Transcript Group 7 part 1

How light bulbs work!
Construction
Light bulbs have a simple structure. They have 2
metal contacts that connect to the end of a circuit
These are attached to stiff wires which instead are
attached to a thin metal filament. The wires & the
filament are enclosed in a glass bulb filled with inert
gas such as argon to prevent the filament from
burning out.
Working
On connecting the bulb to a power supply,
 Electric current flows from 1 contact to the other,
through the wires & filaments.
 As the electrons pass through the filament, they
constantly bump into the atoms that make up the
filament & this causes the atoms to vibrate.
 In other words, heat is generated and this is
converted to light energy.
Light Emitting Diodes
A Light emitting diode is a semiconductor device
that emits Incoherent narrow-spectrum light when
electrically biased in the forward direction.
The color of the light depends on the chemical
composition of the semiconductor material used,
and can be near ultraviolet, visible or infrared.
They are made up of materials characterized by a wider gap between
the conduction band and the lower orbitals. The size of the gap
depends on the frequency of the photon and hence plays a major role
in determining the color of the light.
LEDs are specially designed to release a large no. of photons outward.
They are housed in a plastic bulb that concentrates the light in a
particular direction. Most of the light bounces off the sides of the bulb,
traveling on through the rounded end.
THE BASIC LED
White LED specifications
Voltage: 3.6 VDc
Current: 30 mA
Power: 100 mW
Construction
Positive power is applied to 1 side of the LED semiconductor
through a lead (anode) and a whisker. The Other terminal
(cathode) is attached to the anvil. The entire unit is totally
embedded in epoxy which makes the LED indestructible.

The epoxy resin enclosure has 3 functions:
Designed to allow most light to escape from the
semiconductor
It focuses the light i.e. view angle

Protects the assembly from outside elements

Hence, it is essentially a PN junction semiconductor diode that emits light
when current is applied.
Working of Diodes
In the case of LEDs, the conductor material is typically aluminum-gallium-arsenide (AlGaAs).
In pure aluminum-gallium-arsenide, all of the atoms bond perfectly to their neighbors, leaving
no free electrons (negatively-charged particles) to conduct electric current. In doped material,
additional atoms change the balance, either adding free electrons or creating holes where
electrons can go. Either of these additions make the material more conductive.
At the junction of a p-type and an n-type semiconductor there forms a region called the
depletion zone which blocks current conduction from the n-type region to the p-type region,
but allows current to conduct from the p-type region to the n-type region. Thus when the
device is forward biased, with the p-side at higher electric potential, the diode conducts current
easily; but the current is very small when the diode is reverse biased.
Principle
The approach is based on the encapsulation of semiconductor quantum dots
and engineering their surfaces so that they emit visible light when excited by
UV light emitting diodes (LEDs)
The Quantum dots strongly absorb light in the near UV range and re-emit
visible light. The color of the light depends on the Size and Surface chemistry
of the Quantum dots.
Quantum dot LEDs
•
Similar but versatile
QLEDs (Quantum Light Emitting Diodes) made out of networks of quantum dots would
operate in a manner similar to a traditional LED, but with much greater versatility.
•
Current encounters discretized energy bands specific to Q. dots
Electrical current would still be driven through the quantum dot network, but instead of
encountering traditional semiconductor energy bands, the current would encounter the
discretized energy bands specific to quantum dots. The discretized nature of quantum dot
bands means that the energy separation between the valence and conduction bands (the
bandgap) can be altered with the addition or the subtraction of just one atom – making for a
size dependent bandgap.
•
Size
Predetermining the size of the QLED’s dots would fix the emitted photon wavelength at the
appropriate specified color, even if it is not naturally occurring – an ability limited only to
dots. In addition, the extremely small size and versatility of form for quantum dots allows
them to be inserted into any medium necessary – paint, water, plastics and more.
Size plays an important role in affecting material properties; electrical, nonlinear optical etc.
If excited; smaller the dot  higher the energy & intensity of the emitted light.