Basics of light-emitting diodes

2. Yashpal Singh Katharria
Semiconductor Light Emitting Diodes

3. History
Captain Henry Joseph Round, England
First observation of Electroluminescence from SiC crystals in
the year 1907

4. History
Captain Henry Joseph Round, England
First observation of Electroluminescence from SiC crystals in
the year 1907
Oleg Vladimirovich Losev, Imperial Russia
Reported light emission from rectifier diode in 1927
A talented scientist who spent his life working as technician
Published 43 articles in top Russian, British and German journals as a sole author and
was granted 16 patents without getting any formal education
Died in 1942 at the age of 39 during the blockade of Leningrad

5. History
Captain Henry Joseph Round, England
First observation of Electroluminescence from SiC crystals in
the year 1907
Oleg Vladimirovich Losev, Imperial Russia
Reported light emission from rectifier diode in 1927
A talented scientist who spent his life working as technician
Published 43 articles in top Russian, British and German journals as a sole author and
was granted 16 patents without getting any formal education
Died in 1942 at the age of 39 during the blockade of Leningrad
Nick Holonyak, GE New York, USA
The father of the light-emitting diode: invented the first practically useful visible LED in
1962.
His father worked in a coal-mine & he was the first member of his family to receive any
formal schooling.
First PhD student of John Bardeen

6. Shuji Nakamura, Nichia Corp., Japan
Inventor of high-brightness GaN based LEDs
P-type doping of GaN, high quality InGaN thin films, blue green, white LEDS and blue
laser diodes
The general conclusion among scientists at this time is that Dr. Nakamura’s inventions are
so reliable and energy efficient that they are destined to replace Thomas Edison’s light bulb
and save the world billions of dollars in energy costs.
History

7. What is Light Emitting Diode (LED)?
 A device that converts electrical energy into Light
 A p-n junction diode that emits monochromatic light
when forward biased.
 Requires 1.5 ~ 4 V and 10 mA to turn ON
 One needs to use a resistor to prevent overloading
10 V
0 V
Symbol

8. How LED works?
Ec
Ev
Eg
e
hole
Ec
Ev
hn = Eg
Semiconductors
Indirect band gapDirect band gap
Photon carries negligibly small momentum :
p = h/l ...vertical transitions
Indirect transitions: electron moving from CB
to VB involves a phonon to satisfy momentum
conservation; less probably transitions.
Direct band gap : more light
Indirect band gap : less light

9. How LED works?
p-type
(holes)
n-type
(electrons)
Depletion
region
- -
- -
- -
+
+
+
p-n Junction
p-type : holes and acceptors (-ive ions)
n-type : electrons and donors (+ive ions)
A region p-n junction is depleted of free
carriers…Depletion Region (DR)
Diffusion or built-in potential VD appears
across DR.

10. How LED works?
p-type
(holes)
n-type
(electrons)
Depletion
region
- -
- -
- -
+
+
+
hn
Under forward bias; voltage drop across
Depletion region
Holes will move towards n-region
Electrons towards p-region
Carrier recombination can occurs in the
depletion region and photons are emitted.
Normally,
Eg = hn
However there are other situations like:
-
-
-
+
+
+
EC
EV
ED
EA
EC
EV
EC
EV
hn1
hn2 hn3
Active region
EA
ED

11. Optical emission spectra of LED
Dispersion curves for electrons and holes can be given by the following equations
Energy of the emitted photon can be given by a joint
dispersion relation
Joint density of states will be given by
The distribution of carriers in allowed bands is given by a Boltzman function

12. Optical emission spectra of LED
The emission intensity as a function of energy
I(E) = r(E) . FB(E)
The maximum intensity occurs at

13. Ge
Si
GaAs
Current-Voltage Characteristics
For current flow through the junction
Applied bias V = VD ~ Eg

14. Radiative transitions : a transition leading to the emission of a photon.
Non-radiative transitions : no photon emission
1. Defects (dangling bonds, impurities, point defects such as vacancies, interstitials)
related transitions are usually called Shockley-Read-Hall recombinations; Most active at
the surface of the devices.
2. Auger transitions: Important mechanism in narrow band gap materials
Radiative and Non-radiative transitions
EC
EV
ED
ED
EC
EV
EC
EV
hn1
hn2 hn3

15. Answer: to reduce TIR…
Why do we need the dome?
Dome
+
-

16. Total Internal Reflection (TIR)
 For GaAs-air interface, C = 16o which means that much
of the light suffers TIR.
16o
LED

17. To solve the problem we could:
 Shape the surface of the semiconductor into a dome or
hemisphere so that light rays strike the surface angles < C
therefore does not experience TIR. But expensive and
not practical to shape p-n junction with dome-like structure.
 Encapsulation of the semiconductor junction within a
dome-shaped transparent plastic medium (an epoxy) that
has refractive index higher than air and lower than that of
the semiconductor.
Solving TIR problem
n
Electrodes
Electrodes
pn
junction
Plastic
dome
p

18. Reabsorption problem
 Light emission takes place from depletion region (active region).
However, the emitted light can be re-absorbed in p-type region
because the band gap energy of p-type material is same the energy of
the emitted photon.
hn = Eg
n-type
p-type
Dep. Region
Metal-contact
Solutions:
1. Thin p-layer : e can escape the p-region and recombine at surface non-radiatively.
2. Hetrostructure LED

19. Hetrostructure or Quantum-well LEDs
Eg1 Eg2
Quantum well (QW)
Quantum wells help in:
 Carrier confinement
 Tuning the wavelength of the emitted light
GaN band gap ~ 3.4 eV
InN band gap ~ 0.7 eV
InxGa1-xN (0 < x <1) band gap : 0.7 – 3.4 eV
Advantages
(1) Do not suffer from poor surface conditions since the active region is not near the surface.
(2) Increased carrier injection efficiency due to the low doping active layer.
(3) The photons emitted are also not absorbed in the top or bottom region because the
photon energy is smaller than the bandgap of the n- or p-region.
2eV1.4eV
n-AlGaAs
p-GaAs
p-AlGaAs

20. Quantum-well LEDs
However, LEDs with single QW show carrier overflow when currents injected in the LEDs
are exceeded above a certain value. Therefore, multiple-QWs are, now a days, employed in
GaN based visible and UV-light LEDs.

21. Other LED Problems
n-type
p-type
Dep. Region
Metal-contact
1. Reflection at top metal-semiconductor interface
Minimized by small area of the metal contact
2. Absorption in the top metal contact
Minimized by small area of the metal contact
3. Current crowding at the top contact
Minimized by using Current spreading layers
4. Efficiency Droop
No definite solution yet

22. 1. Quantum Efficiency (QE)
Number of Photons emitted per injected electron.
The active region of an ideal LED emits 1 photon per injected electron. Thus
QE=1 for ideal LED
2. Internal quantum efficiency (IQE)
Is defined by
3. Extraction Efficiency (EE)
Number of photons extracted into free space per photon emitted from the active region.
Photons emitted from active region should escape from LED. In an ideal LED, all the
photons emitted from the active region are emitted into the free space. Therefore,
EE=1 for ideal LED
4. External Quantum Efficiency (EQE)
Is defined by
Basic Terms

23. 5. Power Efficiency (or wall-plug efficiency)
Is defined as
I.V is power provided to the LED and P is output power.
Basic Terms

24. Traffic signals, street light
Residential
Information boards
Buildings
Common application: Digital clock, battery level indicator, torch
Outdoor:
runway in airports
Applications

25. Thank you very much for you attentions !!
Any Questions??

27. • For now, between 25 & 50% efficiency, but some researchers think it’s
possible to have 90% efficiency! Contrary to the traditional light bulb
which has 5% efficiency and no perspective to do better!

28. Good efficiency & durability
• LEDs can provide 50 000 hrs of life compared to 1000 hrs with
incandescent light bulbs
• Associated with perfect material and devices, LEDs would require
only 3 Watts to generate the light obtained with a 60-Watt
incandescent bulb
LED vs. conventional light sources degradation in light output over time

29. 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

30. http://lighting.sandia.gov
http://www.loe.org
http://www.enn.com
http://www.netl.doe.gov
http://www.nichia.com
http://cree.com
www.lumileds.com

31. • Similarities
– Has anode and cathode
• Electrons flow out through anode and in through cathode
– Operates like diode; must be forward-biased
– Photons are emitted when electrons fill holes in the emissive layer
• Differences from conventional LED:
– Emissive and conductive layers are organic compounds or polymers
– Substrate can be plastic
Similarities and differences between OLEDs and LEDs

32. 11 Environment
Result doubly environment-friendly
• Less current consumption
(less electricity burned)
• Less heat produced
Less CO2 emissions
Less light pollution
Positive impact on global warming
Incandescent traffic lights
replaced by LEDs in USA:
economy of
2.5 billion kWhours
= US$ 200 million
= 3 billion kilos of CO2
released in the
atmosphere