Photodetectors
PHOTO DIODE:-
– The device consists of a p and n semiconductor region separated by a very lightly n-doped intrinsic region.
– The optical power decreases exponentially as the incident light is absorbed in depletion region.
– The electron hole pairs generated has a large electric field and drift towards p or n side in opposite direction.
– Thus a reverse bias pn junction is formed and acts as a photodetector called pn photodiode.
AVALANCHE PHOTO DIODE:-
-It multiplies the primary signal photo current before it enters the input circuitary following the amplifier.
– The multiplication effect is achieved using a very high electric field across the photo diode.
– When a photon generated electron encounters this high electric field,it acquires sufficient energy to kick more electrons from the valence to conduction band thereby creating more electron hole pairs.
-These secondary electron hole pairs get accelerated to higher energy thereby generating more pairs.
-This is called avalanche multiplication and the device is called avalanche photodiode.
PHOTODETECTOR NOISE:-
– It is the maeasure of the photodetector capacity to remove the unwanted signals and is defined by
SNR= signal power from photocurrent
Photodetector noise power+ amplifier noise power
-For higher signal to noise ratio the numerator should be maximum and denominator should be minimum. 
– Thus the following conditions should be met:-
• It must have high quantum efficiency to generate large signal power.
• The amplifier noise should be kept low.
DETECTOR RESPONSE TIME:-
– The detector responsivity R can be expressed in terms of a fundamental quantity N called the quantum efficiency and defined as
N= Electron generation rate/ Photon incidence rate
AVALANCHE MULITPLE NOISE:-
Losses in an optical fibre
The types of losses in a optical fibre are
• Attenuation loss
• Absorption
• Scattering
• Bending loss
• Dispersion loss
• Coupling loss
EXPECTED QUESTION WITH ANSWER
1) WHAT IS PHOTODIODE?
ANS:- – The device consists of a p and n semiconductor region separated by a very lightly n-doped intrinsic region.
– The optical power decreases exponentially as the incident light is absorbed in depletion region.
– The electron hole pairs generated has a large electric field and drift towards p or n side in opposite direction.
– Thus a reverse bias pn junction is formed and acts as a photodetector called pn photodiode.
2) WHAT IS AVALANCHE PHOTODIODE?
ANS:- -It multiplies the primary signal photocurrent before it enters the input circuitary following the amplifier.
– The multiplication effect is achieved using a very high electric field across the photodiode.
– When a photon generated electron encounters this high electric field,it acquires sufficient energy to kick more electrons from the valence to conduction band thereby creating more electron hole pairs.
-These secondary electron hole pairs get accelerated to higher energy thereby generating more pairs.
-This is called avalanche multiplication and the device is called avalanche photodiode.
3) WHAT IS PHOTODETECTOR NOISE?
ANS:- -It multiplies the primary signal photocurrent before it enters the input circuitary following the amplifier.
– The multiplication effect is achieved using a very high electric field across the photodiode.
– When a photon generated electron encounters this high electric field,it acquires sufficient energy to kick more electrons from the valence to conduction band thereby creating more electron hole pairs.
-These secondary electron hole pairs get accelerated to higher energy thereby generating more pairs.
-This is called avalanche multiplication and the device is called avalanche photodiode.
GATE Syllabus-
1.Gate Syllabus for Electronics and Communication 2014
2. Gate Syllabus for Engineering Science 2014
IES Syllabus-
1. IES Syllabus for Electronics and Telecommunication
2. IES Syllabus for General Ability
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