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Application of photodiodes

A brief overview

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Quantum devices

• Absorption of a photon of sufficient energy elevates an electron into the conduction band and leaves a hole in the valence band.

• Conductivity of semi-conductor is increased.• Current flow in the semi-conductor is induced.

Conduction band

Energy gap

Valence band

Energylevel

+

-

Photon(hv)

Hole

Electron

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Photodiode structure

n- region

p+ Active AreaInsulation

Depletion region

Back Metalization

n+ Back Diffusion

FrontContact

RearContact

Incident light

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Photodiode fundamentals

• Based on PN or PIN junction diode– photon absorption in the depletion

region induces current flow

• Spectral sensitivityMaterial Band gap

(eV)Spectral sensitivity

silicon (Si) 1.12 250 to 1100 nm

indium arsenide (InGaAs) ~0.35 1000 to 2200 nm

Germanium (Ge) .67 900 to 1600 nm

I

P

N

+

-

h

RLIL

electron

hole

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Photodiode characteristics

• Circuit model– I0 Dark current (thermal)

– Ip Photon flux related current

• Noise characterization– Shot noise (signal current related)

– q = 1.602 x 10–19 coulombs– I = bias (or signal) current (A)

– is = noise current (A rms)

– Johnson noise (Temperature related)– k = Boltzman’s constant = 1.38 x 10–23 J/K– T = temperature (°K)– B = noise bandwidth (Hz)– R = feedback resistor (W)

– eOUT = noise voltage (Vrms)

qiis 2

kTBReout 4

Ip Rj Cj

Rs

I0

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Photodiode current/voltage characteristics

Isc (light level dependent)

CurrentV

olta

ge

Increasing Light level

Dark current

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Trans-impedance amplifier function

• Current to voltage converter (amplifier)• Does not bias the photodiode with a voltage as

current flows from the photodiode (V1 = 0)

• Circuit analysis

sf II

0oI

01 V

sffff IRIRV +

-

+

-

IsVout

Vf

Io

+

IfV1

sffout IRVV

–Note: current to voltage conversion

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Diode operating modes

• Photovoltaic mode– Photodiode has no bias voltage– Lower noise– Lower bandwidth– Logarithmic output with light intensity

• Photoconductive mode– Higher bandwidth– Higher noise– Linear output with light intensity

+

-

+

-

Vout

+

-

+

-

VoutVs

-

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For the photovoltaic mode

• I = thermal component + photon flux related current

• where I = photodiode current

V = photodiode voltage

I0 = reverse saturation current of diode

e = electron charge

k = Boltzman's constant

T = temperature (K)

= frequency of light

h = Plank’s constant

P = optical power

= probability that hv will elevate an electron across the band gap

h

ePeII kT

eV

10

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Circuit Optimization

• Burr-Brown recommendations (TI)• Photodiode capacitance should be as low as possible.

• Photodiode active area should be as small as possible so that CJ is small and RJ is high.

• Photodiode shunt resistance (RJ ) should be as high as possible.

• For highest sensitivity use the photodiode in a “photovoltaic mode”.

• Use as large a feedback resistor as possible (consistent with bandwidth requirements) to minimize noise.

• Shield the photodetector circuit in a metal housing.

• A small capacitor across RF is frequently required to suppress oscillation or gain peaking.

• A low bias current op amp is needed to achieve highest sensitivity