Photodetectors are an integral part of the photo-receivers in the fiber optic communication systems.

2. OUTLINE
 PRINCIPLE
 CHARACTERISTICS
 p-i-n, APD, InGaAs APD
 NOISE
 RESPONSE TIME
 MERITS
 DEMERITS
 APPLICATIONS

3. PRINCIPLE
 Designed to operate in Reverse Bias
condition.
 Photoelectric effect:

4. CHARACTERISTICS
 The primary photocurrent resulting from absorption is:
 Quantum Efficiency:
 Responsivity:
 S/N:
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0
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[A/W]
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N
S

5. p-i-n PHOTODIODE
 The high electric field present in the depletion region causes
photo-generated carriers to separate and be collected across the
reverse –biased junction. This give rise to a current flow in an
external circuit, known as photocurrent.
w

6. Energy Band Diagram

7. AVALANCHE PHOTODIODE
(APD)

8.  Internally multiplies the primary photocurrent
before entry.
 photogenerated electrons and holes travel through
high field region & gain enough energy to ionize
bound electrons in VB upon colliding with them.
This multiplication is known as impact ionization.
 The newly created carriers in the presence of high
electric field result in more ionization called
avalanche effect.

9. InGaAs APD
Metal contact
InP multiplication layer
InGaAs Absorption layer
InP buffer layer
InP substrate
LIGHT

10. NOISE
 Quantum (Shot) noisephoto-generated
electrons.
 Dark current noise: current continues to flow
through the bias circuit in the absence of the light.
=bulk dark current + surface dark current
thermally generated e
and h in the pn junction
surface defects, bias
voltage and surface area

11. RESPONSE TIME
 Depends on:
Transit time of photocarriers.
Diffusion time of photocarriers outside depletion
region.
RC time constant large value limits the
detector response time.
d
d
v
w
t 

12. MERITS
 PIN  detect high energy charged or uncharged particles &
also electromagnetic radiation
 no gain, so useful in some applications
 no high operating voltage required
 low noise & cost
 high efficiency
 long lifetime
 APD  large gain
 greater level of sensitivity
 high speed

13. DEMERITS
 PIN:
Less sensitivity
No internal gain
Small area
Slow response time
 APD:
Much higher operating voltage required
Much higher level of noise
Output is not linear
Requires high reverse bias for operation
Not as widely used due to low reliability

14. APPLICATIONS
 PIN photodiode  most widely used
Audio CD players
DVD players
Computer CD drives
Optical communication systems
Nuclear radiation detectors
 APD  level of gain is of importance for high voltage
requirement.
Laser range finders
fast receiver modules for data communications
high speed laser scanner (2D bar code reader)
speed gun
PET Scanner
microscopy and particle detection.

15. THANK
YOU