Semiconductor Optical Communication Components and …nptel.ac.in/courses/117104022/Lectures/Lec32.pdf · Semiconductor Optical Communication ... Amplifier Quantum Efficiency ...

Professor, Department of Electrical Engineering,

Laser Technology Program,

Indian Institute of Technology, Kanpur

Prof. Utpal Das

http://www.iitk.ac.in/ee/faculty/det_resume/utpal.html

Lecture 32: Photodiode Responsivity and Noise

Semiconductor Optical Communication

Components and Devices

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 500 1000 1500

Sp

ec

tral R

esp

on

siv

ity (

A/W

)

Wavelength (nm)

Si

AlGaAs/GaAs

InGaAs

Ge

InGaAs/InP

=100%

0

20

40

60

80

100

200 400 600 800 1000 1200

Qu

an

tum

Eff

icie

ncy (

%)

Wavelength (nm)

APD

Photodiode

PMT

Quantum Efficiency

Reverse-Bias

photocurrent

Dark current,

Responsivity (A/W),

Bandwidth

PIN: Highly linear,

Low Idark Detector is

followed by a

Transimpedance

Amplifier

Quantum Efficiency

(l) = No. of e- created / photon,

l

gl

p+

p+

p+

n+

n+

n+

n+

GaAs

In0.1Ga0.9As

In0.19Ga0.81As

Undoped In0.36Ga0.64As/GaAs

SLS

In0.19Ga0.81As

In0.1Ga0.9As

In0.01Ga0.99As

GaAs Substrate

1.0 mm

0.4 mm

0.2 mm

0.2 mm

0.6 mm

0.2 mm

Lz = 100 , LB = 177

2.3 mm

e

ph

rNo. of electrons collected= =

No. of incident photons r

o

op

h

h

p

p

oe h

Pr =r =

Pr

qPI =

h

h

h

=

int1 1 WdR e

oP oRP

Responsivity

0.01

0.1

1

0.8 0.9 1 1.1 1.2 1.3 1.4

Re

sp

on

siv

ity (

A/W

)

Wavelength (mm)

1gl

2gl

l

r gives the

respective rates

Valence

Bands

Conduction

Bands

Energy

0.6 0.8 1 1.2 1.4 1.6 1.8

Res

po

nsiv

ity

(A

/W)

Wavelength (mm)

Si

InGaAs

Ge

10%

30%

50%

70%

90%Quantum

Efficiencies

Responsivity

Absorption only for

gE

Top View

Contact

Pad

A A

Back Contact

n-GaAs

substrate

i-GaAs

absorbing

layer

p-AlGaAs

contact layer

Metal

contact ring

Anti-reflection

coating

x

d

0

hn

AR coating

p electrode

n+ InGaAsP (filter B)

n+ InGaAsP (filter A)

n- InP buffer

n- InGaAs absorption layer

n- InP window

InP substrate

p electrode

p+ layer

Passivation

1.3 mm

1.55 mm

Mesa Vertical PIN Photodiode (VPD)

Optical Window with

Silicon Nitride based

AR coating

Polyamide Fill on

Silicon NitrideAir Bridge

PIN Photodiode Noise - I

Req= Equivalent Load Resistance

kB= BoltzmansConstant, T = Temperature, B = Bandwidth, and

d) Additional noise sources are 1/f noise (flicker),

(typically = 2 and b = 1, f is the frequency of

operation.)

I: Detector Generated Noise

a) Thermal (or Nyquist or Johnson Noise):

due to random path of a carrier in conduction

b) Shot Noise: Due to random nature of

carriers overcoming a potential barrier

e) Photon Noise: due to random nature of photon

emission giving a probability P of having

exactly N photons for an average of k photons.

Other less dominant noise mechanisms are (f) Temperature fluctuations,

(g) Mechanical Vibrations, and (h) Background radiation.

2th B en

e

qB

q

4k TB=i 4k T[

RBG] =

2sh average darkn[i ] =2q i +i B2

eq2G-R 2 2n

c

4q i G B[i ] =

1+

21/f n

K i Bi =

f

k -NN e

P(k|N)=k!

c) Generation-Recombination Noise: Due

to random nature of the generation and

recombination process. tc is the

recombination time constant.

PIN Photodiode Noise - II

i 0 JP P 1 m f t W

ph 0 0i P 1 mf t A The photocurrent is

The average photocurrent is

Where I = Id + Iph, the sum of the dark current and the photocurrent.

As an example, assume that the light wave is intensity modulated with modulation

index m

ph 0 0I P

s phi t =I m.f t A

2 2 22

phsh

2

n d ph

I m f ti tS

N i t 2q I I B

and the signal component of the photocurrent is

The signal-to-noise ratio is defined as the ration of signal

power to noise power or the ratio of the squared currents.

Then

Let the incident optical power be:

ph dI I

2

ph 2 2IS

m f tN 2qB For

Where f(t) is the modulating signal and mJ the modulation depth

2 2 2 2sig ph

2 2n

B d ph background 2 2c

i t I m f tS= =

N i t 4q i G4k TG+2q I +I + P + B

1+

Or

S

SNR dB 10N

log

PIN Photodiode

Noise - III1/f

G-R (less in PIN)

Shot

Thermal (for low temp)

Where the responsivity is:

If the temperature (ToK) is kept low for shot noise limited operation at high

frequency operation of PIN diodes.

ln(f)

2Nln[ i ( ) ]t

2 1f (t) =

2

And for a Sinusoidal signal

in 0 m mP t P P Cos t

0 mP P

ph 0 0 m mI P P Cos t

If one also assum

es the dark current to be negligible and

PIN Photodiode Noise - IVMinimum detectable signal is at S/N=1 and

Pm=Noise Equivalent Power (NEP), for B=1, and for f(t)=Cosmt,

Specific Detectivity(D*) = (A) / (NEP), where A is the detector area.

And Detectivity is defined as: D=[NEP]-1

Where the the field of view is important, i.e. background limited detection

Optics-IndependentSpecific Detectivity :

(D**) = D*.NA, {cm.Hz1/2Ster1/2}/W, where NA is theNumerical Aperture.

Needless to say that when the other noise terms are non-negligible NEP

has to be calculated from a solution of a quadratic equation for NEP

and the NEP is usually nW/Hz pW/Hz 10fW/Hz

o o4qP 4hcPNEP= W/ Hz

1 1

** * 2 2D D sin cmHz ster / W

Considering only shot noise to be present

NEP 4qConsidering Po=Pm, i.e. mJ=1, The Noise equivalent power

Carrier transit Time (uniform generation)

e ee e

e d

Nqvqi N

Wt h h

h

d

Nqvi

W

As the number of excess carriers,Ne&Nh are decreasing function of

time, the currents ie&ih are also decreasing functions of time

and

ph e e h hd

qit NtvNtv

W

Let Ne(0) = Nh(0) = No 0o

ph e h

d

qNi vv

W

1e o eNtNtt & 1h o hNtNtt 1 0oee e e

d

qNvit t t

Wt t

& 1 0ohh h hd

qNvit t t

Wt t

Hole Velocity is less than electron velocity

11

ohh

d

Wd e h

qIvi t

W

e t t

n

Cu

rren

t

Time

iph

ie

ih

te

th

Uniform

illumination

e hpho

i ii t

I

Review Questions1. A photon of l=1.55mm is absorbed by a lattice matched InGaAs/InP

PIN photodiode at room temperature. If the kinetic energy of the generated hole

is 0.5kBT, then find the kinetic energy of the generated electron. Given that the

electron and hole effective masses are 0.042mo and 0.5mo, respectively.

2. An optical signal Pin=[1.0mW]{1+0.1Cost} of l=1.5mm is normally incident ona DH InGaAs/InP PIN photodiode. The two semiconductors having band gaps

Eg(InP)=1.5 eV and Eg(InGaAs)=0.75eV. The absorption coefficients at

l=1.55mm are (InP)=1.0x102m-1 and (InGaAs)=1.0x106m-1. Assuminginternal quantum efficiency to be 0.8, calculate the total external quantum

efficiency of the detector for WInGaAs=0.5m, nr(InP)=3.4, nr(InGaAs)=4.2.

3. An incoherent detection system operates with a PIN photodiode R=0.45A/W at

l=1.55mm at 300oK with a dark current iD=1.0nA and when connected to a loadresistance of 50W the operational bandwidth is B=6.0MHz. The incident power

is Pi=Po+ PmCos(mt) and that the background radiation is neglected.Assuming Po=Pm, plot the SNR for 1mW > Po > 1nW.

4. A load resistance RL=100W is connected to a PIN detector of responsivity

0.4A/W at a l=1.0m and dark current of 1.0nA. Assuming the equivalent loadresistance (including the diode resistance and the input resistance of the

amplifier) RL, calculate the NEP of the detector. What is the minimum

detectable power if the operational bandwidth of the photodiode is 250MHz?