LIGHT EMITTING DIODE – Design Principles

EBB 424ELecture 2 – LED 1Dr Zainovia Lockman

1907 Publication report on Curious Phenomenon

H.J. Round, Electrical World, 49, 309, 1907

On applying a potential to a crystal

of carborundum (SiC), the material gave out

a yellowish light

3 Lectures on LED

OBJECTIVES:To learn the basic design principles of LED

To relate properties of semiconductor material to the principle of LED

To be able select appropriate materials for different types of LED

To be able to apply knowledge of band gap engineering to design appropriate materials for a particular LED

To acknowledge other materials that can and have been used in LED

4 Main Issues

1. The device configuration

2. Materials requirements

3. Materials selection

4. Material issues

By the end of this lecture you must be able to …

Draw a typical construction of an LED. Explain your drawing. State all the issues regarding the materials

selection of an LED. State all of the possible answers regarding your

materials issues.Explain band gap engineeringExplain the isoelectronic doping in GaAsP systemState examples of materials that emit, UV, Vis, IR

lights

For the LED lectures you need:

1. Complete set of notes (3 lecture presentation and lecture notes)

2. A photocopy from Kasap (p.139-150)

3. A photocopy from Wilson (p-141-155)

4. Some reading materials

LED are semiconductor p-n junctions that under forward bias conditions can emit radiation by electroluminescence in the UV, visible or infrared regions of the electromagnetic spectrum. The qaunta of light energy released is approximately proportional to the band gap of the semiconductor.

Semiconductors

bring quality

to light!

What is LED?

Applications of LEDs

Your fancy telephone, i-pod, palm pilot and digital camera

Getting to know LED

Advantages of Light Emitting Diodes (LEDs)Longevity: The light emitting element in a diode is a small conductor chip rather than a filament which greatly extends the diode’s life in comparison to an incandescent bulb (10 000 hours life time compared to ~1000 hours for incandescence light bulb)Efficiency: Diodes emit almost no heat and run at very low amperes.Greater Light Intensity: Since each diode emits its own lightCost:Not too badRobustness:Solid state component, not as fragile as incandescence light bulb

LED chip is the part that we shall deal with in this course

Luminescence is the process behind light emission

• Luminescence is a term used to describe the emission of radiation from a solid when the solid is supplied with some form of energy.

• Electroluminescence excitation results from the application of an electric field

• In a p-n junction diode injection electroluminescence occurs resulting in light emission when the junction is forward biased

Excitation

Electron (excited by the biased forward voltage) is in the conduction

band

Hole is in valance band

Normally the recombination takes place between transition of electrons between the bottom of the conduction band and the top of the valance band (band exterma). The emission of light is therefore;hc/ = Ec-Ev = Eg(only direct band gap allows radiative transition)

E

k

How does it work?

P-n junction

Electrical Contacts

A typical LED needs a p-n junctionA typical LED needs a p-n junction

Junction is biased to produce even more e-h and to inject electrons from n to p for recombination to happen

Junction is biased to produce even more e-h and to inject electrons from n to p for recombination to happen

There are a lot of electrons and holes at the junction due to excitationsThere are a lot of electrons and holes at the junction due to excitations

Electrons from n need to be injected to p to promote recombinationElectrons from n need to be injected to p to promote recombination

Recombination produces light!!

Injection Luminescence in LED

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 For example, electrons in n-type (majority carriers) enter the p-type

to become minority carriers The minority carriers will be larger minority carrier injection Minority carriers will diffuse and recombine with the majority carrier. For example, the electrons as minority carriers in the p-region will

recombine with the holes. Holes are the majority carrier in the p-region.

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

Recombination and Efficiency

eVo

Eg

p n+

h =Eg

Eg

p n+(a) (b)

Electrons in CB

Holes in VB

EC

EV

EF

◘Ideal LED will have all injection electrons to take part in the recombination process

◘In real device not all electron will recombine with holes to radiate light

◘Sometimes recombination occurs but no light is being emitted (non-radiative)

◘Efficiency of the device therefore can be described

◘Efficiency is the rate of photon emission over the rate of supply electrons

Emission wavelength, g

◘ 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, g = hc/(EC-EV)

◘ EC-EV = Eg

◘ g = hc/Eg

Calculate

• If GaAs has Eg = 1.43ev

• What is the wavelength, g it emits?• What colour corresponds to the

wavelength?

Construction of Typical LED

Substrate

n

Al

SiO2

Electrical contacts

p

Light output

LED Construction

Efficient light emitter is also an efficient absorbers of radiation therefore, a shallow p-n junction required.

Active materials (n and p) will be grown on a lattice matched substrate.

The p-n junction will be forward biased with contacts made by metallisation to the upper and lower surfaces.

Ought to leave the upper part ‘clear’ so photon can escape.

The silica provides passivation/device isolation and carrier confinement

Efficient LED

Need a p-n junction (preferably the same semiconductor material only different dopants)

Recombination must occur Radiative transmission to give out the ‘right coloured LED’

‘Right coloured LED’ hc/ = Ec-Ev = Eg

so choose material with the right Eg

Direct band gap semiconductors to allow efficient recombination

All photons created must be able to leave the semiconductor

Little or no reabsorption of photons

Materials Requirements

Correct band gap Direct band gap

Material can be made p and n-type

Efficient radiative pathways must exist

Candidate Materials

Direct band gap materials

e.g. GaAs not Si

UV-ED ~0.5-400nm

Eg > 3.25eV

LED - ~450-650nm

Eg = 3.1eV to 1.6eV IR-ED- ~750nm- 1nm

Eg = 1.65eV

Readily doped n or p-types

Materials with refractive index that could allow light to ‘get out’

Typical Exam Question

Describe the principles of operation of an LED and state the material’s requirements criteria to produce an efficient LED.

(50 marks)

Visible LED

Definition:LED which could emit visible light, the band gap of the materials that we use must be in the region of visible wavelength = 390- 770nm. This coincides with the energy value of 3.18eV- 1.61eV which corresponds to colours as stated below:

Violet ~ 3.17eVBlue ~ 2.73eVGreen ~ 2.52eV Yellow ~ 2.15eVOrange ~ 2.08eVRed ~ 1.62eV

Colour of an LED should emits

The band gap, Eg that the

semiconductor must posses to emit each light

Electromagnetic Spectrum

Visible lights V ~ 3.17eV

B ~ 2.73eV

G ~ 2.52eV

Y ~ 2.15eV

O ~ 2.08eV

R ~ 1.62eV

The appearance of the visible light will be the results of the overlap integral between the eye response curve and the spectral power of the device the peak of the luminous curve will not in general be the same as the peak of the spectral power curve

Candidate Materials for LED’s

Question 1

• Indicate the binary compounds that can be selected for red, yellow, green and blue LED.

Candidate Materials Group III-V & Group II-VI

iviii v

ii

Periodic Table to show group III-V and II-V binaries

Group II Group III Group IV Group V

Al

Ga

In

N

P

As

Group III-V (1950)

The era of III–V compound semiconductors started in the early 1950s when this class of materials was postulated and demonstrated by Welker (1952, 1953). The class of III–V compounds had been an unknown substance prior to the 1950s that does not occur naturally. The novel man-made III–V compounds proved to be optically very active and thus instrumental to modern LED technology.

Group III-V LED materials

Al

Ga

In

N

P

As

AlN, AlP,AlAs

GaN, GaP, GaAs

InN, InP, InAs

GaAs GaP

GaAl

GaAsP

GaAsAl

Questions to ask when choosing the right material:1. Can it be doped or not?

2. What wavelength it can emit?3. Would the material able to allow radiative recombiation?

4. Direct or indirect semiconductor?

Questions to ask when choosing the right material:1. Can it be doped or not?

2. What wavelength it can emit?3. Would the material able to allow radiative recombiation?

4. Direct or indirect semiconductor?

Ternarycompounds

Binary compounds

Announcement

Evening classes