used for vehicle brake lights, and also for traffic lights. Here the use of LEDs was of particular interest because of their increased reliability over the incandescent lights that had been previously used.

A year after the first AlGaAs LEDs were introduced another variant, AlInGaP (aluminium Indium Gallium Phosphide) were manufactured. These LEDs gave a significant improvement over the previous AlGaAs diodes by doubling the light output.

Later, in 1993 HP started to use GaP (gallium phosphide) to provide high output green LEDs. Also further developments of this technology allowed the production of high output orange lamps. These were ideal for use as car direction indicators - again their reliability in being turned on and off as well as their efficiency proved to be a major improvement.

There is a wide variety of different LEDs available on the market. The different LED characteristics include colours light / radiation wavelength, light intensity, and a variety of other LED characteristics.

The different LED characteristics have been brought about by a variety of factors, in the manufacture of the LED. The semiconductor make-up is a factor, but fabrication technology and encapsulation also play major part of the determination of the LED characteristics.

LED colours

One of the major characteristics of an LED is its colour. Initially LED colours were very restricted. For the first years only red LEDs were available.

However as semiconductor processes were improved and new research was undertaken to investigate new materials for LEDs, different colours became available.

The diagram below shows some typical approximate curves for the voltages that may be expected for different LED colours.

Typical (approximate) LED voltage curves

LED voltage drops

The voltage drop across an LED is different to that of a normal silicon LED. Typically the LED voltage drop is between around 2 and 4 volts.

The actual LED voltage that appears across the two terminals is dependent mainly upon the type of LED in question - the materials used.

As would be expected the LED voltage curve broadly follows that which would be expected for the forward characteristic for a diode. However once the diode has turned on, the voltage is relatively flat for a variety of forward current levels. This means that in some cases designers have used them as very rough voltage stabilisers - zener diodes do not operate at voltages as low as LEDs. However their performance is obviously nowhere near as good.

Summary of LED characteristics

WAVELENGTH RANGE (NM)	COLOUR	VF @ 20MA	MATERIAL
< 400	Ultraviolet	3.1 - 4.4	Aluminium nitride (AlN) Aluminium gallium nitride (AlGaN) Aluminium gallium indium nitride (AlGaInN)
400 - 450	Violet	2.8 - 4.0	Indium gallium nitride (InGaN)
450 - 500	Blue	2.5 - 3.7	Indium gallium nitride (InGaN) Silicon carbide (SiC)
500 - 570	Green	1.9 - 4.0	Gallium phosphide (GaP) Aluminium gallium indium phosphide (AlGaInP) Aluminium gallium phosphide (AlGaP)
570 - 590	Yellow	2.1 - 2.2	Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium phosphide (GaP)
590 - 610	Orange / amber	2.0 - 2.1	Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaUInP) Gallium phosphide (GaP)
610 - 760	Red	1.6 - 2.0	Aluminium gallium arsenide (AlGaAs) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium phosphide (GaP)
> 760	Infrared	< 1.9	Gallium arsenide (GaAs) Aluminium gallium arsenide (AlGaAs)

When choosing LEDs for particular applications it is necessary to comprehend the different LED specifications or LED parameters.

There is a variety of different LED specifications, each of which will have an effect on the choice of the particular LED used. With such a variety of different LEDs available, the LED specifications cane be matched to the requirements for the particular application rather than having to make do.

Some of the major LED specifications are outlined in the paragraphs below.

LED colour

Obviously the colour is a major LED specification or LED parameter. LEDS tend to provide a single colour. The light emission extends over a relatively narrow light spectrum.

The colour emitted by an LED is specified in terms of its peak wavelength (lpk) - i.e. the wavelength which has the peak light output. This is measured in nanometers (nm).

The colour of the LED, i.e. the peak wavelength of the emission from the LED is a function mainly of the chip material and its fabrication. Typically process variations give peak wavelength variations of up to ±10nm.

When choosing colours within the LED specification, it is worth remembering that the human eye is most sensitive to hue or colour variations is around the yellow / orange area of the spectrum - around 560 to 600 nm. This may affect the choice of colour, or position of LEDs if this could be a problem.

LED light intensity value, Iv

The LED specification for light intensity is important. The light intensity is governed by a variety of factors including the LED chip itself (including the design, individual wafer, the materials, etc.) , the current level, encapsulation and other factors.

The LED light intensity specification is not of crucial importance for most indicator applications, but with LEDs being used for lighting, this parameter is needed to be able to specify exactly what is needed in many situations.

The light output from an LED is quantified in terms a single point, on-axis luminous intensity value (Iv). This is specified as millicandella, mcd.

The lv measurement for LEDs cannot easily be compared with the values of mean spherical candle power, MSCP used for incandescent lamps.

The luminous intensity value for an LED must be quoted for a given current. Many LEDs will operate at currents of around 20mA, but the light output of an LED increases with increasing current.

LED current / voltage specification

LEDs are current driven devices and the level of light is a function of the current - increasing the current increases the light output. It is necessary to ensure that the maximum current rating is not exceeded. This could give rise to excessive heat dissipation within the LED chip itself which could result in reduced light output and reduced operating lifetime.

In operation, LEDs will have a given voltage drop across them which is dependent upon the material used. The voltage will also be slightly dependent upon the level of current, so the current will be stated for this.

Most LEDs require an external series current limiting resistor. Some LEDs may include a series resistor and will state the overall operating voltage.

LED reverse voltage

LEDs are not tolerant to large reverse voltages. They should never be run above their stated maximum reverse voltage, which is normally quite small. If they are then permanent destruction of the device will almost certainly result.

If there is any chance of a reverse voltage appearing across the LED, then it is always best to build in protection into the circuitry to prevent this. Normally simple diode circuits can be introduced and these will adequately protect any LED.

LED angle of view specification

In view of the way in which LEDs operate, the light is only emitted over a certain angle. While this LED specification may not be important for some applications, it is of great importance for others.

The angle of view is normally defined in degrees - °. For early devices, the angle of view was normally relatively small. More recent devices may have a much wider angle of view.

LED specification for operational life

The light intensity of a LED does diminish gradually with time. This means that a LED has an operational life.

This LED specification is of particular importance when a LED or LEDs are to be used for lighting applications. It is not normally as crucial when the LED is used as an indicator - here a catastrophic failure is of greater importance.

The LED specification for its operational life is generally defined in the following terms:

L_70% = Time to 70% of illumination (lumen maintenance)

L_50% = Time to 50% of illumination (lumen maintenance)

The standards state that during these times, the LED should not exhibit any major shifts in chromaticity.

The rationale behind these figures is that 70% lumen maintenance equates to a 30% reduction in light output. This is around the figure for the threshold for detecting gradual reductions in light output.

Where light output is not critical, the 50% lumen maintenance figure may be more applicable. However for applications where lights may be placed side by side the 80% lumen maintenance figure should be considered.

Figures for LED operational life may be of the order of 50 000 hours or more dependent upon the lumen maintenance figure used.

The standard inorganic styles of LEDs can come in many different varieties. These include different packages and configurations.

The variety of packages and styles enables them to be used in many different applications and in many different ways.

Also the different configurations of LEDs enable them to be used in many different ways - as indicators, as displays and as monitors.

LED configurations

There are a number of different formations or configurations that LEDs in which LEDs can be obtained:

• Single colour:   This is the standard format or configuration for an LED. It has two leads, one a cathode and the other an anode. The LED comes on and off according to when a current is passed through the diode.
• Bi-colour LEDs :   This format for LEDs uses a pair of LEDs wired in an inverse parallel formation. This enables one LED to be illuminated at a time dependent upon the polarity of the voltage applied.
  
  LEDs do not withstand significant reverse voltages. Although the diodes in this configuration will experience a reverse bias, this is limited to the forward voltage of the other diode, and this is not sufficient to damage the reverse biased diode.
• Tri-colour LEDs :   This LED configuration again uses two separate LEDs, but in a different configuration. Each LED has a different colour. There are two anode connections and a single cathode. It is therefore possible to turn each LED on separately, giving a choice of two colours - the third is provided by turning both LEDs on together and giving a third colour by addition. It is also possible to have different intensities of both LEDs to further vary the colour.
• Flashing LEDs:   This form of LED is relatively easy to implement. The package contains not only the LED but also a simple IC that provides a timing function to enable the LED to flash.

Traditional LED packages

The traditional LED has been available since the early 1960s and has been produced in quantities of billions.

Typical LED package

LEDs are available in a variety of package sizes. Possibly the most widely used is the 5mm diameter one, although a host of others are available ranging from 1.8mm, 3mm, 4mm, 8mm, and 20mm. There are also and rectangular LEDs available - almost any size to fir a wide variety of requirements. 

Surface mount, SMT LED packages

Like all components, LEDs are used in vast quantities in LED SMT packages. They are available in the industry outline packages. However they have not been subject to quite the same level of miniaturisation as many components such as resistors and capacitors that are available in 0201, etc. This is partly because they need to be a certain size to see!

The most common LED SMT sizes with dimensions are given below.

PACKAGE DESIGNATOR	LENGTH (MM)	WIDTH (MM)	HEIGHT (MM)
1206	3.2	1.5	1.1
0805	2.0	1.25	0.8
0603	1.6	0.8	0.6
0402	1.0	0.5	0.45

LED displays

Although liquid crystal and other forms of display have taken over many alphanumeric display applications, LED technology is still used in a number of applications. It has advantages that it does not need external light as in the case of an LCD. However they are less versatile and costs for customisation normally prohibit this type of use. It is often only possible to display numeric characters, and sometimes some limited graphics

Nevertheless LED displays are used in many areas, particularly where power is not an issue. Alarm clocks, test instrumentation, and other forms of mains powered electronic apparatus use LED alphanumeric displays.

The LED alphanumeric displays use a variety of approaches to display the characters:

• Seven segment:   This form of display can be used to create digiots between "0" and "9". An additional LED can also provide a decimal point, making this basic form of LED display applicable for a variety of basic numeric display applications. One example may be clock radios where power is not an issue.
• Star-burst:   The starburst format for an LED display has the ability to illuminate fifteen lines and in this way it gives a considerable improvement in flexibility over that of the basic seven segment display. It can be programmed to create numeric characters as well as certain limited graphics.
• Dot matrix:   The dot matrix display is the most flexible format. The LED dot matrix display is made up from a matrix of LEDs, each providing a dot. It can be obtained in a variety of formats although 5 rows and five columns, or 7 rows and five columns are common formats.

LED alphanumeric package styles

LEDs are a specialised form of p-n junction diode that have been designed to optimise their electroluminescence. As a result the LED structure and LED fabrication techniques need to ensure that the light output is optimised.

There are a number of different aspects to the LED structure and LED fabrication. These include not only the LED fabrication itself, but also the packaging of the LED once the semiconductor chip itself has been fabricated.

LED die structure

There are two basic configurations for the LED structure.

• Surface emitting LED structure:   This form of LED structure emits light perpendicular to the plane of the PN junction.
• Edge emitting LED structure:   This form of LED structure emits light in a plane parallel to the junction of the PN junction. In this configuration the light can be confined to a narrow angle.

The active films of the LED structure are normally grown epitaxially - often by liquid phase or vapour phase epitaxy. The substrates are chosen to have a close lattice match to the active layers.

Common substrates are GaAS, GaP, InP. The PN junction can be created by either impurity diffusion, ion implantation, or it can be incorporated during the epitaxial growth phase.

Commercially, LEDs exist in a variety of forms, ranging from individual LED indicators where there is just one LED per package, through a variety of displays, right up to vast arrays of LEDs in LED screens.

For some limited applications, it is possible to use a variety of LED PN diode junction types. These can include Schottky contacts and MIS (metal-intrinsic-semiconductor) junctions. However these are normally less efficient and sometimes more difficult to form reliably.

Final LED package structure

There are obviously many different styles of LED that are available. These range from the simple LED indicators through the more complicated LED alphanumeric displays to the LED screens that are now appearing. All these types of LED will have their own package structure. However the simple LED indicators tend to have a fairly common structure and this serves to indicate the constraints on any LED device.

The structure of the LED package can be split into a number of different elements:

• Semiconductor die :  This is the light emitting diode itself formed from the semiconductor.
• Lead frame:  This houses the die and acts as the connection to it.
• Encapsulation:  This surrounds the assembly and acts as protection as well as dispersing the light.

The die is bonded into a recess in one half of the lead frame, called the anvil due to its shape. This is done using conductive epoxy. The recess in the anvil is shaped to throw the light radiation forward. The top contact from the die is then wire-bonded to the other lead frame terminal which is often called the post.

Typical LED package

Organic LEDs or OLEDs are now an established area of the overall LED market.

OLEDS are being used in many applications from television set screens, and computer monitors, along with other small, portable system screens such as mobile smartphones to watches, advertising, information, and indication. OLEDs are also used in large-area light-emitting elements for general illumination.

Currently OLEDs emit less light than their in-organic counterparts, but their flexibility means that they can be used in a much greater number of applications.

Organic LED overview

the organic LED, OLED has many of the properties of a traditional organic LED. It is a PN junction cross which light flows.

However, rather than using the traditional in-organic materials, OLEDs utilise organic compounds for the PN junction. These materials include a variety of substances, but materials such as Aluminium 8-hydroxyquinoline and diamene are often used.

OLED advantages & disadvantages

OLED technology is finding its niche in a variety of applications because it is able to provide a number of advantages:

• Flexible:   It is possible to make OLED displays flexible by using the right materials and processes.
• Very thin:   OLED displays can be made very thin, making them very attractive for televisions and computer monitor applications.
• Colour capability:   It is possible to fabricate OLED displays that can generate all colours.
• Power consumption:   The power consumed by an OLED display is generally less than that of an LCD when including the backlight required. This is only true for backgrounds that are dark, or partially dark.
• Bright images:   OLED displays can provide a higher contrast ratio than that obtainable with an LCD.
• Wide viewing angle:   With many displays, the colour becomes disported and the image less saturated as the viewing angle increases. Colours displayed by OLEDs appear correct, even up to viewing angles approaching 90°.
• Fast response time:   As LCDs depend upon charges being held in the individual pixels, they can have a slow response time. OLEDs are very much faster. A typical OLED can have a response time of less than 0.01ms.
• Low cost in the future:   OLED fabrication are likely to be able to utilise techniques that will enable very low cost displays to be made, although these techniques are still in development. Current costs are high.

OLED displays do have their disadvantages:

• Moisture sensitive:   Some types of OLED can be sensitive to moisture.
• Limited life:   The lifetime of some displays can be short as a result of the high sensitivity to moisture. This has been a limiting factor in the past.
• Power consumption:   Power consumption can be higher than an equivalent LCD when white backgrounds are being viewed as the OLED needs to generate the light for this which will consume more power. For images with a darker background power consumption is generally less.
• Lifespan:   The lifespan of the OLED displays is a major problem. Currently they are around half that of an LCD, being around 15 000 hours.
• UV sensitivity:   OLED displays can be damaged by prolonged exposure to UV light. To avoid this a UV blocking filter is often installed over the main display, but this increases the cost.

Types of OLEDs

Organic LED, OLED technology can be divided into two types of organic LED technologies:

• Small Molecule OLED, SM-OLED:   The small molecule type of organic LED was originally championed by Kodak, and is often the type referred to be the name OLED.
• Polymer LED, PLED:   Polymer LEDs, PLEDs, may also be known as Light Emitting Polymers, LEPs.

OLEDs are being seen used increasingly in view of the advantages which they possess. While they still have some disadvantages, significant development effort is being focussed on this technology around the globe because of its potential which is starting to be taped.

Although OLED technology has a number of basic similarities with those of the traditional inorganic LEDs, there are some major differences.

Not only does organic LED technology differ in the materials that are used, but aspects of OLED operation are also different.

OLED technology basics

there are a number of different variations on the basic OLED technology structure.

The basic OLED comprises an anode and a cathode deposited in a substrate, and sandwiched between these is a layer of organic material.

The organic material is electrically conductive because of what is termed the delocalisation delocalization of pi electrons caused by conjugation over all or part of the molecule.

When used in this way, these organic materials can range from insulators to conductors and are therefore classed as semiconductors.

There are two definitions required, highest unoccupied molecular orbital, HUMO, and the lowest unoccupied molecular orbital, LUMO of organic semiconductors. These are analogous to the valence and conduction bands of inorganic semiconductors.

The OLED display consists of a number of layers. A typical stack may include:

• Anode
• Emissive layer
• Conductive layer
• Cathode

Diagrammatic structure of an OLED

The PMOLED, passive matrix organic light emitting diode, is a form of organic light emitting diode, OLED that is starting to be used within a number of products including cellphones, automobile applications, and even some audio equipment.

The PMOLED is one of around six different types of organic LED display that have been developed and are starting to be used.

The PMOLED displays are suited to applications where text and icons are displayed.

PMOLED basics

The PMOLED structure is arranged in a format that has rows and columns. There are columns of organic cathodes superimposed on rows of anode material.

With this type of format, the row and column lines can be turned on to activate the individual pixels at the intersection points.

PMOLED Structure

The organic material is set down between the anode and cathode - with both the organic material and cathode metal regions being deposited using relatively standard processes. This enables large scale manufacturing to be achieved in a relatively cheap manner.

In terms of their structure, the layers of organic material are set down in a form of ribbed structure with columns and rows obviously running in different directions.

Although the concept of the PMOLED structure is relatively straightforward to design and fabricate, they require relatively complicated drive arrangements because each line needs to have the current limited for each diode - this will change according to the number of diodes in a given row that are activated.

In addition to this, the PMOLEDs require a significantly higher power consumption level than their active AMOLED counterparts. As a result PMOLEDs are normally most suited to display applications where the display size is less than about 50 mm to 80 mm across the diagonal or where there are less than about 100 rows.

The AMOLED or Active Matrix Organic Light Emitting Diode, has many similarities to the PMOLD.

However the AMOLED incorporates more elements of the drive circuitry, making it easier to address individual elements as well as providing more flexibility and a greater level of overall performance and capability.

As a result the AMOLED display is being used more widely and in more exacting applications where higher levels of performance are required.

AMOLED basics

The AMOLED provides a considerably greater degree of flexibility and control when compared to its passive relation.

The AMOLED display consists of a matrix of OLED pixels, each having an anode, cathode and a layer of organic material between them. These pixels are activated by a thin film transistor array which controls the current to each pixel, enabling it to be activated and when current flows through it, light is generated.

Typically two transistors are used for each pixel - one to turn the charge to the pixel on and off, and a second to provide the constant current. This eliminates the need for the very high currents required for passive matrix OLED operation

TFT backplane technology is an essential element for AMOLED display fabrication. Two primary TFT backplane technologies, namely polycrystalline silicon (poly-Si) and amorphous silicon (a-Si), are used today in AMOLEDs. These technologies offer the potential for fabricating the active matrix backplanes at low temperatures (below 150�C) directly onto flexible plastic substrates for producing flexible AMOLED displays.

The TFT array uses a very small amount of energy, yet unlike an LCD, it is able to refresh very fast. This means that the AMOLED display is very suited to television and other displays where moving graphics are to be seen.

AMOLED life expectancy

One of the chief issues with AMOLED displays is the lifetime. Although much has been achieved to improve the displays, the first ones to be introduced commercially only had lifetimes of around 15 000 hours.

Figures of this order are quite acceptable for mobile phones which are replaced fairly regularly and the display may only be on for relatively short periods, but for televisions or computer monitors that have much higher usage rates, the half life becomes an issue.

AMOLED applications

The AMOLED has a number of advantages over its passive relation. This means that AMOLED displays can be used in many more areas.

The first commercially available AMOLED display was produced as early as 2003. It was manufactured by SK Display and was used in a number of products including camera and personal media players.

Since then AMOLED displays have been used in a number of televisions. Currently sizes are not as large as those available with LCD or Plasma displays, but in view of the anticipated cost advantages that are likely to be gained from the use of AMOLED displays, much investment is being directed towards the development of AMOLEDs.

As such the major application for AMOLEDs is likely to be within televisions and computers, although the life of the display is currently an issue.

Comparison of Screen technologies

There are a number of contenders for screen technology within the television, computer monitor and other related areas. Although cathode ray tubes are now well out of the picture, developers of equipment have a choice of technologies of which the AMOLED is one.

AMOLED	LCD	PLASMA
Potentially the lowest cost Consumes lowest power (when backlight of others included) Self emissive Displays wider colour range than LCD No screen burn potential Shorter overall life (red and green half life ~ 10 - 40 k hours, blue ~ 1-k hours	Medium cost Lower power consumption than plasma Requires backlight Colour range not good No screen burn potential Backlight bulb typically requires replace at around 60 k hours	Highest cost Highest power consumption Requires backlight Displays a very deep black Screen burn potential Half life ~ 60 k hours

High brightness LEDs, also known as HBLEDs, are now entering the marketplace.

As the name suggests these high brightness LEDs offer much higher levels of luminosity than the standard LEDs.

In view of their performance high brightness LEDs are now entering many areas where other technologies had reigned supreme before.

HBLED advantages

High brightness LEDs have a number of advantages over the standard LEDs that were previously used in applications including indicators, and displays.

• Brighter
• Longer life
• Low cost
• RoHS manufacturing compatibility (lead free)

What is an HBLED?

With many LEDs being introduced onto the market, and available light levels increasing, it is useful to be able to define an HBLED and understand what differentiates it from a standard LED.

Obviously it is dependent upon the brightness of the LED itself.

One overview of what a high brightness LED is that it is a LED that produces over 50 lumens (1 candela = 12.75 lumens).

High brightness LEDs should not be confused with high power LEDs. Although they may be one and the same, high power refers to the power consumption and not the light output. Generally it is assumed that a high power LED consumes more than 1 watt in power.

One of the chief reasons for using high brightness LEDs is their improved efficiency over other types of lamp. It is worth comparing HBLEDs with other lamps in terms of lumens per watt.

LAMP TYPE	TYPICAL EFFICIENCY (LUMENS PER WATT)
High brightness LED	>100 and improving
Tungsten filament lamp	~18
CFL	~60
Sodium street lamps	~100 - 200

HBLED technology

There are several enhancements that have been made to basic LED technology to enable the high brightness LEDs to be manufactured.

The initial indicator LEDs used a traditional through hole style wired package. A standard 5mm LED would produce a light output of around two or three lumens for an input of 100 mW - equivalent to 20 or 30 lumens per watt.

Surface mount technology allowed development of LEDs in such a way that the printed circuit board could act as a heat sink - with LEDs mounted onto the board, any heat could be removed reasonably effectively, and this allowed light levels to be increased.

The next development was to add a thermal heat slug directly into the bottom of the surface mount package. Being located directly under the LED junction, this allowed heat to be removed far more effectively.

High brightness LEDs utilise this effective heat removal to enable the HBLED junction to remain within its safe limits while still producing he light output required. In addition to this, more effective manufacturing processes have enabled the efficiency to be improved.

Unlike many other semiconductor devices, LEDs have a limited operation lifespan.

The LED lifetime, or LED lifespan, although long, is nevertheless limited.

In view of their long lifespan, LEDs are considered as reliable light sources, both as indicators and for lighting.

LED MTBF

For any component or system, the MTBF is the mean time between failures. The MTBF is the elapsed time which is predicted between inherent failures of a component or system during operation.

The MTBF is a figure used in calculations for the reliability of items of equipment. In order to be able to calculate the MTBF of the equipment, it is necessary to know the MTBF of the individual components, e.g. the LED MTBF in this case.

The failure rate for a component, and the MTBF are linked. MTBF can be calculated as the inverse of the failure rate if it is assumed that there is a constant failure rate, which is not unreasonable as a first order assumption.

MTBF   =   Hours of operation   / Number of failures

The MTBF figures are often quoted in the manufacturers data sheets. However the MTBF can be considerably reduced by operating components close to their rated limits. Hostile environments such as high temperature and vibration also reduce the MTBF.

However when run within their limits, the LED lamps have a long lifetime, and do not fail very often.

Expected LED lifespan

LED light reduces over time. This form of LED lifespan or LED life expectancy is particularly important for applications such as lighting.

A term called lumen deprecation is used to describe this.

The LED life or LED life is the time to when the light output falls to a given level. The generally accepted levels are 70% and some use 50% of the original value. The LED life expectancy may be quoted in the format L70 or L50, for the life to when the light output falls to 70% or 50% respectively.

The L70 value was chosen because a power LED industry group called the Alliance for Solid-State Illumination Systems and Technologies, ASSIST, undertook tests which demonstrated people generally did not notice a gradually diminishing LED light output until it had dropped by 30% of its original brightness, i.e. to 70% of initial light output. This then gave rise to the L70 figure. However for non-critical areas the L50 figure may also be used.

As a rough guide, most LEDs intended for lighting applications offer L70 values of 50,000 to 60,000 hours, although some are quoting figures of 100 000 hours.

Factors affecting LED lifespan

There are a number of factors that affect the useful LED lifetime. By ensuring that the LED is protected from adverse conditions it is possible to ensure the maximum lifetime is maintained.

• Temperature:   One of the major issues in ensuring the maximum life is obtained from a LED is keeping the temperature down. Excess temperature will considerably shorten the life. To prevent the LED chip running over temperature there are a number of elements that can be included within the design
  
  
  • Good thermal path from LED chip to mount:   It is necessary to ensure that the heat can be effectively removed from the LED semiconductor itself. This is the first step in ensuring the LED junction temperature does not rise to high and adversely affect the LED lifetime.
  • Good bonding between LED and external mount:   It is necessary to ensure that the LED package is effectively bonded to the element on which it is mounted. The thermal resistance should be as low as possible, possibly using thermal mounting grease t ensure complete contact.
  • Good heatsink:   In order that heat is removed effectively from the overall assembly the heatsink on which the LED is mounted should have a low thermal resistance. It should also be located so that heat will flow away from the heatsink. For LED lighting, this is particularly important because often lamps will be located within small light fittings and this will not aid cooling, and hence the LED lifetime will be reduced.
• LED drive level:   To obtain the best LED lifetime, the LED should be driven well within its ratings. Overdriving a LED will drastically reduce its lifetime, although it will increase the light output.
• Power supply:   The power supply needs to match the light emitting diode for optimum LED life expectancy. Not only should the voltage be regulated, but the current also needs to be closely controlled to ensure the LED does not run outside its ratings, or even too close to its maximum ratings.
• Environment:   General conditions such as vibration, and temperature extremes - even when not operating - place mechanical stresses on the diode which will reduce the LED lifetime. Ideally, a LED should be operated within a stable dry environment. When this is not possible, a shorter LED lifetime should be anticipated.

Although it may appear obvious at first sight that the LED life should be as long as possible, this may not always be the main requirement. It is possible that in some cases light output is more important than LED lifetime, and in this case it may be permissible to overdrive the LED to obtain the additional light. Additionally budgetary constraints may limit the inclusion of more effective thermal management, and in this case a decision can be made to balance LED life expectancy against the cost.

Surface Mount Technology (SMT)

In recent years there has been a drammatic change from the use of leaded components to surface mount technology. These SMT components make the manufacturing process much easier and faster.

• Introduction to surface mount technology SMT

Passive components

Capacitor markings Summary of capacitor types Quartz crystals, oscillators,filters Resistor types Inductor types

Resistor colour code Standard or preferred resistor values and the E series Thermistor Light dependent resistor, LDR SMD resistor

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Transistors and diodes are in widespread use today. Many millions are used each day apart from those that are incorporated in integrated circuits.

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