Photodetectors

Jun 17 • Notes • 2056 Views • 2 Comments on 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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2 Responses to Photodetectors

  1. Dinesh says:

    This article clearly shows the photodetectors purpose

    and it clearly explains the concept of photodetector and avalanche effect. and the questions which you have provided is very useful at the preparation time

  2. shweta singh says:

    Photodetectors quite an important topic in the field of the electrical as well as in the electronics field….It has been explained in the brief way… 🙂

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