this is a fiber optic communication LED topic ppt

1. SEMICONDUCTOR PHYSICS BACKGROUND
AND LIGHT EMITTING DIODE(LEDS)-
STRUCTURES AND MATERIALS

2. Outline
 n-Type Semiconductor
 p-Type Semiconductor
 Intrinsic & Extrinsic Materials
 The p-n Junction
 Reverse-biased p-n Junction
 Forward-biased p-n Junction
 Direct Band Gap Semiconductors
 Indirect Band Gap Semiconductors
 Basic of LEDs
 Structures
1) Homojunction LED
I) Surface Emitting LED
2) Heterojunction LED
II)Edge Emitting LED
 Material

3. Semiconductor Physics
Background
a) Energy level diagrams showing the excitation of an electron from the valence band to the
conduction band.The resultant free electron can freely move under the application of
electric field.
b) Equal electron & hole concentrations in an intrinsic semiconductor created by the
thermal excitation of electrons across the band gap.

4. n-Type Semiconductor
a) Donor level in an n-type semiconductor.
b) The ionization of donor impurities creates an increased electron concentration
distribution.

5. p-Type Semiconductor
a) Acceptor level in an p-type semiconductor.
b) The ionization of acceptor impurities creates an increased hole concentration
distribution.

6. Intrinsic & Extrinsic
Materials
•Intrinsic material: A perfect material with no impurities.
•
•Extrinsic material: donor or acceptor type semiconductors.
•Majority carriers: electrons in n-type or holes in p-type.
•Minority carriers: holes in n-type or electrons in p-type.
•The operation of semiconductor devices is essentially
based on the injection and extraction of minority carriers.

7. The p-n Junction
Electron diffusion across
a p-n junction creates a
barrier potential (electric
field) in the depletion
region.

8. Reverse-biased p-n Junction
A reverse bias widens the depletion region, but allows minority carriers to move freely with
the applied field.

9. Forward-biased p-n Junction
Lowering the barrier potential with a forward bias allows majority carriers to diffuse
across the junction.

10. Direct Band Gap Semiconductors
The E-k diagram of a direct band gap semiconductor such as GaAs.The E-k curve
consists of many discrete points with each point corresponding to a possible state,
wave function (x), that is allowed to exist in the crystal.The points are so close that
we normally draw the E-k relationship as a continuous curve. In the energy range E,
to E, there are no points ((x) solutions).

11. Indirect Band Gap
Semiconductors
(a) In GaAs the minimum of the CB is directly above the maximum of theVB. GaAs
is therefore a direct band gap semiconductor.
(b) In Si, the minimum of the CB is displaced from the maximum of theVB and Si is
an indirect band gap semiconductor.
(c) Recombination of an electron and a hole in Si involves a recombination center.

12. Light Emitting Diode (LEDs)
•A light emitting diode (LED) is essentially a PN junction
opto-semiconductor that emits a monochromatic (single
color) light when operated in a forward biased direction.
•It is two terminal device, referred as Light Emitting
Diode.
•It is used to emit light.
•Working Principle: It converts electrical signal into light
signal.
•It is used in forward bias and in reverse bias it works
similar to normal PN junction diode.
•LED configurations being used in photonic
communications:
1) Surface Emitters (Front Emitters)
2) Edge Emitters

13. LED Structure
The Common LED structures used in fiber Optic
communiation is as follows.
1. Homojunction LED
 If a p-n junction is made from two mixture of same types. of atoms,
then it is called as homojunction
 Homojunction LED is also Called as Surface emitting LED
2. Heterojunction LED
 If a p-n junction is made from two dissimilar types of atoms, then it is
called as Hetero junction.
 Heterojunction LED is also Called as Edge emitting LED

14. Surface - Emitting LED

15. Basics of Surface Emitting LED
-As name Indiates, light emitted from the surface of LED.
-This surface is placed to the axis of fiber. and lightCan be easily coupled into fiber.
-Sio₂ layer is for insulation of metal & + p-type kaAS .
- Micro Lens can be used to Improve Coupling efficiency.
-By using etching, distance between Active region and emitting Surface is minimized.
Properties of surface Emitting LED
-High radians is obtained (H +Po
- Low terminal Impedance is obtained.
-Due to multiple p-type layer (Double Heterojunction), the Coupling efficiency is
increased.
-Emission pattern is Isotopic with 120° HPBW.
Schematic of high-radiance surface-emitting LED.The active region is limitted
to a circular cross section that has an area compatible with the fiber-core end face.
Disadvantage
-Low life time (Lambeatian pattarn )
-Low Modulation Bandwidth.

16. Edge - Emitting LED

17. Basics of Edge Emitting LED:
-It has two optical guiding layer.
-Both guiding layer has refractive Index lower then active region and higher
then Surrounding material.
-If Numerical Aparture NA is low them show Edge emitting LED Couples more
power then surface emitting LED.
-Modulation BW is in terms of MHz.
Advantages:
-It has high radiance.
-Beam divergance is narrow.
-couple more Optical power
- high modulation Bandwidth It Injects higher currier density.
-Schematic of an edge-emitting double heterojunction LED.The output beam
is Lambertian in the plane of junction (β = 120") and highly directional
perpendicular to pn junction (β=30").They have high quantum efficiency & fast
response.

18. Light Source Materials
•GaAs Gallium Arsenide - Infrared LED - Remote control
•GaP - Gallium Phosphide - Red or Green light
•GaAsP - Gallium Arsenide Phosphide - Red orYellow
light

19. References
1. Semiconductor physics background- Optical fiber communication ,3rd
edition , G.Keiser,MCGrawHill,2000
2. Light Emitting Diodes (LEDs)- By Prof. Hitesh Dholakiya (Engineering
FundaYouTubeVideo)

20. ThankYou!