Download EE 230: Optical Fiber Communication Lecture 11

EE 230: Optical Fiber Communication Lecture 11
Detectors
From the movie
Warriors of the Net
Detector Technologies
Layer Structure
MSM
Simple, Planar,
Low Capacitance
Low Quantum Efficiency
Semiinsulating GaAs
(Metal Semiconductor Metal)
InGaAsP p 5x1018
InGaAs n- 5x1014
InP
n 1x1019
Contact
Absorption
Contact
PIN
Contact
Multiplication
Transition
Absorption
Contact
Substrate
APD
Waveguide
InP
InP
InGaAsP
InGaAs
InP
InP
Absorption Layer
Absorption Layer
Contact layers
Trade-off Between
Quantum efficiency
and Speed
Gain-Bandwidth:
p 1x1018
120GHz
n 5x1016
16
Low Noise
n 1x10
14
Difficult to make
n 5x10
Complex
n 1x1018
Semi insulating
Guide Layers
Key:
Features
High efficiency
High speed
Difficult to couple into
Photo Detection Principles
Device Layer Structure
Bias voltage usually needed
to fully deplete the intrinsic “I”
region for high speed
operation
Band Diagram
showing carrier
movement in E-field
Light intensity as a
function of distance below
the surface
Carriers absorbed here must
diffuse to the intrinsic layer
before they recombine if they are
to contribute to the photocurrent.
Slow diffusion can lead to slow
“tails” in the temporal response.
(Hitachi Opto Data Book)
Current-Voltage Characteristic for a
Photodiode
Characteristics of Photodetectors
• Internal
Quantum Efficiency
i 
Number of Collected electrons
 1  e W 
Number of Photons *Entering* detector
•External
Quantum efficiency
e 
i /q
Number of Collected electrons
 ph
 1  Rp  1  e W 
Number of Photons *Incident* on detector Po / h
• Responsivity
R
i
Photo Current (Amps)
q
 ph 
1 Rp  1 eW 
Incident Optical Power (Watts) Po h
Fraction Transmitted
into Detector
•Photocurrent
i ph
P 
 q  o  1  Rp 1  e W   RPo
 h 

Incident Photon Flux
(#/sec)

Fraction absorbed in
detection region
Responsivity
Output current per unit incident light
power; typically 0.5 A/W
e
R
M
h
Photodiode Responsivity
Detector Sensitivity vs. Wavelength
Absorption coefficient vs. Wavelength
for several materials
Photodiode Responsivity vs. Wavelength
for various materials
(Bowers 1987)
(Albrecht et al 1986)
PIN photodiodes
Energy-band diagram
p-n junction
Electrical Circuit
Basic PIN Photodiode Structure
Rear Illuminated Photodiode
Front Illuminated Photodiode
PIN Diode Structures
Diffused Type
(Makiuchi et al. 1990)
Diffused Type
(Dupis et al 1986)
Etched Mesa Structure
(Wey et al. 1991)
Diffused structures tend to have lower dark current than mesa etched structures although they are
more difficult to integrate with electronic devices because an additional high temperature processing
step is required.
Avalanche Photodiodes (APDs)
• High resistivity p-doped layer increases
electric field across absorbing region
• High-energy electron-hole pairs ionize
other sites to multiply the current
• Leads to greater sensitivity
APD Detectors
 q 
Signal Current i s  M   P
 h 
APD Structure and field distribution (Albrecht 1986)
APDs Continued
Detector Equivalent Circuits
Rd
Iph
Id
Cd
PIN
Rd
Iph
Id
Cd
In
APD
Iph=Photocurrent generated by detector
Cd=Detector Capacitance
Id=Dark Current
In=Multiplied noise current in APD
Rd=Bulk and contact resistance
MSM Detectors
Light
Schottky barrier
gate metal
•Simple to fabricate
•Quantum efficiency: Medium
Problem: Shadowing of absorption
region by contacts
Semi insulating GaAs
Simplest Version
•Capacitance: Low
•Bandwidth: High
Can be increased by thinning absorption layer and
backing with a non absorbing material. Electrodes
must be moved closer to reduce transit time.
•Compatible with standard electronic processes
GaAs FETS and HEMTs
InGaAs/InAlAs/InP HEMTs
To increase speed
decrease electrode spacing
and absorption depth
Absorption
layer
Non absorbing substrate
E Field
penetrates for
~ electrode spacing
into material
Waveguide Photodetectors
•Waveguide detectors are suited for very high bandwidth applications
•Overcomes low absorption limitations
•Eliminates carrier generation in field free regions
•Decouples transit time from quantum efficiency
•Low capacitance
•More difficult optical coupling
(Bowers IEEE 1987)
Carrier transit time
Transit time is a function of depletion
width and carrier drift velocity
td= w/vd
Detector Capacitance
xp
xn
P
N
Capacitance must be minimized for high
sensitivity (low noise) and for high speed
operation
Minimize by using the smallest light collecting
area consistent with efficient collection of the
incident light
C
A
W
Minimize by putting low doped “I” region
between the P and N doped regions to
increase W, the depletion width
p-n junction
w  xp  xn
W can be increased until field required to fully
deplete causes excessive dark current, or
carrier transit time begins to limit speed.
For a uniformly doped junction
1/ 2
A  2q

C
Nd
2 Vo  Vbi 
1/ 2
2(Vo  Vbi ) 
W


 qNd

Where:
=permitivity
q=electron charge
Nd=Active dopant density
Vo=Applied voltage V bi=Built in potential
A=Junction area
Bandwidth limit
C=0K A/w
where K is dielectric constant, A is area, w
is depletion width, and 0 is the
permittivity of free space (8.85 pF/m)
B = 1/2RC
PIN Bandwidth and Efficiency Tradeoff
Transit time
=W/vsat
vsat=saturation velocity=2x107 cm/s
R-C Limitation
 RC  Rin
A
W
Responsivity
R
q
1  Rp  1  e W 

h
Diffusion
=4 ns/µm (slow)
Dark Current
Surface Leakage
Bulk Leakage
Surface Leakage
Bulk Leakage
Ohmic Conduction
Diffusion
Generation-recombination
via surface states
Generation-Recombination
Tunneling
Usually not a significant noise source at high bandwidths for PIN Structures
High dark current can indicate poor potential reliability
In APDs its multiplication can be significant
Signal to Noise Ratio
i p2 M 2
S

2


N 2q I p  I D M F M B  2qI L B  4k BTB / RL
ip= average signal photocurrent level
based on modulation index m where
i
2
p

2 2
p
m I
2
Optimum value of M
M
x2
opt
2qI L  4k BT / RL

xqI p  I D 
where F(M) = Mx and m=1
Noise Equivalent Power (NEP)
Signal power where S/N=1
Units are W/Hz1/2
h
4kT
x
NEP 
2eI D M  2
e
M RL
Typical Characteristics of P-I-N and
Avalanche photodiodes
Comparisons
• PIN gives higher bandwidth and bit rate
• APD gives higher sensitivity
• Si works only up to 1100 nm; InGaAs up
to 1700, Ge up to 1800
• InGaAs has higher  for PIN, but Ge
has higher M for APD
• InGaAs has lower dark current