LED

1. 1) THAKOR JITENDRA(13BCL111)
LED
(Light emitting diode)
Prepared by:-

2. When PN junction is forward
biased,electrons in N-region combine
with the holes in P-region.
Free electrons are in the conduction
band and holes are in the valence band.
Thus electrons go to the valence band
from the conduction band that is
electrons go to the lower energy level
from the higher energy level.

3. So when recombination of electrons
and holes occurs,energy is radiated in
the form of heat and light.
In light emitting diode arrangement is
made such that the energy radiated is
in the form of light.
Colour of the light radiated depends
upon the material of the LED.

4. Construction :
N-type epitaxial layer is
grown on the subtrate.P-
region is formed over it by
the diffusion process.
Recombination of the charge
carries take place in P-region.

5. So P-region is kept on the
upper side.Lead is taken out
from the P-type material such
that is light is not obstructed.
A layer of gold film is kept
below N-layer so that the light
emitted is reflected.

6. Simple circuit of LED

7. Symbol of LED
In the symbol,the arrows point to the
outward side which indicate that the light is
emitted

8. Different types of
LED

9. Semiconductor material used in
semi-transparent.
 Red or amber(yellow) light is
radiated when GaAsP is used
 Green light is radiated when GaP is
used.
 Invisible infrared radiated when
GaAs is used.

10. Working:
LED is forward biased to bring
it into action.
Minimum of 10 mA current
should be passed through it.Value
of current can be kept between
10 mA to 25 mA.

11. Due to the forward
bias,electrons from N-type
material go from the
conduction band and recombine
with the holes in the valence
band of p-type material.
So energy in the form of light
is radiated.

12. Advantages :
1. Long life.
2. Can be operated at
low voltage.
3. Switching the fast.

13. Disadvantages :
1. Power consumption is more
compared to LCD type
display
2. Display becomes dim in
the ambient light.

14. Applications :
1. As pilot indicator.
2. In opto-isolators.
3. In optical communication.
4. In large numric display.
5. Rectangular LEDs are used
in bar graphs.