Lecture 9 Photodetection1

8/13/2019 Lecture 9 Photodetection1

1/9

ECE 4606 Undergraduate Optics Lab

Robert R. McLeod, University of Colorado 111

PhotodetectorsOutline

Lecture 9Photodetectors

Photodiode physics

Responsivity

Biasing

Noise

Shot noise

Dark current Thermal

Figures of merit

Pedrotti3, Chapter 17


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Robert R. McLeod, University of Colorado

DiodesA reminder of the basic physics

112

Anode Cathode

i

+ v

p-doped n-doped

Doping creates free holeson the anode side and free

electrons on the cathode

side.

Conventions

E

++++++

++++++

------

------

Q

x

E

x

V

x

Diffusion of charge across

junction establishes the

insulating depletion layer

The separation of charge

establishes a field which

counteracts diffusion.

The electric field causes

the anode to be at a lower

potential than the cathode.

Forward bias

opposes the built-in field, shrinking

the insulating depletion region,

increasing current flow.

Reverse bias

adds to the built-in field, expanding

the insulating depletion region,

resulting in very low current flow.

Long lead Short lead

Lecture 9Photodiode physics


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pn & pin Photodiodes

Depletion

E

-+

npDiffusion

e-&h

+drift

underEfield

e-&h

+may

diffuseinto

depletionregion

e-&h

+recombineand

donotcontributeto

current

pn: Response time faster than photoconductor (due to E) but limited by diffusion whichmay be as large as carrier recombination lifetime.

pin: Increased depletion layer width gives:

larger capture area

decreased capacitance (faster response)

dominated by diffusion, not drift

Depletion

E

-+

np

pn:

pin:

i

Diffusion

Diffusion

Diffusion

Lecture 9Photodiode physics


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Robert R. McLeod, University of Colorado

ResponsivityBasic input/output relation

114

W

A

P

iPR

[ ] [ ] [ ]

[ ] [ ]second

photons

photonJoules

second

photons

electronCoulombs

photonelectrons

=

=

hP

eip

[A]Pi

[W]P

We primarily use photodiodes as current sources and thus define the

response as the electrical current generated over the input optical power.

Photocurrent flows from the cathode to the anode, swept out of the

depletion region by the space-charge field.

24.1

]m[

==

hc

e

h

eR

Both quantities can be found by counting of quanta

Yielding

The quantum efficiency, , goesto zero when the photon energy

is less than the bandgap of the

semiconductor.

Lecture 9Responsivity


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Bias of photodiodes3 modes

Open circuitaka Photovoltaic

Solar cells

Low dark current

Slow response

Short circuit

Reversed biased

Drift field incr speed

Lower capacitance

Larger sensitive area

> R gives > sensitivity,< range, < BW

h

Pei

ieii

P

P

TKe

rsB

= 1

v

P

P

P

Lecture 9Biasing


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Shot noisePhotons are discrete

0 5 10 15 20

0

0.1

0.2

0.3

0.4

( )

!n

ennp

nn

=1

5

10

For uncorrelated photon arrival times, the probability of detecting n photons in a time

period T for which the average photon arrival rate is is Poissons distribution:n

( ) ( ) nnpnnn

n =

=0

2 Standard deviation (aka RMS) of

photon count, n

nn

nnSNR

n

Optical ==

Optical power SNR is average photon

count over standard deviation

Curves are different

Bars show 1

nSNRSNR OpticalElectrical =2

Because PElectrical= R i2 n2

n, Number of photons actually received

p(n),Probabilityofreceivingnphotons

n

1or1 == nSNROpticalShot noise limit when signal = noise is

average of 1 received photon per period,

(assuming = 1).

Lecture 9Photodetection noise


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Detector figures-of-meritNoise equivalent power & specific detectivity

RRR

==

2iNoiseRMSi

NEP

Noise equivalent power is incident optical signal required to generate

a photocurrent equal to the RMS noise current:

Variances add

Since both shot noise and Johnson noise variances are proportional to

bandwidth, some sources define NEP/Sqrt[B] :

[W]

RRR

==

B

BB

iNEP iNoiseRMS

B

/2

Hz

W

NEP

BAD

Since NEP is proportional to the square root of BW (B) and area (A), it

is common to define a figure-of-merit, the specific detectivity:

Lecture 9Photodetection noise

Lecture 9 Photodetection1

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