The document discusses light emitting diodes (LEDs). It provides a brief history of LEDs including their invention in 1907 and development of the first practical visible spectrum LED in 1962. It describes the working of LEDs as either direct or indirect combination when a voltage is applied across the p-n junction. LEDs have applications in devices, medical equipment, lighting, signs and more. Advantages include energy efficiency and long life, while disadvantages include higher initial cost and sensitivity to temperature and voltage.

1. LIGHT EMITTING DIODE
GROUP MEMBERS:
ATIQ-UR-REHMAN AWAIS ATHER WALEED

2. TOPICS:
 HISTORY
 WORKING
1.DIRECT COMBINATION
2.INDIRECT COMBINATION
 APPLICATIONS
 ADVANTAGES
 DISADVANTAGES

3. HISTORY:
1.The first known report of led solid state diode was made in 1907 by HJ.ROUND.
2.In 1962,NICK HOLONYAK employ of general electric company developed the first
practical visible-spectrum led.He is also known as the “FATHER OF LIGHT EMITTING
DIODE”

4. WORKING:
I. When a voltage V is applied across the junction then electrons from n side
and holes from p side combine to produce photon in the form of light.
II. These photons should be allowed to escape from the device without being
re-absorbed.

5.  DIRECT GAP:
 Maximum point of valence band and minimum of conduction band occur at same
point This is known as direct gap .
 So photon emitted have not to provide momentum to electron .

6. IN DIRECT GAP :
 Maximum point of valence band and minimum of conduction band do not occur at
same point This is known indirect gap
 In this gap momentum to electron is provided.
𝐸 =
ℎ𝐶
𝜆
 So gallium arsenide compounded diodes are used to emit light.

7. APPLICATIONS:
DEVICES,MEDICAL applications,clothing,toys
Remote control
Lighting
Indicators, signs
Traffic signals
Swimming pool lighting

8. ADVANTAGES
 LED produce more light per watt than normal bulbs; useful for energy saving
devices.
 Led lights up very quickly and achieve full brightness in microseconds.
 Led’s can have a relatively long useful life.
 Led’s being solid state components are difficult to damage with external source.

9. DISADVANTAGES
 LED are currently more expensive
 LED must be supplied correct voltage otherwise a little more voltage will burn the
LED
 LED performance depend upon the environment temperature, increase in temp
effect the life of LED