LED Laser Diodes Laser Diode Structures and Radiation Patterns Photodetectors

1. OPTICAL SOURCES AND DETECTORS

2. Light Sources:
1. Heterojunction -structured semiconductor Laser diodes
(injection laser diodes or ILDs)
2. Light-emitting diodes(LEDs)
Heterojunction  two adjoining semiconductor materials with different
band-gap energies.
They have adequate output power for a wide range of applications
High efficiency
Light emitting region  LEDs and LASER diodes  pn junction
Direct band gap III-V semiconductor materials.
Junction  forward baised  electron and holes are injected into the p
and n regions.
Injected minority carriers can recombine either radiatievely ,
Photon energy hv emittd
This pn junction is  active or recombination region.

3. LED:
Output incoherent
No optical cavity exists  output radiation  broad spectral width.
LASER:
Output coherent
Coherent source optical resonant cavity highly monochromatic
output beam very directional.
Choosing optical source:
Optical waveguide, Characteristics of optical fiber like geometry,
attenuation as a function of wavelength, Group delay distortion, modal
characteristics
Interplay of these factors  optical source power , spectral width,
radiation pattern, and modulation capability.
Laser  use single or multi mode fiber.
LED use only Multimode.

4. a) Energy level diagrams showing the excitation of an electron from the valence band to the conduction band.
The resultant free electron can freely move under the application of electric field.
b) Equal electron & hole concentrations in an intrinsic semiconductor created by the thermal excitation of
electrons across the band gap
-123
JK1038.1 
Bk
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

5. Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
• Inject Arsenic into the crystal with an implant step.
• Arsenic is Group5 element with 5 electrons in its outer shell, (one more than
silicon).
• This introduces extra electrons into the lattice which can be released through
the application of heat and so produces and electron current
• The result here is an N-type semiconductor (n for negative current carrier)
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
-
Si -
-
-
-
Si -
-
-
-
-
+
+
+
As -
-
-
- -
As -
-
-
- -
As -
-
-
- -
+
-
-
-

6. a) Donor level in an n-type semiconductor.
b) The ionization of donor impurities creates an increased electron concentration distribution.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

7. Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
• Inject Boron into the crystal with an implant step.
• Boron is Group3 element is has 3 electrons in its outer shell (one less than silicon)
• This introduces holes into the lattice which can be made mobile by applying heat. This
gives us a hole current
• The result is a P-type semiconductor (p for positive current carrier)
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
Si -
-
-
-
-
Si -
-
-
-
Si -
-
-
-
-
+
+
B -
-
-
+
B -
-
-
+
B -
-
-
+
+
+
+
-
-
- +

8. a) Acceptor level in an p-type semiconductor.
b) The ionization of acceptor impurities creates an increased hole concentration distribution
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

9.  Intrinsic material: A perfect material with no impurities.
 Thermal generation process produce free electron hole pair.
 Recombination Process  free electron releases its energy and
drops into a free hole in the valence band.
 Extrinsic material: donor or acceptor type semiconductors.
 Mass action law two types of carriers constant
 Majority carriers: electrons in n-type or holes in p-type.
 Minority carriers: holes in n-type or electrons in p-type.
 The operation of semiconductor devices is essentially based on
the injection and extraction of minority carriers.
)
2
exp(
Tk
E
npn
B
g
i 
ly.respectiveionsconcentratintrinsic&holeelectron,theare&& inpn
e.Temperaturisenergy,gaptheis TEg
2
inpn 

10. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Electron diffusion across a pn junction
creates a barrier potential (electric field)
in the depletion region.

11. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
A reverse bias widens the depletion region, but allows minority carriers to move freely with the applied field.

12. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Lowering the barrier potential with a forward bias allows majority carriers to diffuse across the junction.

14. E
CB
k–k
Direct Bandgap
(a) GaAs
E
CB
VB
Indirect Bandgap, Eg
k–k
kcb
(b) Si
E
k–k
Phonon
(c) Si with a recombination center
Eg
Ec
Ev
Ec
Ev
kvb VB
CB
Er
Ec
Ev
Photon
VB
(a) In GaAs the minimum of the CB is directly above the maximum of the VB. GaAs is
therefore a direct bandgap semiconductor. (b) In Si, the minimum of the CB is displaced from
the maximum of the VB and Si is an indirect bandgap semiconductor. (c) Recombination of
an electron and a hole in Si involves a recombination center .
© 1999 S.O. Kasap,Optoelectronics(Prentice Hall)

18.  Optical communication requiring data rate(bit rate)
100-200 Mb/s with multimode fibre
Coupled optical power tens of microwatts,
 LEDs are usually the best choice.
 LED less complex than laser diode.
 Since no thermal or optical stabilization.
LED have:
1.High radiance output.
2.Fast emission response time.
3.High quantum efficiency.

19. Radiance(brightness):is a measure ,in watts, of the optical power
radiated into a unit solid angle per unit area of the emitting surface.
High radiances necessary to couple sufficiently high optical power
levels into fiber.
Emission response time: the time delay between the application of
current pulse and the onset of optical emission.
Quantum efficiency: the fraction of injected electron hole pairs that
recombine radiatively.
To achieve High Radiance & High Quantum efficiency :
• confining the charge carriers
• stimulated optical emission to the active region of the pn junction,
where radiative recombination takes place.

20. 1.Carrier confinement :
used to achieve a high level of radiative recombination
yields high quantum efficiency
2.Optical confinement:
Prevent absorption of the emitted radiation by material surrounding the
pn junction.
Achieve carrier and optical confinement
LED configurations  Homojunctions and single and double
heterojunctions
Mostly double heterostructure used  two different alloy layers on
each side of active region .
Dual confinement  leads to both high efficiency and high radiance.

21. Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Cross-section drawing of a typical
GaAlAs double heterostructure light
emitter. In this structure, x>y to
provide for both carrier confinement
and optical guiding.
b) Energy-band diagram showing the
active region, the electron & hole
barriers which confine the charge
carriers to the active layer.
c) Variations in the refractive index;
the lower refractive index of the
material in regions 1 and 5 creates an
optical barrier around the waveguide
because of the higher band-gap
energy of this material.

22. Active light emitting region perpendicular to the axis of the fiber
Well is etched through the substrate of the device which fiber is then
cemented  accepted emitted light.
Circular active area50μm diameter,2.5μm thick
Emission Pattern120°half power beam width.

23. Isotropic pattern from surface emitter is called a Lambertian
pattern.
In this pattern source is equally bright when viewed from any
direction.
But power decrease as cosƟ
Ɵ angle between the viewing direction and the normal to the
surface
Power is down to 50% of its peak 
when Ɵ=60° so that total half power beam width is 120°

24. Active junction Region  incoherent light
Two guiding Layers.
Refractive index of guiding layer < Active region. But higher than
surrounding material
This structure forms  waveguide channel that directs the optical
radiation towards the fiber core.

25. Fiber core diameter50-100 μm
Contact stripes edge emitter 50-70μm
Length of active region  100-150μm
Emitted radiation pattern is  more directional.
No wave guide effect.
Half power beam width is 120

26. Semiconductor material used active layer of optical source
direct band gap.
Direct band gap  electron and hole recombine directly
without needing third particle.
Optical radiation high.
None of the normal single element semiconductor
 direct –gap materials many binary compounds are.
Materials: Group III element  Al, Ga, In
Group V Element P, As, Sb
Various ternary and quaternary combinations of binary
compounds Direct band gap materials
800-900 nm spectrum ternary alloy used.
Ratio x of alumininum arsenide to gallium arsenide
determines the band gap of alloy and wave length of peak emitted
radiation.
AsAlGa xx1

27. Full width half maximum:
The width of the spectral pattern at its half power point.
For LED FWHM36nm
Relation between energy and wavelength:
E=hv=hc/ λ
Eis energy, is in joules. λ wavelength
)eV(
240.1
m)(
gE


29. Quantum Efficiency and LED Power
Due carrier injection at devices contact excess electrons and holes  in
p-type and n-type material respectively.
When carrier injection stops  carrier density returns equilibrium value.
Generally  excess carrier density decays exponentially
n0 initial injected excess electron density.
τ--> Carrier life time.(Milliseconds to fraction of nano seconds)
Excess carrier  recombine  radiatively or nonradiatively.
Radiative Recombination photon energy =bandgap energy.
Non radiative recombination  includes self absorption carrier
recombination in hetero junction interfaces and auger process.

30. Constant current flow into an LED equilibrium condition
established
Total Rate(Carrier Generated)=Externally supplied+ thermally
generated rates.
J/qd Externally supplied
J current density
qelectron charge
d thickness of recombination region.
n/τthermal generation rate.
Rate Equation is
The equilibrium condition

31. Internal quantum efficiency= Ratio of the radiative recombination rate to
the total recombination rate.
τr radiative recombination life time
τnr radiative recombination life time
Recombination life time:

32. Quantum efficiency:
Simple homo junction LEDs50%
Double hetro junction60-80%
if Current injected into the LED  I,
total no.of recombination is  Rr+Rnr=I/q
Rr total no.of photons generated per second and each photon has hv
energy.
optical power is

33. To find the emitted power consider External quantum efficiency ηext
The ratio of the photons emitted from the LED to the number of internally
generated photons.
Consider reflection effects




dT
c
)sin2()(
4
1
0
ext 
2
21
21
)(
4
)0(tCoefficienonTransmissiFresnel:)(
nn
nn
TT


2
11
ext2
)1(
1
1If


nn
n 
2
11
int
intext
)1(
powr,opticalemittedLED


nn
P
PP 

34.  The frequency response of an LED depends on:
1- Doping level in the active region
2- Injected carrier lifetime in the recombination region, .
3- Parasitic capacitance of the LED
 If the drive current of an LED is modulated at a frequency of the
output optical power of the device will vary as:
 Electrical current is directly proportional to the optical power, thus we
can define electrical bandwidth and optical bandwidth, separately.

2
0
)(1
)(
i
P
P




i
currentelectrical:power,electrical:
)0(
log20
)0(
10logBWElectrical
Ip
I
)I(
p
)p(















35. 












)0(
)(
log10
)0(
)(
log10BWOptical
I
I
P
P  [4-17]
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

37. Laser  comes many forms  entire room size to grain of salt.
Lasing medium
Gas,
Liquid,
insulating crystal(Solid state),
Semiconductor.
For optical fiber laser sources  Semiconductor laser diode.
its similar to other lasers conventional solid state and gas Lasers.
Emitted radiation  Spatial and temporal coherence
output radiation highly monochromatic and the light beam very
directional.
Laser action  3 key Process
Photon absorption
Spontaneous emission
Stimulated emission

38. Laser Action

39. The meta stable state E2 has greater lifetime than the lower energy state
or ground state E1. Hence, more electrons are accumulated in the energy
state E2 than the lower energy state E1.

41. Optical communication system Requiring Bandwidth > 200MHz.
Laser diode
Response time <1 ns.
Optical bandwidths 2nm or less
Coupling  several tens of milliwatts power into optical fiber.
MFD small.
Multi layered hetero junction devices
LED
Double hetro junction  achieve optical and carrier confinement.
simpler construction.
Small temperature dependence of the emitted optical power
Absence of catastrophic degradation
Laser
Construction  more complicated
Additional requirement current confinement

42. Radiation in the laser diode generated within a Fabry perot resonator
cavity.

43. Pair of flat partially reflecting mirrors directed toward each other to
enclose the cavity.
Mirror facets constructed by making two parallel clefts along natural
cleavage planes of the semiconductor material.
Purpose of these mirror provide strong optical feedback in the longitudinal
direction.
Converting device to oscillator compensates for optical losses in the cavity.
Laser cavity  many resonant frequency.
Device will oscillate those frequency gain is sufficient to overcome the
losses.
Sides of cavity formed by roughening the edges reduce unwanted
emission in this direction.

45. Cleaved facets  not required for optical feedback.
Lasing action from bragg reflectors or periodic variations of the
refractive index.
Which are incorporated into the multilayer structure along the length of
the diode.
*********************************************************
Optical output needed only from front facet of the laser. one to be
aligned with an optical fiber.
Dielectric reflector can be deposited on the rear facet to reduce the
optical loss in cavity.
To reduce threshold current density and to increase external quantum
efficiency.
Reflectivities >98% have been achieved six layer reflector.
Distributed Feedback Laser

46. Modes of the cavity optical radiation within cavity of a laser diode sets up
a pattern of electric and magnetic field lines.
Two independent sets of TE and TM modes.
Each set of modes can  described in terms of the
1. Longitudinal modes
Related to length L of the cavity.
Determine the principal structure of frequency spectrum of emitted
optical radiation.
L much larger than lasing wavelength(1 um)
Many longitudinal mode exist.
2. Lateral Modes
lie in the plane of pn junction.
Depend on the side wall preparation and the width of the cavity
Determine the shape of the lateral profile of the laser beam.
3. Transverse modes
are associated with electromagnetic field and beam profile in the
direction perpendicular to the plane of the pn junction.
These modes important  determine laser characteristics.

47. Determine lasing condition and the resonant frequencies
Electro magnetic wave propagating in longitudinal direction(in-terms of
electric field phasor )
I(z)optical field intensity.
W optical radian frequency
Lasing condition at which light amplification becomes possible in
laser diode.
Requirement lasing population inversion achieved
Understood  optical field intensity I, absorption co efficient αλ
Gain coefficient –g
Stimulation emission rate proportional to intensity of the radiation.
The radiation intensity at a photon energy hV varies

48. ā  effective absorption coefficient of the material
Г optical confinement factor fraction of optical power in the active
layer.
Optical amplification  feedback mechanism of the optical cavity.
Repeated passes between  two partially reflecting mirror portion
of the radiation associated in these modes  highest optical gain.
Further amplified  each trip through the cavity.
Lasing occurs when gain of one or several guided modes is sufficient to
exceed the optical loss during one roundtrip through the cavity
Z=2L. R1,r2  fraction of optical radiation at lasers end 1& 2.
Reflection coefficient

49. At Lasing Threshold steady state oscillation take place magnitude and
the phase of the wave must be equal to original wave.
Amplitude I(2L)=I(0)
Phase exp(-j2βL)=1
lasing at threshold  optical gain =total loss αt
αend  mirror loss in the lasing cavity.
Lasing occur must g≥gth
Means pumping source maintain  population inversion must be strong
to support.

50. Only spontaneous radiation is emitted
low diode current
Spectral range& lateral beam emission
broad like LED
Lasing occur dramatic & sharply
increase power.
Spectral range& lateral beam both
narrow with increasing drive current.
Final spectral width 1nm
Threshold current Ithextrapolation of the lasing region of the power vs
current currve
High power output slope of the curve decrease because junction heating.
Lasing threshold optical gain
gth=β Jth
Β constant

51. Laser diode rate equation
Total carrier population determined by carrier injection,
spontaneous recombination & stimulated emission
Rate equation  in terms no. of photons
=stimulated emission+ spontaneous emission + photon loss.
in terms no. of electrons
=injection + spontaneous recombination +stimulated emission
C coefficient senrength of optical emission & absorption.
Rsp rate of Spontaneous emission.
τph--> photon life time
τs-->spontaneous recombination life time injection current density.

52. Steady state condition
dΦ/dt=dn/dt=0
n,Φ  non zero.
Rsp negligible,
dΦ/dt  positive when Φ is small.
N must exceed a threshold value nth in order for Φ to increase and
threshold value for the electron density nth
The above threshold value(nth) interms of Jth

53. Steady state condition at lasing threshold

54. External quantum efficiency
The no.of photons emitted per radiative electron –hole pair
recombination above the threshold level.
Experimentally
dP incremental change in the emitted optical power
dIincremental change in the drive current

55. Resonant Frequency
Lasing threshold equation
The cavity resonates when an integer number m of half wavelength spans
the region between the mirrors.
Gain  function of frequency or wavelength
In terms of gaussian form.

56. G(0) maximum gain proportional to population inversion
σ--> spectral width
λ0  wavelength.
Consider frequency or wavelength ,specing betwwen the modes of
multimode fiber.
We consider only logitudional modes.
Find frequency spacing  consider two successive modes of frequency

58. Laser Diode Structures and Radiation Patterns
Basic Requirement of laser diodes 
1. optical and carrier Confinement between hetro junction layers.
2. The current flow must be restricted laterally to a narrow stripe along
the length of the laser.
To achieve this Many methods proposed.
but are limiting  No. of lateral modes 
so that lasing confined single filament,
stabilizing the lateral gain
& ensuring relatively low threshold current.
Three  optical confinement methods used for bounding laser light
in the lateral direction.

59. GAIN GUIDED LASER
Narrow electrode stripe less than 8μm wide
Runs along the length of the diode.
The injection of electrons and holes into the device
alters the refractive index of the active layer directly below the stripe.
Injected carrier creates weak ,complex waveguide  that confines the
light laterally.
This Lasers can emit  optical powers  exceeding 100mW
Strong instabilities
High astigmatic

60. Index guided Lasers
More stable structure
Dielectric waveguide  fabricated in the lateral direction
Variations of real refractive index of various material control the lateral
modes
This lasers supports  fundamental transverse and longitudinal modes.
it is known as single mode laser
Emits  single ,well collimated beam light  intensity profile which is bell
shaped Gaussian curve.
Types 1. positive index wave confining Structures.
2. Negative index wave confining Structures.
In Positive index waveguide
central region  higher refractive index than the outer regions
All of the guided light is reflected at the dielectric boundary(just as core
cladding interfaces)
Proper choice change in the refractive index and width of the higher index
region  can make device supports  fundamental lateral mode.

61. In negative index wave guide
Central region of the active layer  lower refractive index than the outer
regions
Dielectric boundaries part of the light is reflected and rest is refracted into
surrounding material  and thus lost.
Radiative loss will be appear in far field radiation pattern.
Positive index is more popular.

62. Index guided lasers  can be made  using any one of fundamental
structures.
1. Buried hetero structure
2. Selectively diffused construction
3. Varying thickness structure
4. Bent layer configuration
Buried hetero structure:
One etches a narrow mesa stripe(1-2 um wide) in double hetero structure
material
Then mesa is embedded in -> high resistivity lattice matched n-type
material with an appropriate band gap and low refractive index.
800- 900 nm laser  GaAlAs material with GaAs active layer
1300-1600nm laser InP material with InGaAsP active layer
This configuration  strongly traps generated light  in a lateral
waveguide.

64. Selectively diffused construction
Chemical dopant zinc for GaAlAs Lasers and
Cadmium for InGaAsP Laser
Is diffused into the active layer immediately below the metallic contact
stripe.
The dopant changes the refractive index of the active layer to form a lateral
wave guide Channel

65. Varying thickness structure
A channel(mesa or terrace) etched into the substate
Layers of crystal are then regrown into channel  using liquid phase
epitaxy
This process fills depressions and partially dissolves the protrusions
This creating variations in the thickness of active region

66. Bent layer structure
This structure forms using vapor phase epitaxy to exactly replicate the
mesa configuration.
Active layer  constant thickness with lateral bends
Optical wave travels along flat top of the mesa in the active area
Lower index in outside of the material.

67. In addition to optical wave confinement,
Also restrict  the drive current tightly to the active layer so more than
60% of the current contributes to lasing.
Current confine methods 4 methods
1.Preferential dopant diffusion
2. Proton implantation
3. Inner stripe confinement
4. Regrowth of back biased pn junction.
Device architecture  blocks current on both sides of the lasing region
Achieved by high resitivity or reverse biased pn junction.
Which prevent the current from flowing while the devices is forward biased
under normal conditions.
Continuous active layer  current can be confined either above or below
the lasing region.

69. Single Mode Laser
High speed long distance communication  Needs single mode laser
Which must contain only  single longitudinal mode and a single
transverse mode
So thatSpectral width of the optical emission is very narrow.
One way is only one longitudinal modes is reduce the length L of the
lasing cavity to the point
But reduce l (example 250um to 25 um In 1300nm region) spacing will
increase 1 um to 10um.
However these  lengths make device hard to handle limited to optical
output power only few milliwatts.
Another method developed vertical cavity surface emitting laser
That have built in frequency selective grading and tunable lasers.

70. The special feature of vertical cavity surface emitting laser is light
emission is perpendicular to the semiconductor surface.
This feature facilitates integration of multiple lasers onto a single chip in
one or two dimensional array.
Which make them attractive for WDM applications
Active region  very small leads to low threshold current
Equivalent output power  compare edge emitting lasers
Modulation bandwidth  much larger higher photon densities  reduce
radiative lifetimes.

71. The frequency selective reflector is a
corrugated gating which is a passive
waveguide layer adjacent to the active region
Optical wave propagates parallel to this gating.
Operation distributed Bragg phase gating reflector.
Phase gating periodically varying refractive index
ne effective refractive index, k order of the gating
Laser Configurations using built in frequency selective reflector
Distributed Feedback (DFB)Laser Distributed Bragg Reflector (DBR)Laser
Distributed Reflector (DR)Laser

72. 1st order mode(k=1) provides  strongest coupling
But some times 2nd order gratings  used larger corrugation Fabrication
easier.

73. Modulation of Laser Diode
Process of imposing information on light steam  modulation.
1. Directly varying the laser drive current with information stream.
produce varying optical output power.
2. External modulator modify a steady optical power level emitted by
the laser.
External modulation  needed for high speed systems(> 2.5 Gb/s)
Varity of external modulator available  either separate device or
integral part of the laser transmitter package.
Limitations on direct modulation
Modulation rate depends on spontaneous (radiative) & stimulated
carrier life times and photon life times.
Spontaneous lifetime function of semiconductor band structure and
carrier concentration.
At room temperature Radiative lifetime  1 ns, GaAs material dopant
concentration10^19 cm^-3.
Stimulated carrier lifetime depends on optical density in the lasing
cavity order of 10ps.

74. Photon Life time  average time that the photon resides in the lasing cavity
before being lost either by absorption or by emission through the facets.
Laser diode  pulse modulated photon life time is much smaller than
carrier lifetime.
If the laser is completely turned off each pulse spontaneous  carrier life
time will limit the modulation rate.
Is needed  to achieve population inversion necessary to produce gain
sufficient to overcome the optical losses in the cavity.
Pulse modulation  carried out by  modulating laser only in operating
region above threshold.
Carrier life time is shorten to stimulated lifetime high modulation rates
are possible.

75. When using directly modulated laser diode in high speed system
Modulation frequency can be no larger than the frequency of the
relaxation oscillation of the laser field.
Relaxation oscillation both spontaneous lifetime and photon life time.
Analog Modulation  carried out by making drive current above
threshold  proportional to baseband information signal.

76. Temperature Effects
Important factor temperature dependence of the threshold current.
T0 Relative temperature insensitivity
Iz Constant

77. Lasing threshold changes laser ages.
If constant output power level is to be maintained temperature changes
or laser ages
Necessary to adjust dc bias current level
Possible Methods:
Optical feedback
Feedforward schemes
Temperature matching transistor
Predistortion technique

80. Photodetectors senses the luminescent power falling upon and it
converts the variation of this optical power into correspondingly
varying electric current.
Types:
Photomultipliers large size
Pyroelectric detectors speed is limited by detector cooling rate
Semiconductor based photoconductor
Phototransistors
Photodiodes small size, suitable material ,high sensitivity & fast
response time.
Types:pin photodetector & avalanche photodiode

81. The pin Photodetector
Generation of free electron hole pair  Photocarriers
Photocurrent with one electron flowing for every carrier pair generated
Diffusion Legthcharge carrier move a distance Ln,Lp
Time takes to electron recombine carrier lifetime τ
D diffusion Coefficient

82. Particular
semiconductor
material used
limited wave length
only
Si1.06um
Ge1.6um
gaAs0.9um

83. Quantum Efficiency
No.of electron hole pair generated per incident photon of energy
hv
η=No.of electron hole pair generated/No. of incident photon
η= (Ip/q) /(P0/hv)
Responsitivity R=Ip/P0
= ηq/hv

84. Avalanche Photodiodes
APDs internally multiply the primary signal photocurrent before it
enters the input circuitry of the following amplifier.
This  increase sensitivity photo current is multiplied before
encountering the thermal noise associated with receiver circuit.
Photogenerated carriers high electric field present
In this high field region  photogenerated electron or hole can gain
enough energy so that it ionizes bound electrons in the valance
band upon colliding with them.
This carrier multiplication  impact ionization
Newly created carriers  accelerated by high electric field.
Thus gaining enough energy to cause further impact ionization 
avalanche effect.

85. Reach-through arise from photo diode operation.
When low reverse bias voltage is applied  most potential drop is across
in pn+ junction
The depletion layer widens with increasing bias until certain voltage is
reached at which the peak electric field at the pn+ junction about 5-10%
below that needed to cause avalanche breakdown.
At this point depletion layer just reaches through to the nearly intrinsic
π region.
Reach through
Construction
High resistivity p
type material
Deposited on
epitaxial layer on
a p+ substrate
Configuration
is p+πpn+ reach
through
structure

86. Normal usage  RAPD fully depleted mode
Light enter p+ region absorbed in the π material it acts as a
collection region for the photogenerated carriers.
Absorbed  photon gives up its energy  creating electron hole pairs
Separated by electric field in the π region.
The photogenerated electrons drift through the π region in the pn+
junction where high electric field exists.
High field region  carrier multiplication takes place.
Ionization rate  The average no. of electron hole pairs created by a
carrier per unit distance traveled.

87. PHOTODETECTOR NOISE
In Optical communication system photodiode  generally required detect
weak optical signals.
Its required to amplification circuitry optimized ,so that given SNR
maintained
The power signal to noise ratio S/N at the output of an optical receiver
Photodetector noises arise from photon-to-electron conversion process.
Amplifier noise due to thermal noise in amplifier circuitry.
To achieve High SNR
1.Photodetector must  have high quantum efficiency
2.The photodetctor and amplifier noises should be kept as low as possible.

88. Noise Sources
If modulated signal of optical power P(t) falls on the detector ,the primary
photocurrent generated is
Pin/ APD photodiode the mean square signal current is

89. Noise Source:
1.Quantum noise
Arises from  statistical nature of production and collection of
photoelectrons, when optical signal is incident on a photo detector.
F(M) Noise figure
2.Photodiode dark current  current that continues to flow
through the bias circuit of the device.
When no light is incident on the photodiode.
It is combination of bulk and surface currents.
3.Surface dark current or surface leakage current  simply
leakage current.
It is dependent on surfaces defects ,cleanliness, bias voltage, and
surface AREA.
Reducing surface dark current  through the use of guard ring
structure

90. The photodetector load resistor contributes a mean square thermal
noise current
Signal to Noise Ratio:

91. DETECTOR RESPONSE TIME
Light enters  through p layer and Produce electron hole pair.
Those electron hole pair generated in the depletion region or within diffusion
length it will be separated by reverse bias voltage induced electric field.
Leading current flow in the external circuit.
Depletion Layer Photocurrent

92. Under steady state conditions
A photodiode area

94. Response Time
Depends on
1.Transit time of the pohotocarriers
2.Diffusion time of the photocarriers generated outside the depletion region
3. RC time constant of the photodiode and its associated circuit
Photodiode parameter:
1.Absorption coefficient αs
2.Depletion region width w
3. Photodiode junction and package capacitance
4. Amplifier capacitance
5.Detector load resistance
6.Amplifier input resistance
7.Photodiode series resistance
Photodiode series resistance  generally only a few ohms can be neglected
(comparison with 5 & 6)
First  transit time
The response speed of a photodiode is limited by  time takes
photogenerated carriers to travel across the depletion region.

95. Typical high speed silicon photodiode  10um depletion layer width
Response time  0.1ns.
Second diffusion time
Photocarriers should be generated in the depletion region or close to it that
the diffusion times are less than or equal to the carrier drift times.
Response time described by rise time and fall time of the detector output
when the detector is illuminated by step input of optical radiation.
Rise time 10 to 90% change
Fall time  90% to 10% change.

96. Electron hole pairs generated in
the n and p regions must slowly
diffuse to the depletion region
before they can be separated
and collected.
The fast carriers allow the device output to rise to 50% of its maximum value
Slow carriers cause a relatively long delay before the output reaches its
maximum value
To achieve high quantum efficiency 
depletion layer width must be much
larger than 1/αs, so that most of the light
will be absorbed.

97. Photo detector simply act as a simple RC low pass filter

98. Avalanche Multiplication Noise
Avalanche multiplication process statistical in nature
Since not every photogenerated carrier pair undergoes the same
multiplication.
Avalanche noise
Excess noise factor
The ratio of excess noise generated in avalanche photodiode to the noise
that would exist if all carrier pairs were multiplied by exactly M

100. Temperature effects

101. Comparisons of photodetector