Light Emitting Diode

Light Emitting Diode

Jigna Dand

dand.gsl@gmail.com

Abstract—Light emitting diodes, commonly known as LEDs are real heroes in the electronic world. LED is semiconductor light source. Diode emits a narrow bandwidth visible light at different coloured wavelengths, invisible infra-red light or laser type light when a forward current is passed through them. This paper focuses on design and analysis of Light emitting diodes. Present state of development of LED is reviewed in the paper. Also characteristics of LEDs, types of LEDs are discussed in this paper. Along with this, compensations of LEDs are discussed in this paper.

Index Terms—Light emitting diodes, LEDs, p-n junction diodes, semiconductor, lighting technology, solid state devices, Gallium arsenide, organic LEDs, inorganic LEDs.

Across the p-n junction, when a voltage is applied then electrons and the holes are injected across the depletion region and they become more minority carriers. These recombine with majority carriers when these minority carriers diffuse this in to neutral semiconductor region. This recombination process in direct band gap material results the emission of photons. The resulting photon intensity is directly proportional to the ideal diode diffusion current which is proportional to the recombination rate. The wave length of output optical signals depends upon the band gap energy. The resulting wave length can be engineered within definite limits by using compound semiconductors, so that a precise colour can be obtained, provided the output is in visible range.

I. INTRODUCTION

Basically, LED is a small light bulb that fits into an electrical circuit. Light emitting diodes are solid state devices that convert electrical energy into light energy. It is a p-n junction diode. It emits the light when activated. It is a two- lead semiconductor light source. When suitable voltage is applied to leads, electrons recombine with holes within the device, releasing the energy in the form of light. Colour of the light depends on the energy band gap of the semiconductor. LEDs require very less power to light up by comparison, due to which they don’t get hotter like conventional bulbs. They don’t have filament that will burn out. Life of LEDs exceeds the short life of conventional bulbs by thousands of hours. LEDs have many benefits over incandescent light sources including lower power consumption, longer lifetime, improved physical robustness, smaller size, and faster switching. Today, LEDs have gained significance importance in the lighting industry and have captured all the areas of the world for lighting. The lifespan of LED lighting technology is much more than various lighting technologies like Compact Fluorescents Lamps (CFL), Halolux, Incandescent, etc.

II. WORKING OF LED

In LED, electrical energy is converted into optical energy [1]. LEDs are example of electroluminescence. In this process, photons are emitted by recombination of excess electrons and holes in a direct band gap semiconductor.

Fig. 1 Working of LED

III. CHARACTERISTICS OF LEDS – LEDS COLOURS

Initially only red LEDs were available. But as semiconductor processes were improved and due to research 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 [2]..

Fig. 2 LED voltage curves

IV. CHARACTERISTICS OF LEDS – LEDS VOLTAGE DROPS

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 [2].

V. COLOURS AND MATERIALS USED

The wavelength of light, and the colour, depends on the type of semiconductor material used to make the diode [3]. That’s because the energy band structure of semiconductors differs between materials, so photons are emitted with differing frequencies. While the wavelength of the light depends on the band gap of the semiconductor, the intensity depends on the amount of power being pushed through the

 

diode. The output from an LED can range from red to blue- violet. Some LEDs emit infrared energy; such a device is known as an infrared-emitting diode (IRED).

The following Table I give us the ideas of various semiconductor material used to construct the diode of LED and the colour of the light generated [3].

COLOUR	SEMICINDUCTOR MATERIAL
Infrared	Gallium arsenide (GaAs)
	Aluminium gallium arsenide (AlGaAs)
Red	Aluminium gallium arsenide (AlGaAs)
	Gallium arsenide phosphide (GaAsP)
	Aluminium gallium indium phosphide (AlGaInP)
	Gallium(III) phosphide (GaP)
	Gallium arsenide phosphide (GaAsP)
Orange	Aluminium gallium indium phosphide (AlGaInP)
	Gallium(III) phosphide (GaP)
	Gallium arsenide phosphide (GaAsP)
Yellow	Aluminium gallium indium phosphide (AlGaInP)
	Gallium(III) phosphide (GaP)

		Traditional green:
		Gallium(III) phosphide (GaP)
		Aluminium gallium indium phosphide (AlGaInP)
	Green	Aluminium gallium phosphide (AlGaP)
		Pure green:
		Indium gallium nitride (InGaN) / Gallium(III)
		nitride (GaN)
		Zinc selenide (ZnSe)
	Blue	Indium gallium nitride (InGaN)
		Silicon carbide (SiC) as substrate
		Silicon (Si) as substrate—under development
	Violet	Indium gallium nitride (InGaN)
		Dual blue/red LEDs,
	Purple	blue with red phosphor,
		or white with purple plastic
		Diamond (235 nm)
		Boron nitride (215 nm)
	Ultraviolet	Aluminium nitride (AlN) (210 nm)
		Aluminium gallium nitride (AlGaN)
		Aluminium gallium indium nitride (AlGaInN)—
		down to 210 nm
		Blue with one or two phosphor layers: yellow with
	Pink	red, orange or pink phosphor added afterwards,
		or white phosphors with pink pigment or dye over
		top.
	White	Blue/UV diode with yellow phosphor
		Table I

VI. TYPES OF LED

LEDs are available in various types. Below is the summary of LED types [4].

A. Traditional inorganic LEDs

Traditional inorganic LEDs are manufactured from inorganic materials. Some of the more widely used are compound semiconductors such as Aluminium gallium arsenide, Gallium arsenide phosphide, etc. 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 in- organic LED category, there are many different styles of LED: Single colour 5 mm, Surface mount LEDs, Bi-colour and multicolour LEDs - the types of LEDs contain several individual LEDs that are turned on by different voltages, Flashing LEDs - with a small time integrated into the package, Alphanumeric LED displays, etc. The traditional types of LED utilise traditional inorganic semiconductors with varying dopant levels and they produce light from the defined PN junction - often this is a point of light [4].

B. Organic LEDs

As the name indicates, this type of LED used organic materials.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 [4].

C. High brightness LEDs

High brightness LEDs are a type of inorganic LED that are used for lighting applications. This type of LED is essentially the same as the basic inorganic LED, but has a much more 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 heat sink to remove the unwanted heat [4].

Application specific LEDs are available. Flashing LEDs resemble standard LEDs but they contain an integrated multivibrator circuit that causes the LED to flash with a typical period of one second. In diffused lens LEDs this is visible as a small black dot. Most flashing LEDs emit light of one colour, but more sophisticated devices can flash between multiple colours and even fade through a colour sequence using RGB colour mixing. Bi-colour LED has two different LED emitters in one case.Tri-colour LED has three different LED emitters in one case. RGB LED has three red, green and blue emitters. Decorative multicolour incorporates several emitters of different colours supplied by only two lead-out wires. Alphanumeric LEDs are available in seven-segment, starburst and dot-matrix format.Seven-segment displays handle all numbers and a limited set of letters. Starburst displays can display all letters. Dot-matrixdisplays typically use 5x7 pixels per character.

VII. LEDS CONFIGURATIONS

LEDs configurations are as below.

	SINGLE	Single colour LEDs has two leads, one a cathode
		and the other an anode. The LED comes on and
	COLOUR	off according to when a current is passed through
		the diode [5].
	BI- COLOUR	Bi-colour LEDs uses a pair of LEDs wired in an
		inverse parallel formation. This enables one LED
	LEDs	to be illuminated at a time dependent upon the
		polarity of the voltage applied [5].
		This LED configuration again uses two separate
		LEDs, but in a different configuration. Each LED
		has a different colour. There are two anode
	TRI-COLOUR	connections and a single cathode. It is therefore
		possible to turn each LED on separately, giving a
	LEDs	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 [5].
	FLASHING	The package contains not only the LED but also a
		simple IC that provides a timing function to
	LEDs
		enable the LED to flash [5].

VIII. BENEFITS

LEDs are widely used today. Benefits of using LEDs are low power consumptions, longer life, physical robustness, smaller size and faster switching. Light emitting diodes are

used in automotive headlamps, advertising, general lighting, traffic signals, camera flashes, LED wallpapers, video displays, etc. Small LEDs are replacing the tubes that light up LCD HDTV to make thinner televisions. They help to form numbers on digital clock, transmit information from remote controls, and brighten a traffic light. LEDs can emit light of projected colour. Due to tiny size, it can be easily attached to published circuit boards. They can quickly light up due to which can be used in the communication devices having quick response time. LEDs are shock resistant. Life of LEDs bulbs is much more than conventional bulbs. Solid package of LED can be designed to focus its light. High density LEDs are used in automotive headlights, spotlights, and accent lightings.

IX. DRAWBACKS

Costing of installing LEDs is too high than the conventional lighting technologies. The performance of LED depends on the temperature of the operating environment. Over-driving LED in high ambient temperatures could result in overheating the LED package, eventually failing the device. As the electric current increases, the luminous efficacy of LEDs decreases. The life of LEDs decreases as heating also increases with higher currents. LED lights used for traffic control can have snow obscuring them, leading to accidents. Insects get attracted towards LEDs so much so that there has been speculative concern about the possibility of disruption to food webs. White LEDs emit proportionally more blue light than conventional outdoor light sources, the strong wavelength dependence of Rayleigh scattering means that white LEDs can cause more light pollution than other light sources. LEDs will only light with correct electrical polarity. The additional cost of rectifiers should be added to match the source polarity with LED devices. Reduced lumen output over time for LED is one of the drawbacks. LEDs cannot provide divergence below a few degrees. LEDs must be supplied with the voltage above the threshold and a current below the rating for its proper functioning.

X. APPLICATIONS

Light emitting diodes are used in motorcycle lights, bicycle lights, traffic lights and signals and automotive lighting. They are also used in message displaying boards, advertising, general lighting. Light emitting diodes are used in aviation lighting. Light bulbs made up of LEDs are used. LEDs are consumed in exit signs, light weight message, displaying boxes. Communications are main area of non- visual applications of LEDs.

Infrared LEDs can be found in the remote controls of several consumer products such as TVs, DVD players, washing machines and other household electronics and appliances. LEDs have allowed new text, live video, video displays and sensors to be developed. Specialized white LEDs are used in flat-panel computer displays. Ease of modulation allows wide communications bandwidth with minimal noise, resulting in high speed and accuracy, hence used in Fibre optics data transmission. LEDs are used as motion sensors, for example in optical computer mice. Some flatbed scanners use arrays of RGB LEDs. LEDs are also used in touch sensing devices. Grow lights use LEDs to increase photosynthesis in plants. Bacteria and Viruses can be removed from water and other substances using UV LEDs for sterilization.

XI. LEDS EFFECT ON HEALTH

Majority of the devices containing LEDs are safe under all the conditions of the normal use. Only few devices containing LEDs may cause blindness because of extremely bright LEDs. LEDs have advantage over fluorescent lamps as they do not contain mercury, they may contain other hazardous metals such as lead and arsenic. LEDs are not dangerous except for low-intensity red LEDs, which have Lead at levels exceeding normal limits. Low intensity yellow LEDs are not hazardous. Excessive levels of copper, silver, nickel, lead in LEDs can be harmful to the health. LEDs can flicker at high frequencies causing a strobe effect.

XII.	LIFETIME AND FAILURE OF LEDS

Life of LEDs increases too much if they are operated at low temperatures and low currents. The common symptom of LED device failure is is the gradual lowering of light output and loss of efficiency. Sudden failure of LED products is very rare, but may occur. LED performance is temperature dependent. Most producers publish operating temperatures of LEDs devices. LED light output rises at lower temperatures, levelling off, depending on type used. They are an energy- efficient technology for uses such as in freezers and refrigerators because they emit less heat. However, the life time of LED lighting is more and chances of failures are very less. So the trends of using LED lighting systems by consumers have grown in this decade.

XIII. LED VS LCD

LED stands for Light emitting diodes and LCD stand for Liquid crystal display. The basic technology is the same in that both types have two layers of polarized glass through which the liquid crystals both block and pass light. LCDs use fluorescent lights while LEDs use those light emitting diodes [6]. Example: LED TV, LCD TV. The fluorescent lights in an LCD TV are always behind the screen. On an LED TV, the light emitting diodes can be placed either behind the screen or around its edges. LED TVs are thinner than LCD TVs because the difference in lights and lighting placements. LED TVs run with greater energy efficiency and can provide a clearer, better picture than the general LCD TVs [6]. LED TV are more better than LCD TV because LED TVs work with a colour wheel or distinct RGB-coloured lights (red, green, blue) to produce more realistic and sharper colours. Second, light emitting diodes can be dimmed [6].

XIV. CONSIDERATIONS WHEN COMPARING LED WITH

CONVENTIONAL LIGHTING TECHNOLOGY

When comparing LED performance with conventional lighting, the end-user must consider energy efficiency, operating life and lumen depreciation, light output/distribution, colour quality, and expected lifetime. The energy efficiency of any lighting system depends on more than the efficacy of the light source itself. It is important to consider mean or end-of-life efficacy, and not initial efficacy. Lumen depreciation is an essential consideration in evaluating the design life of LED lamps. To maximize energy efficiency and lighting quality, it is important to evaluate each luminaire against the specific light level and uniformity requirements of each application. The two most common measurements that provide useful guidance on LED colour quality are correlated colour temperature (CCT), which

measures relative colour appearance of white light from lower (warm/yellow) to higher (cool/blue) values on the Kelvin (K) scale; and colour rendering index (CRI), which provides a comparison of how colours are rendered by LEDs relative to a reference light source (either incandescent or daylight) [7].

XV. FUTURE OF LED TECHNOLOGY

LED innovations have changed the lighting systems of the world. With its innovations, LED technology requires a change in thinking by planners, installers and users. Over other lighting technology, LED technology has very long operating life, excellent colour saturation and great efficiency.

Various new designs with coloured or white light make LED a more decorative and efficient highlight at home. Different situations in daily life require different lighting scenarios. For example white light with high proportion of blue increases concentration at work whereas white light increases a good atmosphere in a bar. Whether it is home, office, industry, bar or any place, it improves the quality of the life. LED lighting systems are used to light up buildings, bridges, monuments, etc. Due to their flexibility and the possibility to set any desired colour, LEDs offer lighting solutions, which are not possible with any other lighting technologies, such as colour wall washing, sharp light/shadow lines and avoiding light pollution on windows, etc. In leisure industry like hotels, restaurants, etc. are using LED lighting for decorative purposes. LED technologies are used in retail sectors. LEDs are finding place in commercial freezers, store rooms, etc. After the invention of the LED technology, the lighting system of the world has changed and it usage has grown. LED technology is offering today one of challenging and best business opportunities for many people across the world. Due to the better qualities of LED lighting, it usage throughout the world will keep on growing.

XVI. CONCLUSIONS

LED is the technology of the today and the future. Because of their advantages, LEDs provide great advantage for the user. LED technology, however, also asks for better communication from the industry, in order to demonstrate the advantages of the LED over conventional technologies. The long life and attractive design of LED luminaires and the many possibilities for use make the LED unique and confirm the decision to purchase. Today, LED has captured the market of the lighting technology in almost all the areas of Business, Retails, Governments, Hospitality, Infrastructure development, Automotive and Aviation Industry, Homes, Leisure Industry, Corporate worlds etc. LED creates individuality. LED is itself a giant industry providing good business for the people and good products for the user. And that is exactly what everyone needs today and in the future until some new innovation takes place.

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