led - device performance characteristics

DEVICE PERFORMANCE CHARACTERISTICS OF LIGHT-EMITTING DIODES[Type the document subtitle]

[Pick the date][Type the company name]Mehjabin Abdurrazaque

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INTRODUCTION

Light Emitting Diodes are the wonder component of electronic devices. Blinking

indicators and light sources in every color of the rainbow make the black boxes of our

profession capable of communicating with their users.

To state the obvious, LEDs are diodes. They are P-N junctions that emit energy at a

wavelength that corresponds to visible light when forward biased. Combinations of

different semiconductor materials and dopants vary the energy drop across the

junction, and the color emitted corresponds to that energy drop. Much work has gone

into making every color of the spectrum, and some LEDs incorporate an extra step to

mix colors to produce white light. These components use phosphors to absorb blue or

ultraviolet light emitted from the die and re-emit it as yellow. Blue and yellow are at

sufficiently opposite ends of the visible spectrum to fool our eyes into seeing white light.

LEDs are simple and ubiquitous components in electronic assemblies. They have their

own electrical and optical properties, knowing which can increase their functionality

and decrease their failures. This project focuses on different characteristics of various

LEDs to determine the factors influencing them either by experimentation or by a

detailed literature search.

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TOPIC OF STUDY

Device Performance Characteristics of Light-Emitting-Diodes

OBJECTIVES OF STUDY

Understand basic LED device operation and LED device structures based on the

p-n junction.

Understand the device performance characteristics of various LEDs.

Verify some of the characteristics through experimentation.

Analyze the factors influencing the characteristics of an LED.

METHOD OF STUDY

Basis of Experimentation of Sample Light-Emitting-Diodes

Experiments had been carried out to study the device characteristics of different light-

emitting-diodes.

TOOLS AND TECHNIQUES USED FOR DATA COLLECTION

1. Literature search based on the topic

2. Experimentation

METHOD OF DATA COLLECTION

A single LED from each category available is taken for investigation. Experimentation

was carried out to determine Forward and Radiation Flux characteristics. For the other

characteristics, cannot be done in the given period of time with the available

instruments, were determined from a literature search.

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LITERATURE REVIEW

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A literature search has been carried out, to collect the information and data for various

experiments. The required information and data has been from the standard books,

monographs and internet sites.

Books and Monographs

The library at M.E.S Kalladi College, Mannarkkad has a good collection of books

dedicated to Semiconductor devices and Electronics. Besides, so many magazines on

physics, especially on electronics are also available in the library consistent with the

emerging technologies and trends.

On-line Open Literature and Internet

The Internet also offers a vast open literature for the collection of data. There are so

many websites on new trends in LED lighting. Besides, blogs are available on light-

emitting-diodes.

Electronic Books

A vast collection of books are available online. In addition, lectures by eminent persons

are also available.

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Light-emitting diodes (LEDs) are small but powerful devices in terms of their diverse

applications. LED lights assume greater significance in the context of need for electrical

energy conservation and pollution control world over. These are used as indicators in various

equipments, for lighting and decorations at homes, and in flashlights, signboards and car

lights.

The LED has been available for many years now, initially as a red coloured indicator. Later,

yellow/amber, green and finally blue coloured LEDs became available, which triggered an

explosion in applications. Applications included traffic lights, vehicle lights and wall-washes

(mood lighting). Recently blue coloured LEDs have been combined with yellow phosphor to

create white light. The amount of light available from LEDs has also increased steadily, and

now power levels of 1 W, 3 W and 5 W are fairly common.

LED lights differ from traditional light sources in the way these produce light. In

incandescent bulbs, a tungsten filament is heated by electric current until it glows or emits

light. In fluorescent lamps, an electric arc excites mercury atoms, which emit UV radiation.

After striking the phosphor coating on the inner side of the glass tubes, the UV radiation is

converted into visible light and emitted. On the other hand LEDs are diodes made from

semiconductor materials. This is why they are referred to as solid-state devices. They rely on

indium-gallium nitride to convert electricity into photons when current flows through it.

An LED is basically a small-area source, often with extra optics added to the chip that shapes

its radiation pattern. The specific wavelength or colour emitted by the LED depends on the

materials used to make the diode. It depends on the composition and condition of the

semiconducting material used, and can be infrared, visible or near-ultraviolet.

LED-based light technology is a subject of intense research and development. It is predicted

to radically change the energy consumption patterns throughout the world over the next few

years. Therefore a phenomenal amount of resources are being devoted to improve the LED

technology worldwide.

2.1. THE ELECTROLUMINESCENT PROCESS

The LED converts input electrical energy into output optical radiation in the visible or

infrared portion of the spectrum, depending on the semiconductor material. The energy

conversion takes place in two stages: first, the energy carriers in the semiconductor are

excited to higher states by electrical energy, and second, most of these carriers after having

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lived a mean lifetime in the higher state, give up their energy as spontaneous emission of

photons with energy nearly equal to the band-gap, εg of the semiconductor. If the

semiconductor can be doped p- and n- type, then the energy of the current carriers can be

increased by applying a forward bias to a p-n junction. Under forward bias, minority charge

carriers are injected on both sides of the junction and these excess minority carriers diffuse

away from the junction, recombining with the majority carriers.

Fig. 2.1: Band diagram of p-n junction diode

The photons emitted have energy hν = εg.

A small fraction of the excess minority

carriers do recombine non-radiatively and

the excess energy of these carriers are

dissipated as heat in the lattice. The rate of

radiative recombination is proportional to

the forward bias injection rate, and hence

to the diode current. Most of the

recombination occurs close to the junction,

although some minority carriers diffuse

away from the junction. The internal

quantum efficiency is defined as the rate of

emission of photons divided by the rate of supply of electrons.

Fig 2.2: cross section of a traditional LED

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2.2. TYPES OF LED

There are two basic types of LED structures: edge emitters and surface emitters.

Fig 2.3: Types of LED

Edge emitters are more complex and expensive devices, but offer high output power levels and

high speed performance. The output power is high because the emitting spot is very small,

typically 30-50 µm, allowing good coupling efficiency to similarly sized optical fibres. Edge

emitters also have relatively narrow emission spectra. The full-width, half-maximum (FWHM) is

typically about 7% of the central wavelength. Another variant of the edge emitter is the super-

radiant LED. These devices are a cross between a conventional LED and a laser. They usually

have a very high power density and possess some internal optical gain like a laser, but the

optical output is still incoherent, unlike a laser. Super-radiant LEDs have very narrow emission

spectra, typically 1-2% of the central wavelength and offer power levels rivalling a laser diode.

These devices are popular for fibre optic gyroscope applications. The second type of LED is the

surface emitter. Surface emitters have a comparatively simple structure, are relatively

inexpensive, offer low-to-moderate output power levels, and are capable of low-to-moderate

operating speeds. The total LED chip optical output power is as high as or higher than the edge-

emitting LED, but the emitting area is large, causing poor coupling efficiency to the optical fibre.

Adding to the coupling efficiency deficit is the fact that surface-emitting LEDs are almost perfect

Lambertian emitters. This means that they emit light in all directions. Thus, very little of the

total light goes in the required direction for injection into an optical fibre.

Infrared LEDs

An IR LED, also known as IR transmitter, is a special purpose LED that transmits infrared rays in

the range of 760 nm wavelength. Such LEDs are usually made of gallium arsenide or aluminum

gallium arsenide. They, along with IR receivers, are commonly used as sensors. The appearance

is same as a common LED. Since the human eye cannot see the infrared radiations, it is not

possible for a person to identify whether the IR LED is working or not, unlike a common LED. To

overcome this problem, the camera on a cell phone can be used. The camera can show us the IR

rays being emanated from the IR LED in a circuit.

http://www.engineersgarage.com/content/led

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Organic Light-Emitting Diodes

In an OLED (organic light-emitting diode) the emissive electroluminescent layer is a film of

organic which emits light in response to an electric current. This layer of organic

semiconductor material is situated between two electrodes. Generally, at least one of these

electrodes is transparent. The conductive Indium tin oxide is used for this purpose. When a

positive bias is applied to the ITO, electrons and holes are injected into the device and under

influence of the applied electrical field these carriers travel through the organic layer or layers.

The current in OLEDs is typically space charge limited, i.e., limited by the bulk of the

semiconductor. In other words, the measured current is a drift current, determined by the

mobility of charge carriers. Charge transport occurs in the vertical direction, perpendicular to

the stack of the organic layers, and the carrier mobility in the organic semiconductor element of

OLEDs is typically low. When a pair of oppositely charged carriers meets, excitons are formed

and radiative relaxation of these excitons results in light-emission.

Fig 2.4: Schematic illustration of the

working principle of a basic two-layer

OLED, comprising a hole-transporting layer

– HTL and an electron-transport layer – ETL

Source: Device Architecture and Materials for

Organic Light-Emitting Devices - Sarah Schols

Quantum Dot Light-Emitting Diodes

Quantum-dot-based LEDs are characterized by pure and saturated emission colours with

narrow bandwidth, and their emission wavelength is easily tuned by changing the size of the

quantum dots. Moreover, QD-LED combine the colour purity and durability of QDs with

efficiency, flexibility, and low processing cost of organic light-emitting devices. QD-LED

structure can be tuned over the entire visible wavelength range from 460 nm (blue) to 650 nm

(red).

The structure of QD-LED is similar to basic design of OLED. The major difference is that the light

emitting centres are cadmium selenide (Cd Se) nano-crystals, or quantum dots. A layer of

cadmium-selenium quantum dots is sandwiched between layers of electron-transporting and

hole-transporting organic materials. An applied electric field causes electrons and holes to move

into the quantum dot layer, where they are captured in the quantum dot and recombine, and

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emitting photons. The spectrum of photon emission is narrow, characterized by its full width at

half the maximum value.

2.3. MATERIALS USED IN LED

Wavelength

(nm)

Colour Name

Fwd Voltage

(at 20mA)

Intensity5mm LEDs

ViewingAngle

LED Dye Material

940 Infrared 1.516mW

@50mA15°

GaAIAs/GaAs -- Gallium Aluminium

Arsenide/Gallium Arsenide


@50mA15°


Arsenide/Gallium Arsenide


@50mA15°


Arsenide/Gallium Aluminum Arsenide

660Ultra Red

1.82000mcd@50mA

15°


Arsenide/Gallium Aluminum Arsenide

635High Eff.

Red2.0

200mcd @20mA

15°GaAsP/GaP - Gallium Arsenic Phosphide / Gallium Phosphide

633Super Red

2.23500mcd@20mA

15°InGaAIP - Indium

Gallium Aluminum Phosphide

620Super

Orange2.2

4500mcd@20mA



612Super

Orange2.2

6500mcd@20mA



605 Orange 2.1160mcd @20mA


595Super Yellow

2.25500mcd@20mA



592 Super Pure

2.1 7000mcd@20mA

15° InGaAIP - Indium Gallium Aluminum

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Yellow Phosphide

585 Yellow 2.1100mcd @20mA


4500K"Incan-

descent"White

3.62000mcd@20mA

20°SiC/GaN -- Silicon Carbide/Gallium

Nitride

6500KPale

White3.6

4000mcd@20mA

20°SiC/GaN -- Silicon Carbide/Gallium

Nitride

8000KCool

White3.6

6000mcd@20mA

20°SiC/GaN - Silicon Carbide / Gallium

Nitride

574SuperLime

Yellow2.4

1000mcd@20mA



570SuperLime Green

2.01000mcd@20mA



565

High Efficienc

y Green

2.1200mcd@20mA

15°GaP/GaP - Gallium Phosphide/Gallium

Phosphide

560SuperPure

Green2.1

350mcd@20mA



555Pure

Green2.1

80mcd@20mA

15°GaP/GaP - Gallium

Phosphide/ Gallium Phosphide

525Aqua Green

3.510,000mcd

@20mA15°

SiC/GaN - Silicon Carbide / Gallium

Nitride

505Blue

Green3.5

2000mcd@20mA


Nitride

470Super Blue

3.63000mcd@20mA


Nitride

430Ultra Blue

3.8100mcd@20mA


Nitride

Table 2.1: Materials used and corresponding wavelengths emitted

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The band gap in the (Al, Ga, In) N-based materials system ranges from 1.9 eV (In N) to 3.4 eV

(GaN) to 6.2 eV (Al N). The band structure is currently thought to be a direct band gap across

the entire alloy range. Therefore, almost the entire visible range of wavelengths is spanned in

the Group III nitride alloy system. This direct band gap is especially fortuitous as it allows for

high quantum efficiency light emitters to be fabricated in this system. Group III nitrides possess

several remarkable physical properties that make them particularly attractive for reliable solid

state device applications. The wide band gap materials possess low dielectric constants with

high thermal conductivity pathways. Group III nitrides exhibit fairly high bond strengths and

very high melting temperatures. The large bond strengths could possibly inhibit dislocation

motion and improve reliability in comparison to other II-VI and III-V materials. In addition, the

nitrides are resistant to chemical etching and should allow GaN-based devices to be operated in

harsh environments. These properties may lead to devices with superior reliability.

2.4. DEVICE PERFORMANCE CHARACTERISTICS

2.4.1. Current – Voltage Characteristics of LED

A semiconductor diode’s behaviour in a circuit is given by its current–voltage characteristic,

or I–V graph. The shape of the curve is determined by the transport of charge carriers through

the so-called depletion layer or depletion region that exists at the p–n junction between

differing semiconductors. When a p–n junction is first created, conduction-band (mobile)

electrons from the n-doped region diffuse into the p-doped region where there is a large

population of holes (vacant places for electrons) with which the electrons "recombine". When

a mobile electron recombines with a hole, both hole and electron vanish, leaving behind an

immobile positively charged donor (dopant) on the n side and negatively charged acceptor

(dopant) on the p side. The region around the p–n junction becomes depleted of charge and

thus behaves as an insulator.

However, the width of the depletion region (called the depletion width) cannot grow without

limit. For each electron–hole pair that recombines, a positively charged dopant ion is left

behind in the n-doped region, and a negatively charged dopant ion is left behind in the p-

doped region. As recombination proceeds more ions are created, an increasing electric field

develops through the depletion zone that acts to slow and then finally stop recombination. At

this point, there is a "built-in" potential across the depletion zone. This potential difference is

termed as the barrier potential.

http://en.wikipedia.org/wiki/Dopant

http://en.wikipedia.org/wiki/Electron%E2%80%93hole_pair

http://en.wikipedia.org/wiki/Depletion_width

http://en.wikipedia.org/wiki/Nonconductor



http://en.wikipedia.org/wiki/P%E2%80%93n_junction

http://en.wikipedia.org/wiki/Depletion_region

http://en.wikipedia.org/wiki/Depletion_zone

http://en.wikipedia.org/wiki/Current%E2%80%93voltage_characteristic

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Fig 2.5: p-n junction under

(a) Zero bias

(b) Forward bias

Under forward bias minority

carriers diffuse into the neutral

regions where they recombine.

If an external voltage is placed across the diode with the same polarity as the built-in

potential, the depletion zone continues to act as an insulator, preventing any significant

electric current flow (unless electron/hole pairs are actively being created in the junction).

This is the reverse bias phenomenon. However, if the polarity of the external voltage opposes

the built-in potential, recombination can once again proceed, resulting in substantial electric

current through the p–n junction (i.e. substantial numbers of electrons and holes recombine at

the junction). For silicon diodes, the built-in potential is approximately 0.7 V (0.3 V for

Germanium and 0.2 V for Schottky). Thus, if an external current is passed through the diode,

about 0.7 V will be developed across the diode such that the p-doped region is positive with

respect to the n-doped region and the diode is said to be "turned on" as it has a forward bias.

For efficient operation it is important to know the forward threshold voltage, the reverse

breakdown voltage and the reverse leakage current at breakdown.

Fig. 2.6.: i-v characteristics

Curve shows typical characteristics of a low

resistance LED. Compared to curve 2 which

is a normal LED, it is clear that the forward

voltage required to produce the same

current value is lower. ‘Resistance’ referred

here does not mean the term electrical

resistance but instead indicates the slope of

a tangent for the characteristic curve at the

specified current or voltage – differential resistance.

http://en.wikipedia.org/wiki/P%E2%80%93n_junction#Forward_bias

http://en.wikipedia.org/wiki/P%E2%80%93n_junction#Reverse_bias

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In general, using an LED with a lower forward voltage allows easier circuit design. For an LED

with a larger forward voltage, the power consumption will be larger when operated at the same

current value. This will cause a subsequent rise in temperature in the diode, resulting in

detrimental effects such as a decrease in the output power, peak emission wavelength shift and

deterioration of the LED. specified current or voltage – differential resistance.

2.4.2. Radiation Flux and Forward Current Characteristics of LED

An increase in the current through the diode implies a growth in the electron flow at the

junction. This will results in increased number of recombination and hence, the amount of

radiation. The linearity of the radiation flux – current characteristics is important for

modulation in analog transmission. Some LEDs can exhibit non-linearity depending on material

properties and device configuration. It is also important to note that the edge emitters radiate

less optical power into air than surface emitters, although the radiation intensity at the emitting

face can be very high in the former. This is because interfacial recombination and re-absorption

can play a more dominant role in edge-emitting LEDs. However, because of the higher radiation

intensity from them, coupling into fibres with smaller numerical apertures is more efficient.

Fig. 2.7: Radiant output power vs. DC drive current characteristic

The linearity between the radiation output power vs. forward current characteristics enables

estimation of the approximate radiant power at a different current value, if the radiant power at

a certain value is measured. However, if the temperature of the emission area increases due to

the ambient temperature and heat generated from the LED chip itself, the radiant power

decreases and saturation is seen in the characteristic graph. In pulsed operation, the saturation

state varies according to the pulse width and duty ratio.

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2.4.3. Spectral Response

The spectral response of an LED is the plot of the relative output intensity against the

wavelength in nanometers. The spectral line-width is measured in units of wavelength between

the half maximum intensity points and termed the full width at half maximum.

Fig 2.8: spectral response

2.4.4. Performance Deterioration

When an LED is used for long periods of time, performance deterioration may take place.

Common deterioration phenomena include a decrease in the output power and variations of the

forward voltage. It is thought that these deteriorations result from the crystal dislocation and

shift caused by heat generation in the emission area. These can be observed as a dark line or

dark spot in the emission pattern.

Fig 2.9: Output power vs. Time characteristic

Deterioration may possibly

occur from an external stress.

If the LED is driven with stress

applied to the LED chip, its

performance may unduly

deteriorate. This stress may

also issue from mechanical

distortion on the package.

Sufficient care must be

exercised when mounting the

LED.

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2.5. CHARACTERISTICS PLOTTED BY OTHER RESEARCHERS

Fig 2.10: Super red LED

Fig 2.11: Super bright Green LED

Fig 2.12: Blue LED

Source: KINGBRIGHT

The Super Bright Red source color devices are made with Gallium Aluminum Arsenide

Red Light Emitting Diode. The Super Bright Green source color devices are made with

Gallium Phosphide Green Light Emitting Diode. The Blue source color devices are made

with GaN on SiC Light Emitting Diode.

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AN OUTLINE OF EXPERIMENTATION

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Experiment No. 1

FORWARD CHARACTERISTICS OF LIGHT-EMITTING-DIODES

To plot the forward characteristics curve of various LEDs – drive current vs. forward voltage

Apparatus and Accessories:

A variable DC supply (0 – 12V), a micro-ammeter, a mille-ammeter, multimeter, resistance and

LEDs of different colours and ratings

Circuit Arrangement:

LEDs are current controlled devices. i.e., the intensity of their light output is proportional to the

current passing through them. They also have a maximum current rating which may not be

exceeded, otherwise they can be damaged. To limit the amount of current through an LED a

current limiting resistor is typically inserted in series with it.

Value of the resistance is calculated as per Ohm’s law. Most LED datasheets will tell you the

forward voltage drop. Given the voltage of the source - Vs, the forward voltage drop - Vf, the

maximum LED current - Imax, it is easy to compute the resistor based on Ohms Law:

…E1.1

The maximum voltage given is 12V; the voltage rating is taken as 3V and maximum current

through the diode as 15mA. Thus, value of the resistor is 600Ω.

For convenience, we had chosen 1KΩ resistor for all diodes in every experiments.

A micro-ammeter is used to measure current through the diode since the maximum current

through the diode is in the order of mille-amperes.

Procedure:

The resistor is attached to the anode of the LED. Connect it in forward bias - the anode of the

LED to the positive terminal. A micro-ammeter is connected in series at the cathode wire so as

to read the current through the diode – cathode wire connected to the positive terminal of the

ammeter. Connect the negative terminal of the ammeter to the negative terminal of the voltage

source.

R = (Vs - Vf) / Imax

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Fig. E1.1: Circuit for V-I characteristics of LED

1. Note down the specifications of the meters and correction values.

2. Turn the voltage source to its minimum settings, and switch it on.

3. Adjust the power supply till the striking voltage is attained.

4. Note the input voltage (VI) and the diode current (IR). Measure the potential differences

across the resistor (VR) and the diode (VD).

5. Calculate the diode voltage from load and input voltage:

VD = VI - VR

Take mean of the two values.

6. Slowly increase the input voltage in convenient steps and repeat step 4.

7. The experiment is repeated till the maximum voltage is reached.

8. Plot forward voltage against the drive current with voltage and current along X and Y

axes respectively.

9. Change LED and repeat the whole experiment.

Precautions:

1. While doing the experiment do not exceed the ratings of the diode. This may lead to

damage of the diode.

2. Connect Ammeter in correct polarity as shown in the circuit diagram.

3. Do not switch ON the power supply unless you have checked the circuit connections as

per the circuit diagram.

4. Too much current may blow the fuse inside the ammeter.

Cleanup and Recycling:

There may be a chance for the diodes to burn out. The dead LEDs cannot be reused. Package the

burnt out LED bulbs safely, and ship them to LED Light Bulb Recycling. Because LED bulbs don't

contain mercury and are not considered a hazardous material, they may be safely recycled.

Everything else in the experiment can be reused.

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Experiment No. 2

RADIANT OUTPUT POWER AND DC DRIVE CURRENT

To plot the variation of radiant output power with forward current

How to measure radiant output power?

For radiant flux measurement, the radiant power is measured when a specified forward current

flows into the diode. Flux along the vertical direction is measured by covering the horizontal

part. The radiant power is then measured using an optical sensor.

Fig. E2.1: Measuring radiant power using an optical sensor


A variable DC supply (0 – 12V), a micro-ammeter, a mille-ammeter, multimeter, resistance and

LEDs of different colours and ratings, optical sensor


The circuit for the experiment No.: 1 is used for this experiment.

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Fig. E2.2: Circuit for lighting LED for flux measurement

Procedure:

1. Turn the voltage source to its minimum settings, and switch it on.

2. Adjust the power supply till the striking voltage is attained.

3. Note the input voltage (VI) and the diode current (IR). Measure the potential

differences across the resistor (VR) and the diode (VD).

4. Measure the radiant power using an optical sensor.

5. Slowly increase the diode current in convenient steps and repeat steps 3 and 4.

6. The experiment is repeated till the maximum voltage is reached.

7. Change LED and repeat the whole experiment.

8. Plot forward current against the radiant flux with current and flux along X and Y

axes respectively.

Precautions:

1. Prevent stray radiation by covering the contact region of LED and the optical sensor.

2. Note down the equilibrium value of power. Avoid fluctuating readings.

Cleanup and Recycling:

The dead LEDs cannot be reused. Give them to LED Light Bulb Recycling. Everything else in the

experiment can be reused.

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Experiment No. 3

WAVELENGTH OF THE LIGHT EMITTED BY LEDs

To determine the wavelength of the light emitted by LEDs


A variable DC supply (0 – 12V), multimeter, resistance and LEDs of different colours and ratings,

optic sensor device, grating, holder, meter stick


Fig. E3.1: Lighting the LED

A suitable resistance is connected to the anode as per Ohm’s law. For convenience, again 1KΩ is used.

Experimental Setup for Wavelength Determination Using Diffraction

Grating:

Fig. E3.2: Arrangement for diffraction

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The equipment consists of an LED, di raction grating with holder and a screen with meter stickff

attached. The whole experiment is done in a dark room to reduce noise in the experiment. The

theory is similar to Double slit case, except here, instead of just using two slits, the light beam

will pass through the multiple slits of the di raction grating. By measuring the angles at whichff

the interference peaks or maxima occur, we can determine the wavelength of the LED light by

knowing the spacing of the grating (grating constant). We can geometrically construct the case

of light di raction from a di raction grating; this gives the condition for nth maxima to beff ff

…E3.1

where θn = atan(x/Z), n is the order number (n = 0, 1, 2...), x is the distance of the nth maxima

from the central maxima (n = 0), N is the number of lines per mm in the grating, and Z is the

distance between the grating and the screen.

Procedure:

1. Place the LED, grating and the stick (horizontally) in a line. The grating must adjusted so

that the light passes through its centre and touches the reference point in the metre

stick.

2. Measure the distance between the central maximum and the centre of the grating, Z.

3. Measure the distance between the central and secondary maxima on both sides, x and x.

4. Calculate the mean distance, x.

5. Compute Θn = atan(x/Z).

6. The wavelength emitted, = sin λ Θn /(n N)

7. Find the wavelength by changing Z and determine the mean value.

8. Repeat the experiment for other LEDs.

Precautions:

1. To get a clear image pass light emitted from the LED through a small hole or pinhole.

2. Prevent stray radiation as much as possible.

3. The image may not be pointed, so determine the centre of image and calculate the

distance between image centers.

4. Use an opaque white coloured scale.

Nn = sinλ θn

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Fig E3.3 – Experimental setup - wavelength of the light emitted by an LED

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AC LINE-POWERED LED CIRCUITS

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Figure E4: Circuits for driving LED's directly from the AC line

Figure E4 shows two circuits that can be used to drive LED's directly from the AC line without a need for a transformer-based DC supply. Both circuits employ a series non-polarized capacitor to achieve a large voltage drop that causes the LED's to see a lower voltage across them. A 1-K series resistor is also used to limit the current through the circuit, especially during the periods when the capacitor is minimally charged and acts like a short circuit. For this circuit to work properly, the capacitor must conduct current in both AC directions to prevent it from becoming fully charged permanently and causing the circuit to become open. This is the purpose of the 1N914 diode in the first circuit - to provide a current path during the negative cycle. It also limits the reverse voltage across the LED. The LED lights up during each positive cycle. In the second circuit, the ordinary diode is replaced by another LED that lights up during the negative cycle to complement the one that lights up during the positive cycle.

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Colour of LED

Input Voltage Vi (Volt)

Load Voltage VL (Volt)

Diode Current ID (Ampere)

Diode Voltage VD (Volt)Theoretical

= Vi - VLPractical Mean

Transparent

Blue

2.75 0.015 16 x 10-6 2.74 2.72 2.7275

3.00 0.050 52 x 10-6 2.95 2.98 2.9650

6.91 2.970 2 x 10-3 3.94 3.91 3.9250

8.73 4.620 4 x 10-3 4.11 4.07 4.0900

11.38 7.080 6 x 10-3 4.30 4.24 4.2700

Green

1.78 0.025 24 x 10-6 1.755 1.709 1.73201.85 0.053 54 x 10-6 1.797 1.740 1.7685

1.94 0.089 92 x 10-6 1.851 1.764 1.8075

2.00 0.112 114 x 10-6 1.888 1.771 1.8295

4.68 2.700 2 x 10-3 1.980 1.965 1.9725

7.45 5.370 4 x 10-3 2.080 2.050 2.0650

9.25 7.120 6 x 10-3 2.130 2.100 2.1150

11.50 9.280 8 x 10-3 2.220 2.160 2.1900

Red

1.50 0.002 2 x 10-6 1.50 1.490 1.4940

1.60 0.010 10 x 10-6 1.59 1.592 1.5910

1.70 0.027 26 x 10-6 1.67 1.631 1.6520

1.84 0.078 68 x 10-6 1.76 1.679 1.7205

1.93 0.114 120 x 10-6 1.82 1.698 1.7570

4.10 2.250 2 x 10-3 1.85 1.820 1.8350

4.50 2.650 2 x 10-3 1.85 1.830 1.8400

5.12 3.260 3 x 10-3 1.86 1.840 1.8500

6.50 4.630 4 x 10-3 1.87 1.860 1.8650

7.01 5.120 4 x 10-3 1.89 1.860 1.8750

8.00 6.100 5 x 10-3 1.90 1.870 1.8850

9.00 7.080 6 x 10-3 1.92 1.880 1.9000

10.00 8.070 7 x 10-3 1.93 1.890 1.9100

11.00 9.040 8 x 10-3 1.96 1.890 1.9250

Yellow

1.89 0.098 102 x 10-6 1.792 1.680 1.73604.37 2.520 2 x 10-3 1.850 1.830 1.8400

7.14 5.240 4 x 10-3 1.900 1.875 1.8875

8.99 7.050 6 x 10-3 1.940 1.896 1.9180

10.95 9.020 8 x 10-3 1.930 1.913 1.9215

White 2.50 0.085 92 x 10-6 2.415 2.48 2.4475

P a g e | 31

2.75 0.110 128 x 10-6 2.640 2.50 2.5700

5.80 2.990 2 x 10-3 2.810 2.79 2.8000

8.00 5.100 4 x 10-3 2.900 2.88 2.8900

10.00 7.030 6 x 10-3 2.970 2.94 2.9550

11.93 8.880 8 x 10-3 3.050 3.00 3.0250

Opaque

Green

1.80 0.043 40 x 10-6 1.757 1.697 1.7270

1.90 0.085 88 x 10-6 1.815 1.735 1.7750

2.00 0.128 130 x 10-6 1.872 1.755 1.8135

4.90 2.940 2 x 10-3 1.960 1.949 1.9545

6.97 4.940 4 x 10-3 2.030 2.020 2.0250

9.30 7.210 6 x 10-3 2.090 2.050 2.0700

11.53 9.420 8 x 10-3 2.110 2.100 2.1050

Red

1.75 0.029 31 x 10-6 1.721 1.694 1.7075

2.00 0.122 126 x 10-6 1.878 1.754 1.8160

2.03 0.134 136 x 10-6 1.896 1.756 1.8260

2.06 0.145 150 x 10-6 1.915 1.764 1.8395

4.70 2.750 2 x 10-3 1.950 1.910 1.9300

5.08 3.140 2 x 10-3 1.940 1.927 1.9335

6.78 4.780 4 x 10-3 2.000 1.972 1.9860

8.02 5.970 5 x 10-3 2.050 2.000 2.0250

9.00 6.940 6 x 10-3 2.060 2.020 2.0400

10.10 8.080 7 x 10-3 2.020 2.040 2.0300

11.08 8.960 8 x 10-3 2.120 2.060 2.0900

Yellow

1.72 0.035 36 x 10-6 1.685 1.630 1.6575

4.75 2.900 2 x 10-3 1.850 1.828 1.8390

7.21 5.310 4 x 10-3 1.900 1.873 1.8865

9.36 7.450 6 x 10-3 1.910 1.905 1.9075

11.40 9.420 8 x 10-3 1.980 1.929 1.9545

Table 4.1: Forward characteristics

P a g e | 32

FORWARD CHARACTERISTICS OF TRANSPARENT LEDs

Fig 4.1

Fig 4.2

P a g e | 33

Fig 4.3

Fig 4.4

P a g e | 34

Fig 4.5

P a g e | 35

FORWARD CHARACTERISTICS OF OPAQUE LEDs

Fig 4.6

P a g e | 36

Fig 4.7

P a g e | 37

Fig 4.8

P a g e | 38

The forward characteristics of LEDs shows that the curve varies for different LEDs depending

on the nature of the diode – dimensions, materials used etc. But all the curves are increasing

linearly with the diode voltage, if the errors are neglected.

The linearity of forward characteristic accounts for the enhanced electron flow at the junction.

When electrons do not have sufficient energy to overcome the barrier potential, no current will

flow through the diode. But when external voltage is applied in forward bias, electrons gain

kinetic energy under the external electric field:

K.E = eV .

If this energy is sufficient for overcoming the opposing force by the barrier, the electron will

flow to p – region. But as electrons flow across the junction and recombine with the holes,

depletion region widens and current flow is limited. To increase the current flow then external

potential has to be increased.

THRESHOLD VOLTAGE

The forward potential difference across the diode at which significant emission of photons

initiates is called the striking voltage. From experiments, striking voltage for different LEDs is:

Colour of LED Striking Voltage - voltsBlue about 2.7

Green about 1.7Red about 1.4

White about 2.4Yellow about 1.7

Table 4.2: Colour and striking voltage

Striking voltage also indicates the value of forward bias required to give significant forward

current. DC drive current is in micro-amperes nearer to striking voltage. Then it increases

rapidly in the mille-ampere region for a small change in diode voltage. LEDs are normally used

in this region below the maximum current rated for the diode.

The diode turn-on is distributed over a range of voltages rather than occurring abruptly at the

threshold voltage. The non-abrupt turn-on is referred to as sub-threshold turn-on or

premature turn-on. The sub-threshold current can be caused by carrier transport through

surface states or deep levels in the bulk of the semiconductor.

P a g e | 39

Fig 4.9: I-V with clearly discernable sub-threshold turn-on, caused by defects or surface states

PARASITIC RESISTANCE

Frequently a diode has unwanted or parasitic resistances. The effect of a series resistance

and a parallel resistance is shown in Fig. 4.10. A series resistance can be caused by excessive

contact resistance or by the resistance of the neutral regions. A parallel resistance can be

caused by any channel that bypasses the p-n junction. This bypass can be caused by damaged

regions of the p-n junction or by surface imperfections.

Fig 4.10: Effect of series and parallel resistance

DIFFERENTIAL RESISTANCE

For the transparent blue LED used in the study, at approximately 2.7V diode current is 16µA.

For an increase of 0.2375V then, current increased by 36µA. Due to the unavailability of higher

range micro-ammeters, the next value had been taken in mille-amperes. A current of 2mA

needed approx. 3.9V. i.e., an increase of 1.948mA needed an increase of 0.96V. Thus, resistance

can be calculated as:

R = ∆V/∆I

P a g e | 40

Change in voltage – volts Change in current – ampere Differential resistance - ohm

0.2375 0.000036 6597.2

0.9600 0.001948 492.8

0.1650 0.002000 82.5

0.1800 0.002000 90

Table 4.3: Differential resistance of blue transparent LED

For the green transparent LED striking voltage is around 1.7V and current 24µA. It shows that

the energy needed for the electrons to recombine is much lesser than in the case of a blue LED

with higher frequency. Similarly the differential resistances are:


0.0365 0.000030 1216.6

0.0390 0.000038 1026.3

0.0220 0.000022 1000

0.1430 0.001886 75.8

0.0925 0.002000 46.2

0.0500 0.002000 25

0.0750 0.002000 37.5

Table 4.4: Differential resistance for green transparent LED

For red transparent LED, striking voltage is about 1.4V and current is 2µA.


0.0970 0.000008 12125

0.0610 0.000016 3812.5

0.0685 0.000042 1630.9

0.0365 0.000052 701.9

0.0780 0.001880 41.4

0.0150 0.001000 15

0.0150 0.001000 15

Table 4.5: Differential resistance for red transparent LED

The last two rows are intentionally left same to show that for a same amount of change in the

voltage, the change in current remains constant. i.e., an increase from 1.835V to 1.85V and 1.85V

P a g e | 41

to 1.865V resulted in raising current by 0.001A. This enables to determine the forward current

at a certain value of diode voltage, if a voltage-current value at another point is known. It can be

inferred that increase in the number of electron flow or excited electrons per unit voltage

increase is a constant. From this external potential needed for exciting an electron in the diode

can be calculated.

It can also be seen from the tables of differential resistance that in the operating region, the

resistance decreased with increase in voltage. It even decreased from thousands of ohms to a

few ohms.

Evaluation of parasitic resistances from differential resistance

The diode parallel resistance can be evaluated near the origin of the I–V diagram where

V<<εg /e. For this voltage range, the p-n junction current can be neglected and the parallel

resistance is given by

Rp = dV/dI |near origin

LEDVoltage difference near origin

(Volt)

Current difference near origin (Ampere)

Parallel resistance

(Ohm)

V1 V2 dV I1 I2 dI Rp

Transparent

Blue 2.7275 2.9650 0.2375 0.000016 0.000052 0.000036 6597.22

Green 1.7320 1.7685 0.0365 0.000024 0.000054 0.000030 1216.67

Red 1.4940 1.5910 0.0970 0.000002 0.000010 0.000008 12125.00

Yellow 1.7360 1.8400 0.1040 0.000102 0.002000 0.001898 54.79

White 2.4475 2.5700 0.1225 0.000092 0.000128 0.000036 3402.78

Opaque

Green 1.7270 1.7750 0.0480 0.000040 0.000088 0.000048 1000.00

Red 1.7075 1.8160 0.1085 0.000031 0.000126 0.000095 1142.11

Yellow 1.6575 1.8390 0.1815 0.000036 0.002000 0.001964 92.41

Table 4.6: Parallel parasitic resistance

The series resistance can be evaluated at a high voltage where V > Eg /e. For sufficiently large

voltages, the diode I–V characteristic becomes linear and the series resistance is given by

Rs = dV/dI |at voltages exceeding turn-on

P a g e | 42

However, it may not be practical to evaluate the diode resistance at high voltages due to

device heating effects. But from tables of differential resistances, at moderately high voltages

exceeding turn-on, series resistances can be calculated approximately. It is obvious that series

resistance is much less than parallel resistance.

LED Voltage difference near origin (Volt)

Current difference near origin (Ampere)

Series resistance

(Ohm)

V1 V2 dV I1 I2 dI Rp

Transparent

Blue 4.090 4.2700 0.1800 0.004 0.006 0.002 90.00

Green 2.115 2.1900 0.0750 0.006 0.008 0.002 37.50

Red 1.910 1.9250 0.0150 0.007 0.008 0.001 15.00

Yellow 1.918 1.9215 0.0035 0.006 0.008 0.002 1.75

White 2.955 3.0250 0.0700 0.006 0.008 0.002 35.00

Opaque

Green 2.0700 2.1050 0.035 0.006 0.008 0.002 17.50

Red 2.0300 2.0900 0.060 0.007 0.008 0.001 60.00

Yellow 1.9075 1.9545 0.047 0.006 0.008 0.002 23.50

Table 4.6: Series parasitic resistance

The effect of series resistance is obvious in the characteristics; in order to attain a certain value

of current it needed more voltage than the ideal case.

P a g e | 43

Fig 4.11

Fig 4.12

P a g e | 44

Fig 4.13

From the comparison of characteristics of transparent and opaque LEDs, the curves do not vary

greatly. For yellow LEDs, the curves are approximately parallel. The variations can be

accounted for the usage of different LEDs, but not for the transparency of the shield. It can be

said that if the transparent shield of a diode is replaced by an opaque one of any colour, without

disturbing the solid, the forward characteristic curve remains invariant. It is due to the fact that

for a diode, the current through it depends on the material used and the dimensions.

P a g e | 45

Colour of LED

Input Voltage Vi (Volt)

Load Voltage VL

(Volt)

Diode Current ID (Ampere)

Diode Voltage VD (Volt)Output

Power (dB)Theoretical = Vi - VL

Practical Mean

Transparent

Blue

2.80 0.022 22 x 10-6 2.778 2.77 2.774 -39.3

2.98 0.036 40 x 10-6 2.944 2.89 2.917 -32.6

3.21 0.072 80 x 10-6 3.138 3.03 3.084 -26.0

3.37 0.104 120 x 10-6 3.266 3.10 3.183 -24.0

6.41 2.550 2 x 10-3 3.860 3.86 3.860 -9.3

8.76 4.720 4 x 10-3 4.040 4.08 4.060 -8.6

11.05 6.860 6 x 10-3 4.190 4.26 4.225 -7.5

13.06 8.740 8 x 10-3 4.320 4.36 4.340 -5.8

Green

1.63 0.004 3 x 10-6 1.626 1.612 1.619 -47.6

1.80 0.035 40 x 10-6 1.765 1.710 1.738 -45.7

1.90 0.074 80 x 10-6 1.826 1.788 1.807 -43.8

2.00 0.114 120 x 10-6 1.886 1.757 1.822 -42.3

2.05 0.137 140 x 10-6 1.913 1.766 1.840 -41.2

4.43 2.450 2 x 10-3 1.980 1.942 1.961 -27.1

7.09 5.020 4 x 10-3 2.070 2.030 2.050 -24.1

8.96 6.830 6 x 10-3 2.130 2.090 2.110 -23.1

11.01 8.830 8 x 10-3 2.180 2.150 2.165 -21.9

12.80 10.510 10 x 10-3 2.290 2.180 2.235 -21.4

Red

1.70 0.009 10 x 10-6 1.691 1.670 1.681 -42.8

1.73 0.045 40 x 10-6 1.685 1.708 1.697 -32.6

1.83 0.076 80 x 10-6 1.754 1.748 1.751 -29.9

1.93 0.116 120 x 10-6 1.814 1.750 1.782 -25.8

4.08 2.200 2 x 10-3 1.880 1.860 1.870 -16.5

6.49 4.820 4 x 10-3 1.670 1.920 1.795 -11.4

8.91 7.320 6 x 10-3 1.590 1.970 1.780 -10.7

10.60 8.980 8 x 10-3 1.620 1.980 1.800 -9.9

12.46 10.840 10 x 10-3 1.620 1.950 1.785 -9.1

Yellow1.60 0.009 8 x 10-6 1.591 1.573 1.582 -44.9

1.70 0.033 40 x 10-6 1.632 1.632 1.632 -44.4

1.80 0.068 68 x 10-6 1.767 1.654 1.711 -43.7

1.83 0.074 80 x 10-6 1.756 1.663 1.710 -43.5

1.92 0.114 120 x 10-6 1.806 1.690 1.748 -40.9

P a g e | 46

4.19 2.33 2 x 10-3 1.860 1.815 1.838 -29.6

6.76 4.85 4 x 10-3 1.910 1.860 1.885 -26.4

8.47 6.56 6 x 10-3 1.910 1.870 1.890 -25.4

10.47 8.49 8 x 10-3 1.980 1.890 1.935 -24.4

12.49 10.44 10 x 10-3 2.050 1.910 1.980 -23.2

White

2.44 0.002 1 x 10-6 2.438 2.4 2.419 -35.0

2.66 0.033 36 x 10-6 2.627 2.56 2.594 -21.8

2.84 0.090 92 x 10-6 2.750 2.62 2.685 -17.6

2.90 0.116 120 x 10-6 2.784 2.65 2.717 -17.1

2.93 0.126 134 x 10-6 2.804 2.64 2.722 -16.8

2.96 0.139 142 x 10-6 2.821 2.66 2.741 -15.9

5.58 2.630 2 x 10-3 2.950 2.92 2.935 -13.3

8.04 4.990 4 x 10-3 3.050 3.01 3.030 -9.5

10.18 7.060 6 x 10-3 3.120 3.06 3.090 -8.3

12.15 8.980 8 x 10-3 3.170 3.11 3.140 -7.2Opaque

Green

1.64 0.006 8 x 10-6 1.634 1.618 1.626 -39.5

1.78 0.041 40 x 10-6 1.739 1.699 1.719 -39.0

1.89 0.076 80 x 10-6 1.814 1.730 1.772 -38.4

2.00 0.119 120 x 10-6 1.881 1.754 1.818 -37.4

4.86 2.890 2 x 10-3 1.970 1.945 1.958 -23.1

6.88 4.850 4 x 10-3 2.030 2.000 2.015 -21.0

8.80 6.710 6 x 10-3 2.090 2.040 2.065 -19.9

10.82 8.650 8 x 10-3 2.170 2.080 2.125 -18.7

12.50 10.270 10 x 10-3 2.230 2.120 2.175 -18.1

Yellow

1.65 0.014 20 x 10-6 1.636 1.592 1.614 -38.6

1.72 0.035 40 x 10-6 1.685 1.626 1.656 -37.5

1.82 0.078 80 x 10-6 1.742 1.651 1.697 -36.6

1.92 0.120 120 x 10-6 1.800 1.665 1.733 -35.1

4.58 2.720 1 x 10-3 1.860 1.820 1.840 -23.0

6.96 5.050 4 x 10-3 1.910 1.850 1.880 -20.2

8.72 6.780 6 x 10-3 1.940 1.880 1.910 -19.2

10.72 8.790 8 x 10-3 1.930 1.900 1.915 -18.0

12.64 10.590 10 x 10-3 2.050 1.940 1.995 -17.2

Table 4.8: Radiation output power vs. drive current

P a g e | 47

Fig 4.14

Fig 4.15

P a g e | 48

Fig 4.16

P a g e | 49

Radiation from an LED is due to recombination of electrons and holes. The nature of the

radiation, to which part of the EM spectrum does it belong, is determined by the energy gap of

the material. In fact, energy gap of a material denotes the energy needed for a valence electron

to reach conduction band or to create free electrons. An electron absorbing energy hv to become

free will emit the same radiation during recombination with a hole. Thus radiation from a diode

varies from material to material. If the energy gap is in the region of visible spectrum, the LED

will emit light radiations.

Striking voltage is the external potential at which significant electron flow initiates. It also

indicates the sufficient radiation needed to measure the amount of radiation. So, at striking

voltage LED lights up.

From the evaluation forward characteristics it has been seen that current flow through the

diode varies linearly with the diode voltage. The amount of recombination depends on the

number of electron reaching the p region crossing the barrier and hence intensity of radiation

or radiation flux depends on the number of electrons recombining with holes. This number is

influenced by the external voltage. In fact, the number of electrons crossing the junction per unit

time is the current through the diode. Therefore, when current through the diode increases,

radiation flux also increases. Radiation output power vs. diode current shows this relationship.

In all the characteristic curves, as diode current increased radiation output power also

increased. Of all the diodes, super blue diode – silicon carbide/gallium nitride – showed the

greatest power at a given value of current after the striking voltage.

Fig 4.17: InGaN-based LEDs extend the range of LEDs into blue and achieve efficiencies

exceeding tungsten light bulbs.

P a g e | 50

Brightness and wattage are actually two different things. Lumens are the units used to measure

light intensity or brightness, while wattage is a measure of how much power the light source

uses. Since a light-emitting diode (LED) bulb uses as much as 90 percent less electricity than a

regular incandescent bulb, the LED requires only about 9 watts to produce a comparable

amount of brightness to a 75 watt incandescent.

White LED has highest output power then to super blue LED. This can be accounted as white

LED is actually a mixture of blue and yellow radiations, the opposite extreme ends of the

spectrum and can befool eyes by appearing as white.

A current flow through the diode is sum of current by electron and hole flows. There are cases in

which radiative and non-radiative transition occur. Radiation flux is thus dependent on the

recombination yielding radiative transitions. There exists several recombination processes

dealing with the important radiative transitions between the valence and the conducting bands.

The recombination of an intrinsic free electron or a hole and free excitons is assisted indirectly

by a phonon interaction in the gap materials. In donor impurities, there exist free to bound or

bound excitons recombination for neutral acceptors, and donor acceptor pair recombination.

The non-radiative transitions are mainly involved with auger effect, and also for deep impurities

multi-phonon emission is possible.

From the characteristics, Blue LED gives maximum radiation output power. In other words, it is

the most efficient LED among them. Efficiency of an LED is the rate of photon emission over the

rate of supply of electrons. It is because GaN have the reduced electron leakage and lower

efficiency droop – a phenomenon that provokes LEDs to be their most efficient when they

receive low-density electric currents, but to lose their efficiency as higher density currents are

fed into the device.

The better efficiency of red LED is accounted by its smaller diode ideality factor. The ideality

factor of a diode is a measure of how closely the diode follows the ideal diode equation. The

ideal diode equation assumes that all the recombination occurs via band to band or

recombination via traps in the bulk areas from the device (i.e. not in the junction). However

recombination does occur in other ways and in other areas of the device. These recombinations

produce ideality factors that deviate from the ideal.

The green and yellow LEDs have a greater ideality factor from the I-V characteristics, and also

they encounter relatively high electron leakage and efficiency droop.

P a g e | 51

Apart from these factors, sub-threshold turn on also affects the efficiency. Efficiency is the

greatest for no sub-threshold turn on diodes. Efficiency depends upon wavelength shifts too.

Smaller the shifts, greater is the efficiency.

P a g e | 52

Colour of LEDInput Voltage

Vi (Volt)Output

Power (dB)Wavelength of the

light (nm)Transparent

Blue 12.50 -7.60 540Green 8.79 -11.90 630Yellow 11.00 -7.80 687

Yellow - OrangeYellow

8.76 -7.14663

Orange 728Red 12.47 -9.60 713

White

Blue region 10.91 1.80 307Green region 436

Yellow region

720

Red region 793Opaque

Yellow 10.9 -17.9 693Red 10.1 -8.9 745

Table 4.9: Wavelength emitted by LED

The wavelengths determined through are found to be in the range of visible spectrum. The

diode yellow – orange, on diffraction provided two patterns of yellow 1 and orange.

During experiment, at low radiation power, pattern was not clear and at extremely high power,

pattern showed widened image, overlapped maxima, making it impossible to resolve them. This

increased difficulties to determine the spectral responses of LEDs.

A slight variation can be noted in the case of green transparent LED. This is because the light is

in the region of green and yellow overlapping.

P a g e | 53

LUMINAIRE EFFICACY

The use of light-emitting diodes (LEDs) as a general light source has forced changes in test

procedures used to measure lighting performance. Lighting energy efficiency is a function of

both the light source (the light “bulb” or lamp) and the fixture, including necessary controls,

power supplies and other electronics, and optical elements. The complete unit is known as a

luminaire.

Traditionally, lighting energy efficiency is characterized in terms of lamp ratings and fixture

efficiency. The lamp rating indicates how much light (in lumens) the lamp will produce when

operated at standard room/ambient temperature (25 degrees C). The luminous efficacy of a

light source is typically given as the rated lamp lumens divided by the nominal wattage of the

lamp, abbreviated lm/W. The fixture efficiency indicates the proportion of rated lamp lumens

actually emitted by the fixture; it is given as a percentage. Fixture efficiency is an appropriate

measure for fixtures that have interchangeable lamps for which reliable lamp lumen ratings are

available.

However, the lamp rating and fixture efficiency measures have limited usefulness for LED

lighting at the present time, for two important reasons:

1) There is no industry standard test procedure for rating the performance of LED devices

or packages.

2) The luminaire design and the manner in which the LEDs are integrated into the

luminaire have a material impact on the performance of the LEDs.

Given these limitations, how can LED luminaires be compared to traditional lighting

technologies? As an example, the table below compares two recessed down-light fixtures, one

using a 13-watt CFL and the other using an array of LEDs. The table differentiates data related

to the light source and data resulting from actual luminaire measurements. Luminaire

photometry shows that in this case the LED fixture has input wattage and light output similar to

the CFL fixture, and matches the CFL product’s luminaire efficacy. This example is based on a

currently available, residential-grade, six-inch diameter down-light. LED down-light

performance continues to improve rapidly, with some LED retrofit products surpassing CFL

down-lights in luminaire efficacy.

P a g e | 54

Table 4.10

For all light sources, there is a difference between rated luminous flux of the lamp and actual

performance in a luminaire. However, traditional light sources installed in luminaires operate

relatively predictably because the performance of traditional light sources in a wide range of

luminaire types, applications, and use conditions is well documented and understood. LED

technology is at a far earlier stage of development, so experience and documentation of

performance within luminaires is lacking. The efficiency of LEDs is very sensitive to heat and

optical design, which increases the relative importance of luminaire design.

Ensuring necessary light output and life of LEDs requires careful thermal management, typically

requiring the use of the fixture housing as a heat sink or at least as an element in the heat

removal design. Luminaires therefore have a fundamental and typically large effect on the

luminous flux produced by the LEDs, and on the rate of lumen depreciation over time. LED

“drop-in” replacement lamps, such as Edison-based reflector lamps or MR-16 replacements, are

in theory designed to provide the necessary heat sinking for the LEDs, but given their

installation in fixtures not specifically designed for LEDs, good heat management will be a

challenge.

In summary, luminous flux—and by extension, luminous efficacy—must be measured at the

luminaire level for two primary reasons: 1) no standard procedures are available for rating LED

devices on their own, and; 2) the amount of light emitted by a fixture cannot be predicted

reliably based on available information about LED devices and fixtures. he lighting industry has

adopted luminaire efficacy as the preferred measure of LED performance, as evident in the

development of a new test procedure based on this approach.

P a g e | 55

SECTION FIVE

P a g e | 56

CONCLUSION

Light-emitting diodes are forward biased p-n junctions which emit spontaneous

radiations. The phenomenon behind them is the process of electroluminescence. Of

course, the material used to manufacture the light-emitting diode governs the type

of radiation emitted. Besides, the structure determines the characteristics of these

diodes. Surface imperfections can cause parasitic resistances that deviates the

current-voltage characteristic of a diode from its ideal case.

The radiation output power of an LED is affected by electron leakage, the

efficiency droop, diode ideality factor, wavelength shift, and sub-threshold turn on.

For blue LEDS with smaller efficiency droops and electron leakage, have the better

efficiency.

The performance of the LED is the light produced per diode and it has been shown

that it depends on many factors ranging from the material and size to temperature.

Knowing extends of influence of these factors - disrupting LEDs from the ideal

case - the performance of LEDs can be improved.

While many generally use luminaire efficacy as a measure of performance, it

should be seen as a standalone metric, but should be assessed in the light of other

parameters like temperature, etc, relevant to the application. In general, high

performance LEDs offer a greater light output in a smaller footprint, and high

luminous intensity for low currents.

led - device performance characteristics

Documents

variable dc

prevent stray

power supply

radiant output

narrow emission

forward voltage

direct band

device performance