CB

VB

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

The question is; The question is; where does that energy go?where does that energy go?

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

VB

The question is; The question is; what is the mechanism what is the mechanism

behind photon emission in LEDs?behind photon emission in LEDs?

For example;Silicon is known as an indirect band-gap material.

as an electron goes from the bottom of as an electron goes from the bottom of the conduction band to the top of the the conduction band to the top of the valence band;valence band;

it must also undergo a it must also undergo a significant significant change in change in momentum.momentum.

CB

VB

What this means is thatWhat this means is that

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 This 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

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

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

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!

Mechanism is “injection Mechanism is “injection Electroluminescence”. Electroluminescence”. Luminescence Luminescence part tells us that we are producing photons.part tells us that we are producing photons.

Electro part tells us that Electro part tells us that the photons are being produced the photons are being produced by an electric current.by an electric current.

e-

Injection tells us that Injection tells us that photon production is by photon production is by the injection of current carriers.the injection of current carriers.

e-

Electrons recombine with holes.

Energy of photon is the energy of Energy of photon is the energy of band gap.band gap.

CB

VB

e-

h

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

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.

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.

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.

( )( )

hcnm

E eVλ =

1242( )

( )nm

E eVλ =

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.

GaN

GaN

Zn

Se

Zn

Se

violet blue

GaP

:NG

aP:Ngreen yellow

GaA

sG

aAs .1

4.1

4pp868

6

GaA

sG

aAs .

35.35pp

6565

redorange

GaA

sG

aAs .6.6

pp44

MaterialMaterial Wavelength Wavelength (µm)(µm) MaterialMaterial Wavelength Wavelength

(µm)(µm)

ZnS ZnS ZnOZnOGanGanZnSeZnSeCdSCdSZnTeZnTeGaSeGaSeCdSeCdSeCdTeCdTe

0.33 0.33 0.370.370.400.400.460.460.490.490.530.530.590.59

0.6750.6750.7850.785

GaAsGaAsInPInP

GaSbGaSbInAsInAsTeTe

PbSPbSInSbInSbPbTePbTePbSePbSe

0.84-0.950.84-0.950.910.911.551.553.13.13.723.724.34.35.25.26.56.58.58.5

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

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

• Sensor Applications

• Mobile Applications

• Sign Applications

• Automative Uses

• LED Signals

• Illuminations

• Indicators