EE232$Lecture$18-1 Prof.$Ming$Wu

EE"232"Lightwave"DevicesLecture"18:"Avalanche"Photodiode"(APD)

Reading:"Chuang"15.4"(2nd Ed)

Instructor:"Ming"C."Wu

University"of"California,"BerkeleyElectrical"Engineering"and"Computer"Sciences"Dept.

EE232$Lecture$18-2 Prof.$Ming$Wu

Avalanche)Photodiode)(APD)

EE232$Lecture$18-3 Prof.$Ming$Wu

Typical(APD(Structure:Separate(Absorption(and(Multiplication((SAM)(APD

EE232$Lecture$18-4 Prof.$Ming$Wu

Ideal&APD:&Injection&Impact&Ionization&Only

1

( )( )

: electron ionization coefficient [cm ]

Electron current distribution along the field:

( ) (0)

Multiplication factor:( )(0)

n

n

nn n

n

xn n

Wnn

n

dJ xJ x

dx

J x J e

J WM e

J

!

!

!

! "

=

=

= =

EE232$Lecture$18-5 Prof.$Ming$Wu

( )nJ x

Practical(APD:(Both(Electron(and(Hole(produce(Impact(Ionizations

( )

( )

1

1

( )( ) ( )

( )( ) ( )

: electron ionization coefficient [cm ]

: hole ionization coefficient [cm ]

( ) ( ) 0

( ) ( ) constant

( )( )

nn n p p

pn n p p

n

p

n p

n p

nn p n p

dJ xJ x J x

dxdJ x

J x J xdx

dJ x J x

dxJ x J x J

dJ xJ x J

dx

! "

! "

!

"

! " "

#

#

$ = +%%&%# = +%'

+ =

+ = =

# # =

EE232$Lecture$18-6 Prof.$Ming$Wu

Multiplication+Factor

( )

( )( )

( )

'

0

(1 )

Multiplication factor:1

(0)1 '

1

1 1

Define

1

n p

n p

n p

n

n Wxn

n

nWn

n p

n p

Wn p

p

n

n k W

JM

Jdx e

Me

e

k

kM

e k

! "

! "

! "

!

!

!! "

! "

! "

"!

# #

# #

# #

# #

= =#

=# #

#

#=

#

=

#=

#

$

0 1 2 3 4 51

10

100

1 103!

1 104!

Mn 0 x, ( )

Mn 0.2 x, ( )

Mn 0.9 x, ( )

x

nM

nW!

0k =

0.2k =

0.9k =

EE232$Lecture$18-7 Prof.$Ming$Wu

Response'Time

Response time = transit time in absorption region + multiplication time

Usually

t m

n nm

e h e

abst

h

m t m

M kW M kWWv v v

Wv

! ! !

!

!

! ! ! !

= +

" = + #$$%$ =$&

' #

Gain-Bandwidth Product:

1 12 2

constant2

en n

m n

e

vG BW M MM kW

vG BWkW

!" !

!

# = $ = $

# = =

EE232$Lecture$18-8 Prof.$Ming$Wu

Noise&Figure

2

2

Excess noise factor (due to fluctuation in gain) :

1(1 ) 2

Noise Figure:10log

Small has small under high gain

Minimum noise figure = 3 dB occurs when 0

nn

n

F

MF k M k

MM

NF F

k F Mk

! "= = + # #$ %$ %

& '

=

=

EE232$Lecture$18-9 Prof.$Ming$Wu

K"factor)of)Common)Semiconductors

http://www.albisopto.com/avalanche3photodiode3basics/

EE232$Lecture$18-10 Prof.$Ming$Wu

APD$Noises

( )( )

222

2

2 2

2

2

Signal is amplified by the average gain :

12

Shot noise is amplified by :

2

2

: photocurrent;

: background photocurrent: dark current

4

p opt

S p B D

p B D

p

B

D

BT

M

qi P M

M

i e I I I M dv

e I I I F M dv

I

II

k T vi

!"

# $= % &' (

= + +

= + +

)=

( )

22

2

12

42

opt

Bp B D

RqP M

SNRk T ve I I I F M dvR

!"

# $% &' (=

)+ + +

1 10 6!" 1 10 5!" 1 10 4!" 1 10 3!" 0.01 0.1 11 104"

1 105"

1 106"

1 107"

1 108"

1 109"

1 1010"

1 1011"

1 1012"

ip

Slope=2at$low$Ip

Slope=1at$high$Ip

Photocurrent,$ip

SNR

EE232$Lecture$18-11 Prof.$Ming$Wu

( )

( )

22

2

2

12APD: 42

p-i-n: set 1, 1

12

42

opt

Bp B D

opt

Bp B D

qP MSNR

k T ve I I I F M dvR

M F

qPSNR

k T ve I I I dvR

!"

!"

# $% &' (=

)+ + +

= =

# $% &' (=

)+ + +

ip

1 10 9!" 1 10 7!" 1 10 5!" 1 10 3!" 0.11

10

100

1 103"

1 104"

1 105"

1 106"

1 107"

1 108"

1 109"

1 1010"

ip

SNR$Comparison$of$p/i/n$and$APD

Photocurrent,$ip

SNR

APD

p/i/n