Upload
sourav-sasidharan
View
223
Download
1
Embed Size (px)
Citation preview
8/18/2019 3.Optical Detectors
1/36
Fiber Optic Light Detectors
1
Libish T.M. Associate ProfessorElectronics DepartmentS.C.T. College Of EngineeringTrivandrum
8/18/2019 3.Optical Detectors
2/36
2SCTCE, Electronics Department
8/18/2019 3.Optical Detectors
3/36
3SCTCE, Electronics Department
8/18/2019 3.Optical Detectors
4/36
4SCTCE, Electronics Department
8/18/2019 3.Optical Detectors
5/36
SCTCE, Electronics Department 5
- +
+-
Photodiode structure
• Photodiode = LED
– LED: forward bias
– PD: reverse bias
• Absorption of incident light fast decreasing intensity
creation of electron-holepairs
electrons move to n-typeregion
holes move to p-typeregion
n-p+ n+
E
Depletion region
Light intensity
8/18/2019 3.Optical Detectors
6/36
High sensitivity (responsivity) at thedesired wavelength and low responsivityelsewhere
Low noise and reasonable cost
Fast response time. Insensitive to temperature variations.
Compatible physical dimensions
Long operating life.
High Quantum Efficiency
Low Bias Voltage
SCTCE, Electronics Department 6
8/18/2019 3.Optical Detectors
7/36
7SCTCE, Electronics Department
8/18/2019 3.Optical Detectors
8/36
SCTCE, Electronics Department 8
Photodiodes
Positive-Intrinsic-Negative ( pin) photodiode
No internal gain
Avalanche Photo Diode ( APD)
An internal gain of M due to self multiplication
Photodiodes are reverse biased for normaloperation
8/18/2019 3.Optical Detectors
9/36
SCTCE, Electronics Department 9
pin photodiode circuit
Incident photons trigger a photocurrent I p in
the external circuitry
Photocurrent
Incident Optical Power
8/18/2019 3.Optical Detectors
10/36
SCTCE, Electronics Department 10
8/18/2019 3.Optical Detectors
11/36
SCTCE, Electronics Department 11
http:/ / www.youtube.com/watch?v=U6Wvmrc3akc
http:/ / lmoe.utm.md/pin/pin.html
http://www.youtube.com/watch?v=U6Wvmrc3akchttp://lmoe.utm.md/pin/pin.htmlhttp://lmoe.utm.md/pin/pin.htmlhttp://www.youtube.com/watch?v=U6Wvmrc3akc
8/18/2019 3.Optical Detectors
12/36
pin photodiode (1)• Disadvantage of p-n
photodiode:only absorption in thindepletion layer
• pin diode: intrinsic layer(i) in between p+ and n+
– i-layer is much thicker thanthe depletion region
– almost no charge carriers
at reverse bias
– electric field at reversebias
high responsivity
SCTCE, Electronics Department 12
ip+
E
- +
Charge concentration
n+
Depletion region
electric field
8/18/2019 3.Optical Detectors
13/36
SCTCE, Electronics Department 13
pin energy-band diagram
c
g
h c
E
Cut off wavelength depends on the
bandgap energy
8/18/2019 3.Optical Detectors
14/36
14SCTCE, Electronics Department
8/18/2019 3.Optical Detectors
15/36
SCTCE, Electronics Department 15
Quantum efficiency:
hP
q I p
/
/
photonsincidentof number
generated pairshole-electronof number
0
0
/
/ p
I q
P h
%
0
p I q
P h
mA/mWResponsivity ()
8/18/2019 3.Optical Detectors
16/36
SCTCE, Electronics Department 16
8/18/2019 3.Optical Detectors
17/36
SCTCE, Electronics DepartmentSCTCE, Electronics Department 1717
8/18/2019 3.Optical Detectors
18/36
18http:/ / www.olympusmicro.com/ primer/ java/ photomicrography/ avalanche/ index.html
http://www.olympusmicro.com/primer/java/photomicrography/avalanche/index.htmlhttp://www.olympusmicro.com/primer/java/photomicrography/avalanche/index.html
8/18/2019 3.Optical Detectors
19/36
SCTCE, Electronics Department 19
8/18/2019 3.Optical Detectors
20/36
SCTCE, Electronics DepartmentSCTCE, Electronics Department 2020
8/18/2019 3.Optical Detectors
21/36
SCTCE, Electronics DepartmentSCTCE, Electronics Department 2121
8/18/2019 3.Optical Detectors
22/36
SCTCE, Electronics Department 22
Light enters the device through the p+ region and is absorbed in
the p material, which acts as the collection region for the photo-
generated carriers.
Accelerated by the weak E field in depletion region, the electron drift
toward multipl icat ion region
PhotonsPhotons migratemigrate toto thethe pp--nn junction junction inin picosecondspicoseconds – – highhigh responseresponse timetime
ExactlyExactly thethe samesame accelerationacceleration andand thereforetherefore similar similar multiplicationmultiplication occursoccurs – – lowlow noisenoise andand highhigh--speedspeed responsrespons..l1
8/18/2019 3.Optical Detectors
23/36
Slide 22
l1 lipu, 12/30/2004
8/18/2019 3.Optical Detectors
24/36
SCTCE, Electronics Department 23
Gaining giant energy when drifting into the multiplication region, the
electron wil l impact and ionize the second electron-hole pair, andcontinue the dri ft and impact-ionization process.
APDs internally multiply the primary signal photocurrent in a mechanism
known as impact ionization.
The created carriers are accelerated by the high electric f ield, gaining
enough energy to cause further impact ionization. This phenomenon is theavalanche effect
As a result, one incident photon can generate hundreds of electrons-hole
pairs and form current multiplication
8/18/2019 3.Optical Detectors
25/36
SCTCE, Electronics Department 24
Most materials exhibit different electron ionization rates α and hole
ionization rates β.
The average number of electron-hole pairs created by a carrier per unit
distance traveled is called the ionization rate.
The ratio k = β / α of the electron and hole ionization rates is a measure
of the photo-detector performance.
APDs constructed of materials in which one type of carrier largely
dominates impact ionization exhibit low noise .
. The coefficients increase so rapidly with increasing electric field
strength
The avalanche should be initiated by the carrier with the higher
ionisation coefficient, because otherwise the APD bandwidth is
reduced and its noise factor is increased
8/18/2019 3.Optical Detectors
26/36
SCTCE, Electronics Department 25
Avalanche Photodiodes Avalanche Photodiodes
The multiplicationThe multiplication M M for all carriers generated in thefor all carriers generated in the
photodiode is defined byphotodiode is defined by
M M == I I MM // I I pp
I I MM : average value of the total multiplied current: average value of the total multiplied current..
The performance of an APD is characterized by theThe performance of an APD is characterized by theresponsivityresponsivity given bygiven by
R R APDAPD = (= ( q q //h h n))M M APD PIN
8/18/2019 3.Optical Detectors
27/36
SCTCE, Electronics Department 26
To achieve a high quantum efficiency, the depletion layer must be thicker.
However, the thicker the depletion layer, the longer it takes for the photo-generated carriers to drif t across the reverse-biased junct ion.
Compromise has to be made between response speed and quantum
efficiency.
Factors determining Speed Time it take for photogenerated electron to cross the absorption region to
the multiplication layer
Time it takes for the avalanche process to build-up in the mult iplication
region and generate EHPs
8/18/2019 3.Optical Detectors
28/36
SCTCE, Electronics Department 27
Temperature dependence
When an APD is operated at
a constant bias voltage, the
gain decreases with increase
in temperature. Therefore in
order to obtain constant
output it is necessary to vary
the bias voltage according
to the APD temperature or to keep the APD at a
constant temperature
8/18/2019 3.Optical Detectors
29/36
28SCTCE, Electronics Department
8/18/2019 3.Optical Detectors
30/36
Thursday, August 13, 2015 SCTCE29
8/18/2019 3.Optical Detectors
31/36
Example 1:
A photodiode constructed of GaAs has a band-gap energy of 1.43eV at 300oK.
calculate the cutoff wavelength.
c = hc/Eg
(6.625x10-34J.s)(3x108m/s)= ---------------------------------- = 869 nm.
(1.43eV)(1.6x10-19J/eV)
This GaAs photodiode will not operate for
photons of > 869 nm.
SCTCE, Electronics Department 30
8/18/2019 3.Optical Detectors
32/36
Example 2.
In a 100-ns pulse, 6.0x106 photons at 1300-nm fal l on an
InGaAs photo-detector. On the average, 5.4x106
electron-hole pairs are generated.
Calculate the quantum efficiency
number of e-h pairs generated = -----------------------------------------
number of incident photons
= (5.4x106) / (6x106) = 0.9.
Thus, the quantum efficiency at 1300-nm is 90 %.
SCTCE, Electronics Department 31
8/18/2019 3.Optical Detectors
33/36
Example 3
Photons of energy 1.53x10-19J are incident on
a photodiode which has a responsivity of
0.65A/W.If the optical power level is 10mW, calculate
the photo-current .
Ip = RPo = (0.65A/W)(10mW) = 6.5m A
SCTCE, Electronics Department 32
8/18/2019 3.Optical Detectors
34/36
SCTCE, Electronics Department 33
M M == I I MM // I I pp OR M OR M == I I d d // I I pp
0
p I q
P h
OROR I I p p == I I o o R R
8/18/2019 3.Optical Detectors
35/36
SCTCE, ElectronicsDepartment
34
0
p I q
P h
M M == I I MM // I I pp
8/18/2019 3.Optical Detectors
36/36
SCTCE, ElectronicsDepartment
35
M M == I I MM // I I pp