1. LIGHT EMITTING DIODE – Design Principles
EBB424 USM SMMRE
LIGHT EMITTING DIODE – Design Principles
EBB 424E
Lecture 2 – LED 1
Dr Zainovia Lockman
Dr Zainovia Lockman L2

2. 1907 Publication report on Curious Phenomenon
On applying a potential to a crystal of carborundum (SiC), the material gave out a yellowish light
H.J. Round, Electrical World, 49, 309, 1907

3. 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. 4 Main Issues The device configuration Materials requirements
Materials selection
Material issues

5. 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 engineering
Explain the isoelectronic doping in GaAsP system
State examples of materials that emit, UV, Vis, IR lights

6. For the LED lectures you need:
Complete set of notes (3 lecture presentation and lecture notes)
A photocopy from Kasap (p )
A photocopy from Wilson (p )
Some reading materials

7. Semiconductors bring quality to light!
What is LED?
Semiconductors bring quality
to light!
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.

8. Applications of LEDs

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

10. 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 ( 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 light
Cost:
Not too bad
Robustness:
Solid state component, not as fragile as incandescence light bulb

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

12. 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

13. Excitation E
Electron (excited by the biased forward voltage) is in the conduction band k
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)
Hole is in valance band

14. How does it work? Recombination produces light!!
P-n junction
Electrical Contacts
A typical LED needs a p-n junction
There are a lot of electrons and holes at the junction due to excitations
Electrons from n need to be injected to p to promote recombination
Recombination produces light!!
Junction is biased to produce even more e-h and to inject electrons from n to p for recombination to happen

15. 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.

16. Recombination and Efficiency
eVo Eg p n+
h =Eg
(a)
(b)
Electrons in CB
Holes in VB EC EF EV
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

17. 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

18. Calculate If GaAs has Eg = 1.43ev What is the wavelength, g it emits?
What colour corresponds to the wavelength?

19. Construction of Typical LED
Light output
SiO2 p n
Electrical contacts
Substrate

20. 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

21. 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

22. Materials Requirements
EBB424 USM SMMRE
Correct band gap
Direct band gap
Materials Requirements
Efficient radiative pathways must exist
Material can be made p and n-type
Dr Zainovia Lockman L2

23. Direct band gap materials
 UV-ED  ~ nm
Eg > 3.25eV
 LED -  ~ nm
Eg = 3.1eV to 1.6eV
 IR-ED-  ~750nm- 1nm
Eg = 1.65eV
Direct band gap materials
e.g. GaAs not Si
Candidate Materials
Materials with refractive index that could allow light to ‘get out’
Readily doped n or p-types

24. 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)

25. Visible LED Violet ~ 3.17eV Blue ~ 2.73eV Green ~ 2.52eV
Definition:
LED which could emit visible light, the band gap of the materials that we use must be in the region of visible wavelength = nm. This coincides with the energy value of 3.18eV- 1.61eV which corresponds to colours as stated below:
The band gap, Eg that the semiconductor must posses to emit each light
Violet ~ 3.17eV
Blue ~ 2.73eV
Green ~ 2.52eV
Yellow ~ 2.15eV
Orange ~ 2.08eV
Red ~ 1.62eV
Colour of an LED should emits

26. Electromagnetic Spectrum
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
V ~ 3.17eV
B ~ 2.73eV
G ~ 2.52eV
Y ~ 2.15eV
O ~ 2.08eV
R ~ 1.62eV
Visible lights

27. Candidate Materials for LED’s

28. Question 1
Indicate the binary compounds that can be selected for red, yellow, green and blue LED.

29. Candidate Materials Group III-V & Group II-VI
Group IV
Group V iv
iii v N P As ii Al Ga In
Periodic Table to show group III-V and II-V binaries

30. 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.

31. Group III-V LED materialsGa In N P As
AlN, AlP,AlAs
GaN, GaP, GaAs
InN, InP, InAs
Binary compounds
GaP
GaAl
GaAsP
GaAsAl
Ternary
compounds
GaAs
Questions to ask when choosing the right material:
Can it be doped or not?
What wavelength it can emit?
Would the material able to allow radiative recombiation?
Direct or indirect semiconductor?

32. Announcement
Evening classes