EE 230: Optical Fiber Communication Lecture 11

From the movieWarriors of the Net

Detectors

Detector Technologies

MSM(Metal Semiconductor Metal)

PIN

APD

Waveguide

Contact InP p 1x1018

Multiplication InP n 5x1016

Transition InGaAsP n 1x1016

Absorption InGaAs n 5x1014

Contact InP n 1x1018

Substrate InP Semi insulating

Semiinsulating GaAs

Contact InGaAsP p 5x1018

Absorption InGaAs n- 5x1014

Contact InP n 1x1019

Absorption Layer

Guide Layers

Simple, Planar, Low CapacitanceLow Quantum Efficiency

Trade-off Between Quantum efficiency and Speed

High efficiencyHigh speedDifficult to couple into

Gain-Bandwidth: 120GHzLow NoiseDifficult to makeComplex

Key: Absorption Layer

Contact layers

Layer Structure Features

Photo Detection Principles

(Hitachi Opto Data Book)

Device Layer Structure

Band Diagramshowing carriermovement 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.

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

Current-Voltage Characteristic for a Photodiode

Characteristics of Photodetectors

Number of Collected electrons1

Number of Photons *Entering* detector

/Number of Collected electrons1 1

Number of Photons *Incident* on detector /

Photo Current (Amps)

Wi

ph We p

o

e

i qR e

P h

R

1 1Incident Optical Power (Watts)

1 1ph o

ph Wp

o

Wp

oi RP

i qR e

P h

R ePqh

• Internal Quantum Efficiency

•External Quantum efficiency

• Responsivity

•Photocurrent

Incident Photon Flux (#/sec)

Fraction Transmitted into Detector

Fraction absorbed in detection region

Responsivity

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

Mh

eR

Photodiode Responsivity

Detector Sensitivity vs. Wavelength

Absorption coefficient vs. Wavelength for several materials (Bowers 1987)

Photodiode Responsivity vs. Wavelength for various materials (Albrecht et al 1986)

PIN photodiodes

Energy-band diagram p-n junction

Electrical Circuit

Basic PIN Photodiode Structure

Front Illuminated Photodiode

Rear Illuminated Photodiode

PIN Diode Structures

Diffused Type (Makiuchi et al. 1990)

Etched Mesa Structure(Wey et al. 1991)

Diffused Type(Dupis et al 1986)

Diffused structures tend to have lower dark current than mesa etched structures although they aremore 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

Signal Current

s

qi M P

h

APD Structure and field distribution (Albrecht 1986)

APDs Continued

Detector Equivalent Circuits

Iph

Rd

Id Cd

PIN

Iph

Rd

Id Cd

APD

In

Iph=Photocurrent generated by detectorCd=Detector CapacitanceId=Dark CurrentIn=Multiplied noise current in APDRd=Bulk and contact resistance

MSM Detectors

Semi insulating GaAs

•Simple to fabricate

•Quantum efficiency: MediumProblem: Shadowing of absorption region by contacts

•Capacitance: Low

•Bandwidth: HighCan 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 processesGaAs FETS and HEMTs InGaAs/InAlAs/InP HEMTs

To increase speeddecrease electrode spacingand absorption depth

Absorptionlayer

Non absorbing substrate

E Fieldpenetrates for ~ electrode spacinginto material

Simplest Version

Schottky barriergate metal

Light

Waveguide Photodetectors

(Bowers IEEE 1987)

•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

Carrier transit time

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

td= w/vd

Detector Capacitance

p-n junction

xp xn

For a uniformly doped junction

Where: =permitivity q=electron charge

Nd=Active dopant density

Vo=Applied voltage Vbi=Built in potential

A=Junction area

C A

Ww xp xn

C A

2

2qVo Vbi

Nd

1/ 2

W 2(Vo Vbi)

qNd

1/ 2

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

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.

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 = 1/2RC

PIN Bandwidth and Efficiency Tradeoff

Transit time

=W/vsat

vsat=saturation velocity=2x107 cm/s

R-C Limitation

Responsivity

Diffusion

=4 ns/µm (slow)

RC in

AR

W

1 1 W

pRq

R eh

Dark Current

Surface Leakage

Bulk Leakage

Surface Leakage

Ohmic Conduction

Generation-recombinationvia surface states

Bulk Leakage

Diffusion

Generation-Recombination

Tunneling

Usually not a significant noise source at high bandwidths for PIN StructuresHigh dark current can indicate poor potential reliabilityIn APDs its multiplication can be significant

Signal to Noise Ratio

ip= average signal photocurrent level

based on modulation index m where

LBLDp

p

RTBkBqIBMFMIIq

Mi

N

S

/422 2

22

2

222 pp

Imi

Optimum value of M

where F(M) = Mx and m=1

Dp

LBLxopt IIxq

RTkqIM

/422

Noise Equivalent Power (NEP)

Signal power where S/N=1

Units are W/Hz1/2

L

xD RM

kTMeI

e

hNEP

2

42

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