light emitting diodesx

1

LIGHT EMITTING DIODES

S.Krishnaveni,AP/EEE

2

A light emitting diode (LED) is essentially a PN junction opto-semiconductor that emits a monochromatic (single color) light when operated in a forward biased direction.

LEDs convert electrical energy into light energy. They are frequently used as "pilot" lights in electronic appliances to indicate whether the circuit is closed or not.


WORKING

� Under forward bias – majority carriers from both sides of the junction can cross the depletion region and entering the material at the other side.

� Upon entering, the majority carriers become minority carriers

� The minority carriers will be larger � minority carrier injection

� Minority carriers will diffuse and recombine with the majority carrier.

� The recombination causes light to be emitted. Such process is termed radiative recombination

S.Krishnaveni,AP/EEE 3

�The number of radiative recombination is proportional to

the carrier injection rate.

�Carrier injection rate is related to the current flowing in the

junction .

� If the transition take place between states (conduction and

valance bands) the emission wavelength, λg = hc/(EC-EV)

�EC-EV = Eg AND λg = hc/Eg

WORKING


5

About LEDs

The most important part of a light emitting diode (LED) is the semi-conductor chip located in the center of the bulb as shown at the right. The chip has two regions separated by a junction. The p region is dominated by positive electric charges, and the n region is dominated by negative electric charges. The junction acts as a barrier to the flow of electrons between the p and the n regions. Only when sufficient voltage is applied to the semi-conductor chip, can the current flow, and the electrons cross the junction into the p region.


CB

VB

When the electron falls down from conduction band and fills in a hole in valence band, there is an obvious loss of energy.

The question is;

where does that energy go?


In order to achieve a reasonable efficiency for photon emission, the semiconductor must have a direct band gap.

CB

VB


For example;

Silicon is known as an indirect band-gap

material.

CB

VB

E

k


• As we all know, whenever something changes

state, one must conserve not only energy, but also momentum.

• In the case of an electron going from conduction band to the valence band in silicon, both of these things can only be conserved:

The transition also creates a quantized

set of lattice vibrations,

called phonons, or "heat“ .


• Phonons possess both energy and momentum.

• Their creation upon the recombination of an electron

and hole allows for complete conservation of both

energy and momentum.

• All of the energy which the electron gives up in going

from the conduction band to the valence band (1.1

eV) ends up in phonons, which is another way of

saying that the electron heats up the crystal.


In a class of materials called direct band-gap

semiconductors;

»the transition from conduction band to

valence band involves essentially no

change in momentum.

»Photons, it turns out, possess a fair

amount of energy ( several eV/photon in

some cases ) but they have very little

momentum associated with them.


• Thus, for a direct band gap material, the excess

energy of the electron-hole recombination can either

be taken away as heat, or more likely, as a photon of

light.

• This radiative transition then

conserves energy and momentum

by giving off light whenever an

electron and hole recombine. CB

VB

This gives rise to

(for us) a new type

of device;

the light emitting diode (LED).


Producing photon

Electrons recombine with holes.

Energy of photon is the energy of

band gap.

CB

VB

e-

h


Method of injection • We need putting a lot of e-’s where there are lots of

holes.

• So electron-hole recombination can occur.

• Forward biasing a p-n junction will inject lots of e-’s

from n-side, across the depletion region into the p-side

where they will be combine with the high density of

majority carriers.

n-side

p-side

-

+

I


Notice that: • Photon emission occurs whenever we have injected

minority carriers recombining with the majority

carriers.

• If the e- diffusion length is greater than the hole

diffusion length, the photon emitting region will be

bigger on the p-side of the junction than that of the

n-side.

• Constructing a real LED may be best to consider a

n++p structure.

• It is usual to find the photon emitting volume occurs

mostly on one side of the junction region.

• This applies to LASER devices as well as LEDs. S.Krishnaveni,AP/EEE 15

MATERIALS FOR LEDS • The semiconductor bandgap energy

defines the energy of the emitted

photons in a LED.

• To fabricate LEDs that can emit

photons from the infrared to the

ultraviolet parts of the e.m.

spectrum, then we must consider

several different material systems.

• No single system can span this

energy band at present, although the

3-5 nitrides come close.

CB

VB


• Unfortunately, many of potentiallly useful 2-6 group

of direct band-gap semiconductors (ZnSe,ZnTe,etc.)

come naturally doped either p-type, or n-type, but

they don’t like to be type-converted by overdoping.

• The material reasons behind this are complicated

and not entirely well-known.

• The same problem is encountered in the 3-5 nitrides

and their alloys InN, GaN, AlN, InGaN, AlGaN, and

InAlGaN. The amazing thing about 3-5 nitride alloy

systems is that appear to be direct gap throughout.


Properties of InGaN

• InGaN alloy has one composition at a time only. • This material will emit one wavelength only

corresponding to this particular composition. • An InGaN LED would not emit white light (the whole of

the visible spectrum at once) since its specific composition.

• For a white light source we have to form a complicated multilayer device emitting lots of different wavelengths.


Properties of InGaN

• A LED fabricated in a graded material where

on either side of the junction region the

material changes slowly from InN to GaN via

InGaN alloys.

• Minority carriers need to get through the

whole of this alloy region if efficient photon

production at all visible wavelengths was to

occur.


GaN InN

Concentration:

The highly gallium rich

alloy

The highly indium rich

alloy

Band gap: 3.3eV 2 eV

Wavelength of photons:

376 nm 620 nm

Part of the electromagnetic spectrum:

In the ultraviolet

In the visible

(orange)


Materials for visible wavelength LEDs

• We see them almost everyday, either on calculator displays or

indicator panels.

• Red LED use as “ power on” indicator

• Yellow, green and amber LEDs are also widely available but very

few of you will have seen a blue LED.


Red LEDs

• can be made in the GaAsP

(gallium arsenide phosphide).

• GaAs1-xPx

• for 0<x<0.45 has direct-gap

• for x>0.45 the gap goes indirect

and

• for x=0.45 the band gap energy

is 1.98 eV.

• Hence it is useful for red LEDs.

N-GaAs substrate

N-GaAsP P = 40 %

p-GaAsP region

Ohmic Contacts

Dielectric (oxide or nitride)

Fig. GaAsP RED LED on a GaAs sub.


• Orange (620 nm) and yellow (590 nm) LEDs are commercially made using the GaAsP system. However, as we have just seen above, the required band-gap energy for emission at these wavelengths means the GaAsP system will have an indirect gap.

• The isoelectronic centre used in this instance is nitrogen, and the different wavelengths are achieved in these diodes by altering the phosphorus concentration.

• The green LEDs (560 nm) are manufactured using the GaP system with nitrogen as the isoelectronic centre.

Orange-Yellow & Green LEDs


Blue LEDs • Blue LEDs are commercially available and are fabricated using

silicon carbide (SiC). Devices are also made based on gallium

nitride (GaN).

• Unfortunately both of these materials systems have major

drawbacks which render these devices inefficient.

• The reason silicon carbide has a low efficiency as an LED

material is that it has an indirect gap, and no ‘magic’

isoelectronic centre has been found to date.


Blue LEDs

• The transitions that give rise to blue photon emission in SiC

are between the bands and doping centres in the SiC. The

dopants used in manufacturing SiC LEDs are nitrogen for n-

type doping, and aluminium for p-type doping.

• The extreme hardness of SiC also requires extremely high

processing temperatures.


Gallium Nitride (GaN)

• Gallium nitride has the advantage of being a direct-gap

semiconductor, but has the major disadvantage that bulk

material cannot be made p-type.

• GaN as grown, is naturally n++ .

• Light emitting structures are made by producing an intrinsic

GaN layer using heavy zinc doping. Light emission occurs

when electrons are injected from an n+ GaN layer into the

intrinsic Zn-doped region.


• A possible device structure is shown

in fig.

• Unfortunately, the recombination

process that leads to photon

production involves the Zn impurity

centres, and photon emission

processes involving impurity centres

are much less efficient than band-to-

band processes.

Sapphire

Substrate

(transparent)

n + GaN

i-GaN

Ohmic Contacts Dielectric (oxide or nitride)

Fig. Blue LED

Blue photons


• It is generally true to say that if we order the photon

producing processes (in semiconductors) in terms of

efficiency, we would get a list like the one below.

– band-to-band recombination in direct gap material,

– recombination via isoelectronic centres,

– recombination via impurity (not isoelectronic) centres,

– band-to-band recombination in indirect-gap materials.

• So, the current situation is that we do have low-efficiency blue

LEDs commercially available. We are now awaiting a new

materials system, or a breakthrough in GaN or SiC technology,

for blue LEDs of higher brightness and higher efficiency to be

produced.


Color Name Wavelength (Nanometers)

Semiconductor Composition

Infrared 880 GaAlAs/GaAs

Ultra Red 660 GaAlAs/GaAlAs

Super Red 633 AlGaInP

Super Orange 612 AlGaInP

Orange 605 GaAsP/GaP

Yellow 585 GaAsP/GaP

Incandescent White

4500K (CT) InGaN/SiC

Pale White 6500K (CT) InGaN/SiC

Cool White 8000K (CT) InGaN/SiC

Pure Green 555 GaP/GaP

Super Blue 470 GaN/SiC

Blue Violet 430 GaN/SiC

Ultraviolet 395 InGaN/SiC S.Krishnaveni,AP/EEE 29

Material Wavelength

(µm) Material

Wavelength (µm)

ZnS ZnO Gan ZnSe CdS ZnTe GaSe CdSe CdTe

0.33 0.37 0.40 0.46 0.49 0.53 0.59 0.675 0.785

GaAs InP

GaSb InAs Te

PbS InSb PbTe PbSe

0.84-0.95 0.91 1.55 3.1

3.72 4.3 5.2 6.5 8.5


31

Testing LEDs

Never connect an LED directly to a battery or power supply! It will be destroyed almost instantly because too much current will pass through and burn it out.

LEDs must have a resistor in series to limit the current to a safe value, for quick testing purposes a 1k resistor is suitable for most LEDs if your supply voltage is 12V or less.

Remember to connect the LED the correct way round!


32

How Much Energy Does an LED Emit?

The energy (E) of the light emitted by an LED is related to the electric charge (q) of an electron and the voltage (V) required to light the LED by the expression: E = qV Joules.

This expression simply says that the voltage is proportional to the electric energy, and is a general statement which applies to any circuit, as well as to LED's. The constant q is the electric charge of a single electron, -1.6 x 10-19 Coulomb.


33

Finding the Energy from the Voltage

Suppose you measured the voltage across the leads of an LED, and you wished to find the corresponding energy required to light the LED. Let us say that you have a red LED, and the voltage measured between the leads of is 1.71 Volts. So the Energy required to light the LED is

E = qV or E = -1.6 x 10-19 (1.71) Joule,

since a Coulomb-Volt is a Joule. Multiplication of these numbers then gives

E = 2.74 x 10-19 Joule.


34

Applications

• Sensor Applications

• Mobile Applications

• Sign Applications

• Automative Uses

• LED Signals

• Illuminations

• Indicators


35

Sensor Applications

• Medical Instrumentation • Bar Code Readers • Color & Money Sensors • Encoders • Optical Switches • Fiber Optic Communication


36

Mobile Applications

• Mobile Phone • PDA's • Digital Cameras • Lap Tops • General Backlighting


37

Sign Applications

• Full Color Video • Monochrome Message Boards • Traffic/VMS • Transportation - Passenger Information


38

Automative Applications

• Interior Lighting - Instrument Panels & Switches, Courtesy Lighting • Exterior Lighting - CHMSL, Rear Stop/Turn/Tail • Truck/Bus Lighting - Retrofits, New Turn/Tail/Marker Lights


39

Signal Appications

• Traffic • Rail • Aviation • Tower Lights • Runway Lights • Emergency/Police Vehicle Lighting

LEDs offer enormous benefits over traditional incandescent lamps including: • Energy savings (up to 85% less power than incandescent) • Reduction in maintenance costs • Increased visibility in daylight and adverse weather conditions


40

Illumination (1/2)

�Architectural Lighting

�Signage (Channel Letters)

�Machine Vision

�Retail Displays

�Emergency Lighting (Exit Signs)

�Neon Replacement

�Bulb Replacements

�Flashlights

�Outdoor Accent Lighting - Pathway, Marker Lights


41

Indication

� Household appliances � VCR/ DVD/ Stereo and other audio and video devices � Toys/Games � Instrumentation � Security Equipment � Switches


42

Driving LEDs

• Analog LED Drive Circuits • Digital LED Drive Circuits


43

Analog LED Drive Circuit


44

Digital LED Drive Circuits


45

Colours of LEDs (1/3)

LEDs are available in red, orange, amber, yellow, green, blue and white. Blue and white LEDs are much more expensive than the other colours. The colour of an LED is determined by the semiconductor material, not by the colouring of the 'package' (the plastic body). LEDs of all colours are available in uncoloured packages which may be diffused (milky) or clear (often described as 'water clear'). The coloured packages are also available as diffused (the standard type) or transparent.

LEDs are made from gallium-based crystals that contain one or more additional materials such as phosphorous to produce a distinct color. Different LED chip technologies emit light in specific regions of the visible light spectrum and produce different intensity levels.


46


Tri-colour LEDs The most popular type of tri-colour LED has a red and a green LED combined in one package with three leads. They are called tri-colour because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on. The diagram shows the construction of a tri - colour LED. Note the different lengths of the three leads. The centre lead (k) is the common cathode for both LEDs, the outer leads (a1 and a2) are the anodes to the LEDs allowing each one to be lit separately, or both together to give the third colour.


47


Bi-colour LEDs A bi-colour LED has two LEDs wired in 'inverse parallel' (one forwards, one backwards) combined in one package with two leads. Only one of the LEDs can be lit at one time and they are less useful than the tri-colour LEDs described above.


48

LED Performance (1/8)

• Color • White light • Intensity • Eye safety information • Visibility • Operating Life • Voltage/Design Current

LED performance is based on a few primary characteristics:


49


Colour

Peak wavelength is a function of the LED chip material. Although process variations are ±10 NM, the 565 to 600 NM wavelength spectral region is where the sensitivity level of the human eye is highest. Therefore, it is easier to perceive color variations in yellow and amber LEDs than other colors.


50


White Light When light from all parts of the visible spectrum overlap one

another, the additive mixture of colors appears white. However, the eye does not require a mixture of all the colors of the spectrum to perceive white light. Primary colors from the upper, middle, and lower parts of the spectrum (red, green, and blue), when combined, appear white.


51


Intensity LED light output varies with the type of chip, encapsulation, efficiency

of individual wafer lots and other variables. Several LED manufacturers use terms such as "super-bright," and "ultra-bright“ to describe LED intensity. Such terminology is entirely subjective, as there is no industry standard for LED brightness.


52


Eye Safety

The need to place eye safety labeling on LED products is dependent upon the product design and the application. Only a few LEDs produce sufficient intensity to require eye safety labeling. However, for eye safety, do not stare into the light beam of any LED at close range


53


Visibility

Luminous intensity (Iv) does not represent the total light output from an LED. Both the luminous intensity and the spatial radiation pattern (viewing angle) must be taken into account. If two LEDs have the same luminous intensity value, the lamp with the larger viewing angle will have the higher total light output.


54


Operating Life

Because LEDs are solid-state devices they are not subject to catastrophic failure when operated within design parameters. DDP® LEDs are designed to operate upwards of 100,000 hours at 25°C ambient temperature. Operating life is characterized by the degradation of LED intensity over time. When the LED degrades to half of its original intensity after 100,000 hours it is at the end of its useful life although the LED will continue to operate as output diminishes. Unlike standard incandescent bulbs, DDP® LEDs resist shock and vibration and can be cycled on and off without excessive degradation.


55


Voltage/Design Current LEDs are current-driven devices, not voltage driven. Although drive current and light output are directly related, exceeding the maximum current rating will produce excessive heat within the LED chip due to excessive power dissipation. The result will be reduced light output and reduced operating life.

LEDs that are designed to operate at a specific voltage contain a built-in current-limiting resistor. Additional circuitry may include a protection diode for AC operation or full-bridge rectifier for bipolar operation. The operating current for a particular voltage is designed to maintain LED reliability over its operating life.


56

Bargraph 7-segment Starburst Dot matrix

Some Types of LEDs


LED MATERIALS

� Forward voltages between 1.4V and 3.6V.

� Reverse breakdown voltages between -3V to -10V.

Compound Forward

voltage

Energy gap,Eg Colour

emitted

GaAs 1.5v Infrared

AlGaAs 1.8V 1.62eV Red

GaP 2.4V 2.52eV Green

AlGaInP 2.0v 2.15eV yellow

AlGaInN 3.6v 2.73eV Blue


ADVANTAGES OF LEDS

• Less power consumption

• Very fast action

• Small size and weight

• Long life

• Operating voltage and current is less

• Adaptability to coherent laser operation

• Variety of spectral output colors


DISADVANTAGES OF LEDS

• Sensitive device

• Radiated power output is temperature dependent

• Low efficiency


light emitting diodesx

Documents