Ee 230 optical fiber communication lecture 11
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EE 230: Optical Fiber Communication Lecture 11. Detectors. From the movie Warriors of the Net. Detector Technologies. Features. Layer Structure. Simple, Planar, Low Capacitance Low Quantum Efficiency. MSM (Metal Semiconductor Metal) PIN APD Waveguide. Semiinsulating GaAs.

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EE 230: Optical Fiber Communication Lecture 11

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Ee 230 optical fiber communication lecture 11

EE 230: Optical Fiber Communication Lecture 11


From the movie

Warriors of the Net

Detector technologies

Detector Technologies


Layer Structure

Simple, Planar,

Low Capacitance

Low Quantum Efficiency


(Metal Semiconductor Metal)




Semiinsulating GaAs

Contact InGaAsP p 5x1018

Absorption InGaAs n- 5x1014

Contact InP n 1x1019

Trade-off Between

Quantum efficiency

and Speed



Low Noise

Difficult to make


Contact InP p 1x1018

Multiplication InP n 5x1016

Transition InGaAsP n 1x1016

Absorption InGaAs n 5x1014

Contact InP n 1x1018

Substrate InP Semi insulating

High efficiency

High speed

Difficult to couple into

Absorption Layer

Guide Layers

Absorption Layer

Contact layers


Photo detection principles

Photo Detection Principles

Bias voltage usually needed to fully deplete the intrinsic “I” region for high speed operation

Device Layer Structure

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

Current-Voltage Characteristic for a Photodiode

Characteristics of photodetectors

Characteristics of Photodetectors

• Internal

Quantum Efficiency


Quantum efficiency

• Responsivity


Fraction Transmitted

into Detector

Incident Photon Flux


Fraction absorbed in

detection region



Output current per unit incident light power; typically 0.5 A/W

Photodiode responsivity

Photodiode Responsivity

Detector sensitivity vs wavelength

Detector Sensitivity vs. Wavelength

Photodiode Responsivity vs. Wavelength

for various materials

(Albrecht et al 1986)

Absorption coefficient vs. Wavelength

for several materials

(Bowers 1987)

Pin photodiodes

PIN photodiodes

Energy-band diagram

p-n junction

Electrical Circuit

Basic pin photodiode structure

Basic PIN Photodiode Structure

Rear Illuminated Photodiode

Front Illuminated Photodiode

Pin diode structures

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

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

APD Detectors

Signal Current

APD Structure and field distribution (Albrecht 1986)

Apds continued

APDs Continued

Detector equivalent circuits











Detector Equivalent Circuits


Iph=Photocurrent generated by detector

Cd=Detector Capacitance

Id=Dark Current

In=Multiplied noise current in APD

Rd=Bulk and contact resistance

Msm detectors

MSM Detectors


Schottky barrier

gate metal

  • Simple to fabricate

  • Quantum efficiency: Medium

    • Problem: Shadowing of absorption

    • region by contacts

  • 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

Semi insulating GaAs

Simplest Version

To increase speed

decrease electrode spacing

and absorption depth



E Field

penetrates for

~ electrode spacing

into material

Non absorbing substrate

Waveguide photodetectors

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

Carrier transit time

Transit time is a function of depletion width and carrier drift velocity

td= w/vd

Detector capacitance

Detector Capacitance



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

Minimize by putting low doped “I” region between the P and N doped regions to increase W, the depletion width

W can be increased until field required to fully deplete causes excessive dark current, or carrier transit time begins to limit speed.



p-n junction

Bandwidth limit

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

PIN Bandwidth and Efficiency Tradeoff

Transit time


vsat=saturation velocity=2x107 cm/s

R-C Limitation



=4 ns/µm (slow)

Dark current

Dark Current

Surface Leakage

Bulk Leakage

Surface Leakage

Ohmic Conduction


via surface states

Bulk Leakage




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

Signal to Noise Ratio

ip= average signal photocurrent level

based on modulation index m where

Optimum value of m

Optimum value of M

where F(M) = Mx and m=1

Noise equivalent power nep

Noise Equivalent Power (NEP)

Signal power where S/N=1

Units are W/Hz1/2

Typical characteristics of p i n and avalanche photodiodes

Typical Characteristics of P-I-N and Avalanche photodiodes



  • 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

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