3.Optical Detectors

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Fiber Optic Light Detectors

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Libish T.M. Associate ProfessorElectronics DepartmentS.C.T. College Of EngineeringTrivandrum


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2SCTCE, Electronics Department


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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


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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



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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


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pin photodiode circuit

Incident photons trigger a photocurrent I p 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

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


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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


ip+

E

- +

Charge concentration

n+

Depletion region

electric field


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pin energy-band diagram

c

g

h c

E

Cut off wavelength depends on the

bandgap energy


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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 ()


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SCTCE, Electronics DepartmentSCTCE, Electronics Department 1717


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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


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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 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


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Slide 22

l1 lipu, 12/30/2004


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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


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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


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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


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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 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


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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 SCTCE29


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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.



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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 %.



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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) = 6.5m A



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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


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0

p I q

P h



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3.Optical Detectors

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