Fiber Optic Light Detectors
Libish T.M.
Associate Professor
Electronics Department
S.C.T. College Of Engineering
Trivandrum
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Photodiode structure
Photodiode = LED
-
LED: forward bias
PD: reverse bias
Absorption of incident light
E
+
fast decreasing intensity
creation of electron-hole
pairs
electrons move to n-type
region
holes move to p-type
region
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p+
n-
n+
Depletion region
Light intensity
High sensitivity (responsivity) at the
desired wavelength and low responsivity
elsewhere
Low noise and reasonable cost
Fast response time.
Insensitive to temperature variations.
Compatible physical dimensions
Long operating life.
High Quantum Efficiency
Low Bias Voltage
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Photodiodes
Positive-Intrinsic-Negative (pin) photodiode
Avalanche Photo Diode (APD)
No internal gain
An internal gain of M due to self multiplication
Photodiodes are reverse biased for normal
operation
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pin photodiode circuit
Incident photons trigger a photocurrent Ip in
the external circuitry
Photocurrent Incident Optical Power
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http://www.youtube.com/watch?v=U6Wvmrc3akc
http://lmoe.utm.md/pin/pin.html
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pin
photodiode
(1)
Disadvantage of p-n
photodiode:
only absorption in thin
depletion layer
E
+
pin diode: intrinsic layer
(i) in between p+ and n+
i-layer is much thicker than
the depletion region
almost no charge carriers
at reverse bias
electric field at reverse
bias
high responsivity
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p+
i
n+
Depletion region
Charge concentration
electric field
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pin energy-band diagram
hc
c
Eg
Cut off wavelength depends on the
bandgap energy
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Quantum efficiency:
number of electron - hole pairs generated I p / q
number of incident photons
P0 / h
Ip / q
P0 / h
Responsivity ()
Ip
P0 h
mA/mW
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http://www.olympusmicro.com/primer/java/photomicrography/avalanche/index.html
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Light enters the device through the p+ region and is absorbed in
the p material, which acts as the collection region for the photogenerated carriers.
Accelerated by the weak E field in depletion region, the electron drift
toward multiplication region
Photons migrate to the p-n junction in picoseconds high response time
Exactly the same acceleration and therefore similar multiplication occurs
low noise and high
high--speed respons
respons..
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l1
Slide 22
l1
lipu, 12/30/2004
Gaining giant energy when drifting into the multiplication region, the
electron will impact and ionize the second electron-hole pair, and
continue the drift 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 field, gaining
enough energy to cause further impact ionization. This phenomenon is the
avalanche effect
As a result, one incident photon can generate hundreds of electrons-hole
pairs and form current multiplication
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Most materials exhibit different electron ionization rates and hole
ionization rates .
The average number of electron-hole pairs created by a carrier per unit
distance traveled is called the ionization rate.
The ratio k = / 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
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Avalanche Photodiodes
The multiplication M for all carriers generated in the
photodiode is defined by
M = IM / Ip
IM : average value of the total multiplied current.
The performance of an APD is characterized by the
responsivity given by
RAPD = (h
(hq/hn)M
APD PIN M
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To achieve a high quantum efficiency, the depletion layer must be thicker.
However, the thicker the depletion layer, the longer it takes for the photogenerated carriers to drift across the reverse-biased junction.
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 multiplication
region and generate EHPs
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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
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Thursday, August 13, 2015
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Example 1:
A photodiode constructed of GaAs has a bandgap energy of 1.43eV at 300oK.
calculate the cutoff wavelength.
lc = 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 l > 869 nm.
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Example 2.
In a 100-ns pulse, 6.0x106 photons at 1300-nm fall 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
h = ----------------------------------------number of incident photons
= (5.4x106) / (6x106) = 0.9.
Thus, the quantum efficiency at 1300-nm is 90 %.
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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) = 6.5mA
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M = IM / Ip
OR
M = Id / Ip
Ip
P0 h
OR
Ip = Io R
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M = IM / Ip
Ip
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P0 h
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M = IM / Ip
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