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

2. CB
VB
When the electronWhen the electron
falls down fromfalls down from
conduction band andconduction band and
fills in a hole infills in a hole in
valence band, there isvalence band, there is
an obvious loss ofan obvious loss of
energy.energy.
The question is;The question is;
where does that energy go?where does that energy go?

3. In order to achieve aIn order to achieve a
reasonable efficiency forreasonable efficiency for
photon emission, thephoton emission, the
semiconductor must have asemiconductor must have a
direct band gap.direct band gap. CB
VB
The question is;The question is;
what is the mechanismwhat is the mechanism
behind photon emission in LEDs?behind photon emission in LEDs?

4. For example;
Silicon is known as an indirect band-gap material.
as an electron goes from the bottom ofas an electron goes from the bottom of
the conduction band to the top of thethe conduction band to the top of the
valence band;valence band;
it must also undergo ait must also undergo a
significantsignificant change inchange in
momentum.momentum.
CB
VB
What this means is thatWhat this means is that
E
k

5. 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“ .

6. 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.

7. 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.

8. 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 toThis gives rise to
(for us) a new type(for us) a new type
of device;of device;
the light emitting diode (LED).the light emitting diode (LED).

9. 9
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.

10. 10
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.

11. 11
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!

12. Mechanism is “injectionMechanism is “injection
Electroluminescence”.Electroluminescence”.
LuminescenceLuminescence
part tells us that we are producing photons.part tells us that we are producing photons.
Electro part tells us thatElectro part tells us that
the photons are being producedthe photons are being produced
by an electric current.by an electric current.
e-
Injection tells us thatInjection tells us that
photon production is byphoton production is by
the injection of current carriers.the injection of current carriers.
e-

13. Electrons recombine with holes.
Energy of photon is the energy ofEnergy of photon is the energy of
band gap.band gap.
CB
VB
e-
h

14. 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

15. 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.

16. 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

17. 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.

18. When we talk about light ,it is conventional to
specify its wavelength, λ, instead of its frequency.
Visible light has a wavelength on the order of
nanometers.
Thus, a semiconductor with a 2 eV band-gap
should give a light at about 620 nm (in the red). A
3 eV band-gap material would emit at 414 nm, in
the violet.
The human eye, of course, is not equally
responsive to all colors.
( )
( )
hc
nm
E eV
λ =
1242
( )
( )
nm
E eV
λ =

19. 350 400 450 500 550 600 650 700 750
100
10-1
10-2
10-3
10-4
Relative eye responseRelative eye response
Wavelength in nanometers
The materials which are used for important light emitting
diodes (LEDs) for each of the different spectral regions.
GaNGaN
ZnSeZnSe
violet blue
GaP:NGaP:N
green yellow
GaAsGaAs.14.14
pp
8686
GaAsGaAs.35.35
pp
6565
redorange
GaAsGaAs.6.6
pp
44

20. MaterialMaterial
WavelengthWavelength
(µm)(µm)
MaterialMaterial
WavelengthWavelength
(µm)(µm)
ZnSZnS
ZnOZnO
GanGan
ZnSeZnSe
CdSCdS
ZnTeZnTe
GaSeGaSe
CdSeCdSe
CdTeCdTe
0.330.33
0.370.37
0.400.40
0.460.46
0.490.49
0.530.53
0.590.59
0.6750.675
0.7850.785
GaAsGaAs
InPInP
GaSbGaSb
InAsInAs
TeTe
PbSPbS
InSbInSb
PbTePbTe
PbSePbSe
0.84-0.950.84-0.95
0.910.91
1.551.55
3.13.1
3.723.72
4.34.3
5.25.2
6.56.5
8.58.5

21. 21
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.

22. 22
LEDs not only consume far less electricity than traditional
forms of illumination, resulting in reduced energy costs, but
require less maintenance and repair. Studies have shown that the
use of LEDs in illumination applications can offer:
Greater visual appeal
Reduced energy costs
Increased attention capture
Savings in maintenance and lighting replacements
As white LED technology continues to improve, the use of
LEDs for general illumination applications will become more
prevalent in the industry.

23. 23
• Sensor Applications
• Mobile Applications
• Sign Applications
• Automative Uses
• LED Signals
• Illuminations
• Indicators