Optical Detectors -Principle & Types.ppt

Optical Communication Detectors Types

1. Chapter 5
Optical Detector

2. Introduction
Adetector’sfunctionistoconvertthereceivedoptical
signalintoanelectricalsignal,whichisthenamplified
beforefurtherprocessing.
2
I
Light

3. a)Highsensitivityattheoperatingwavelength.
b)Highfidelity.Toreproducethereceivedsignal
waveformwithfidelity(Example:foranalog
transmissiontheresponseofthephotodetectormust
belinearwithregardtotheopticalsignalovera
widerange).
c)Largeelectricalresponsetothereceivedoptical
signal.Thephotodetectorshouldproducea
maximumelectricalsignalforagivenamountof
opticalpower.
3
Requirements:

4. 4
d)Shortresponsetime.(pn-µsec,PIN/APD-nsec)
e)Minimumnoise.
f)Stability.
g)Smallsize
h)Lowbiasvoltage.
i)Highreliability
j)Lowcost

5. Optical Detection Principles
5
Operationofthep-nphotodiode:(a)thestructureofthereversebiasedp-njunction
illustratingcarrierdriftinthedepletionregion;(b)theenergybanddiagramofthereverse
biasedp-njunctionshowingphotogenerationandthesubsequentseparationofanelectron-
holepair.
Fig. 5.1
(a)
(b)

6. Thisdeviceisreversebiasedandtheelectricfielddevelopacrossthe
p-njunctionsweepsmobilecarriers(holesandelectrons)totheir
respectivemajoritysides(pandn).
Aphotonincidentinornearthedepletionregionofthisdevice
whichhasanenergygreaterthanorequaltothebandgapenergyE
g
ofthefabricatingmaterial(i.e.hf>E
g)willexciteanelectronfrom
thevalencebandintotheconductionband.
Thisprocessleavesanemptyholeinthevalancebandandisknown
asthephotogenerationofanelectron-hole(carrier)pair.
6

7. 7
Carrierspairssogeneratednearthejunctionareseparatedandswept
(drift)undertheinfluenceoftheelectricfieldtoproducea
displacementcurrentintheexternalcircuitinexcessofanyreverse
leakagecurrent(Fig5.1(a)).
Photogenerationandtheseparationofacarrierpairinthedepletion
regionofthisreversebiasedp-njunctionisillustratedinFig.5.1(b).

8. 8
Absorptionoutsidedepletionregion–diffusion
current-reducesspeed.
Absorptioninsidedepletionregion–driftcurrent–
fastduetothelargeelectricalfield.
Thedepletionregionmustbesufficientlythicktoallowalarge
fractionoftheincidentlighttobeabsorbedinordertoachieve
maximumcarrierpairgeneration.(PN1to3µm,PIN20to50µm).
However,sincelongcarrierdrifttimesinthedepletionregionrestrict
thespeedofoperationofthephotodiodeitisnecessarytolimitits
width.

9. 9
Photodetector Characteristics
1. Responsivity
Responsivity -ratio of current output to light input
varies with wavelength
theoretical maximum resposivity: 1.05A/W at 1300nm
typical responsivity: 0.8 -0.9 A/W at 1300nm
formula for theoretical maximum responsivity (quantum efficiency = 100%)
where:
R = theoretical maximum responitivity in Amps/Watt
= quantum efficiency
= wavelength in nanometers1240

R
R = ηeλ
hc
e=1.6e-19, h=6.63e-34, c=3e81
AW
P
I
R
o
p

10. 10
Quantum Efficiency -
ratio of primary
electron-hole pairs
created by incident
photons to the
photons incident
on the diode material
Figure 5.2 Typical Spectral Response of
Various Detector Materials
(Illustration courtesy of Force, Inc.)
2. Quantum Efficiencyp
r
r
e
 
number of electrons collected
Number of incident photons
where r
pis the incident photon rate (photon per second) and r
e
is the corresponding electron rate (electrons per second)

11. 11
3. Capacitance of a detector
dependent upon the active area
of the device and the reverse
voltage across the device.
A smaller active diameter
makes it harder to align the
fiber to the detector.
Also, only the center should
be illuminated
photodiode response is slow
at the edges
edge jitter
Figure 6.2 Capacitance versus Reverse
Voltage
(Illustration courtesy of Force, Inc.)

12. 12
Response Time
Time needed for the photodiode to
respond to optical input and
produce an external current
Dependent on
photodiode capacitance
load resistance
design of photodiode
Measured between 10% and 90%
of amplitude
90%
10%
Vout
Time
Rise
Time
Fall
Time

13. 13
Response Time
Approximate -3 dB frequency formula:
where:
R = Impedance that the detector operates into
C = Capacitance of the detector
Rise or fall time formula:
Formula for and f
-3dBRC
f
dB
2
1
3
 RC2.2 dBf
3
35.0



14. 14
Problems
Calculate the theoretical maximum responsivity of a detector at 1550nm.
Calculate the theoretical maximum responsivity of a detector at 820nm.
Calculate the -3dB frequency and rise time of a detector with a capacitance of 0.5pF
operating into an impedance of 50W.
Calculate the responsively of a detector with quantum efficiency of 10% at
800 nm.
Ans: 6.45 A/W
A detector operating at 800 nm produces an output current of 80 A for an
incident light beam of power 800 W. Calculate the quantum efficiency and
responsivity of the detector.
Ans: 0.1 A/W , 15.5%
Answers: 1.25 Amps/Watt, 0.661 Amps/Watt, 6.4 GHz

15. Semiconductor diodes can be classified
into two categories
1.With internal gain
2.Without internal gain
Semiconductor photodiodes without
internal gain generate a single electron
hole pair per absorbed photon.
15
Semiconductor Photodiodes

16. Semiconductor Photodiodes Without Internal Gain
a) P-N Photodiode as given in figure 5.3
16
In the depletion regionthe carrier pairs separate and drift under
the influence of the electric field, whereas outside this region
the hole diffusestowards the depletion region in order to be
collected .
The diffusion process is very slow compared to the drift process
and thus limits the response of the photodiode.

17. Figure 5.3
17
p-n photodiode showing depletion and diffusion regions.

18. 18
It is therefore important that the photons are absorbed in the
depletion region.
Thus it is made as long as possible by decreasing the doping in
the n type material.
The depletion region width in a p-n photodiode is normally 1-
3µm and is optimized for the efficient detection of light at a
given wavelength.

19. 19
Typical output characteristics for the reverse-biased p-n
photodiode is illustrate in Fig 5.4.
The different operating conditions may be noted moving from no
light input to a high light level.

20. 20
Figure 5.4

21. In order to allow operation at longer wavelengths where the light
penetrates more deeply into the semiconductor material a wider
depletion region is necessary.
To achieved this the n-type material is doped so lightly that it can
be considered intrinsic, and to make a low resistance contact a
highly doped n-type (n
+
) layer is added.
This creates a p-i-n (or PIN) structure as may be seen in Fig. 5.4
where almost all the absorption takes place in the depletion
region.
21
b) p-i-n Photodiode

22. Figure 5.4
22
p-i-n photodiode showing combined absorption and depletion region.

23. 23
Germanium p-i-n photodiodes which span the entire wavelength
range of interest are also commercially available, but the dark
current is relatively high.
Dark current arises from surface leakage currents as well as generation-
recombination currents in the depletion region in the absence of illumination.

24. Semiconductor Photodiode with Internal Gain -
Avalanche Photodiode (APD)
TheAPDhasmoresophisticatedstructurethanthep-i-n
photodiodeinordertocreateanextremelyhighelectricfield
region.
Therefore,aswellasthedepletionregionwheremostofthe
photonsareabsorbedandtheprimarycarrierpairsgenerated
thereisahighfieldregioninwhichholesandelectronscan
acquiresufficientenergytoexcitenewelectron-holepairs.
Theprocessisknownasimpactionizationandisthe
phenomenonthatleadstoavalanchebreakdown.
24

25. 25
Itrequiresveryhighreversebiasvoltage(100-400V)inorder
thatthenewcarrierscreatedbyimpactionizationcanthemselves
produceadditionalcarriersbythesamemechanismasshownin
Fig.5.5(b).
Carriermultiplicationfactorsasgreatas10
5
maybeobtained
usingdefectfreematerialstoensureuniformityofcarrier
multiplicationovertheentirephotosensitivearea.

26. 26
Highreversevoltage.Thisaccelerateselectronsandholes
thereuponacquireshighenergy.Theystrikeneutralatoms
andgeneratesmorefreechargecarriers.Thesesecondary
chargesthenionizeothercarriers.
Primarygeneratedelectronsstrikebondedelectronsatthe
VBandexcitethemtotheCB.KnownasImpact
Ionization.
Themainadvantagecomparedtop-i-nphotodiodeisthe
multiplicationorgainfactor,M.

27. Figure 5.5
27
(a) (b)
(a)Avalanchephotodiodeshowinghighelectricfield(gain)region.(b)Carrierpair
multiplicationinthegainregion.

28. 28
OftenanasymmetricpulseshapeisobtainedfromtheAPD
whichresultsfromarelativelyfastrisetimeastheelectronsare
collectedandafalltimedictatedbythetransittimeoftheholes
travellingataslowerspeed.
Hence,althoughtheuseofsuitablematerialsandstructuresmay
givesrisetimesbetween150and200ps,falltimesofa1nsor
morearequitecommonwhichlimittheoverallresponseofthe
device.

29. Drawbacks of The Avalanche Photodiode
1.Fabrication difficulties due to their more complex structure and
hence increased cost.
2.The random nature of the gain mechanism which gives an
additional noise contribution.
3.The high bias voltages required (100-400 V).
4.The variation of the gain with temperature.
29

30. 30

31. Multiplication Factor
The multiplication factor M is a measure of the internal gain
provided by the APD.
It is define as:
where I is the total output current at the operating voltage and I
P
is the initial or primary photocurrent.
The gain M, increases with the reverse bias voltage, V
d.
where n=constant and V
BRis the breakdown voltage of the
detector which is usually around 20 to 500V.
31

32. Bandwidth: Maximum frequency or bit rate that a photodiode can
detect. Determined by the response time.
Theresponsetimelimitedbythreefactors.
1)Thetransittimeofthecarriersacrosstheabsorptionregion,
=d/V
sat
2)TheRCtimeconstantincurredbythejunctioncapacitance(C
j)
ofthediodeanditsload.C
j=A/d.isthepermittivityofthe
semiconductorandAistheactiveareaofthephotodiode.
3)Thetimetakenbythecarrierstoperformtheavalanche
multiplicationprocess(forAPD).
V
sat=saturationvelocity
32

33. Comparison between PIN and APD
33
Material StructureRisetime λ(µm) R(A/W) Dark Current
(nA)
Gain
Silicon PIN 0.5 0.3-1.1 0.5 1 1
Germanium PIN 0.1 0.5-1.8 0.7 200 1
InGaAs PIN 0.3 0.9-1.7 0.6 10 1
Silicon APD 0.5 0.4-1.0 75 15 150
Germanium APD 1.0 1.0-1.6 35 700 50
InGaAs APD 0.25 1.0-1.7 12 100 20

34. Selection Chart
34
CHOICES 0.6-0.8 µm 0.8-0.9 µm1.2-1.7 µm
SOURCE LED LED LED LED(1.3µm)
LASER VCSEL VCSEL LD
FIBER GLASS 3 dB/km <1 dB/km
MM GRIN MM GRIN MM GRIN
SMF SMF
PLASTIC 160 dB/km
SI SI
DETECTOR MATERIAL Si Si InGaAs
Ge Ge Ge
PIN PIN PIN PIN
APD APD
Q1: Compare the important properties between a PIN photodiode and an
Avalanche Photodiode (APD).