5_photodetectors

8/10/2019 5_PHOTODETECTORS

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September 2004

PHOTODETECTORSPHOTODETECTORS

FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIA

ASMS05 FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIA

CHAPTER 5

http://en.wikipedia.org/wiki/Photodiode
http://upload.wikimedia.org/wikipedia/en/f/f8/Photodiode-closeup.jpg


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September 2004 Prof. John Watson



A detectors function is to convert the received

optical signal into an electrical signal, which is then

amplified before further processing.

Therefore when considering signal attenuation along

the link, the system performance is determined at the

detector.

Improvement of detector characteristics and

performance thus allows the installation of fewer

repeater stations and lowers both the capital

investment and maintenance costs.

INTRODUCTIONINTRODUCTION


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INTRODUCTIONINTRODUCTION



(a) High sensitivity at the operating wavelength.

(b) High fidelity - to reproduce the received signal waveform

with fidelity (eg: for analog transmission the response ofthe photodetector must be linear with regard to the optical

signal over a wide range.

(c) Large electrical response to the received optical signal -

the photodetector should produce a maximum electricalsignal for a given amount of optical power

(d) Short response time. (pn-msec, PIN/APD - nsec)

(e) Minimum noise.

(f) Stability.(g) Small size

(h) Low bias voltage.

(i) High reliability.

(j) Low cost.

Requirements:


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CLASSIFICATION OF PHOTODETECTORSCLASSIFICATION OF PHOTODETECTORS

Classify detectors by mechanism of response to incident light detectors of photons

detectors of heat

Semiconductor detectors

e-h pairs are created by excitation with incident light

two types of semiconductor diode

bulk semiconductor - (LDR change resistance when illuminated)

junction diode - pn diode, pin diode, phototransistor

Photoemissive detectors

electrons ejected from a photosensitive material on irradiation bylight

photomultiplier tube (emits ellectrons when illuminated)

Thermal detectors

heating effect of light, raises the temperature of the irradiatedmaterial

with the subsequent change in its electric properties

thermopile, pyroelectric detector

FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIAASMS05 FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIA


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OPTICAL DETECTION PRINCIPLESOPTICAL DETECTION PRINCIPLES


A photon incident in or near the depletion region of this

device which has an energy greater than or equal to the

bandgap energy Eg of the fabricating material (i.e. hf >

Eg) will excite an electron from the valence band into the

conduction band. This process leaves an empty hole in

the valence band and is known as the photogeneration

of an electron-hole (carrier) pair Absorption





Absorption

)exp(1( dPP ooabs =

The absorption of photons in a

photodiode to produce carrier

pairs and thus a photocurrent:

is dependent on theabsorption coefficient 0 ofthe light in thesemiconductor used tofabricate the device.

0 strongly dependent onwavelength as illustrated.Light falling on aphotodiode - partiallyabsorbed and partiallytransmitted.

Pabs = power absorbed andPo = power incident.

d is the width of theabsorption region.





where :

e is the charge on electron, r is the Fresnel reflection coefficientat the semiconductor-air interface and d is the width of theabsorption region.

When 0 goes to zero, Pabs goes to zero.When 0 goes to infinity Po = Pabs.

At a specific wavelength the photocurrent Ip produced byincident light of optical power P0 is given by:

( ) ( )[ ]dhf

rePI p 00 exp11 =





For a given semiconductor material, the photodiode can

detect only wavelengths

< c = hc/Eg

If Eg is specified in eV , then c can be written as

c = 1.24/Eg (m)

For wavelengths longer than c , the photons will travel

through the material without interaction.Si 1100 nm and

InGaAs 1700 nm.



PERFORMANCE PARAMETERSPERFORMANCE PARAMETERS

Spectral Response

All parameters vary with wavelength

Match peak emission wavelength of source with peakresponse of detector

Quantum Efficiency ( , QE) is defined as the fraction of incident photons which are

absorbed by the photodetector (photogenerated carriers) andgenerate electrons to incident photons.

= number of electrons collectednumber of incident photons

= (re/rp) x 100%

re is the rate of photoelectron generation

rp is the incident photon rate Values in the range 5% to 30% are typical


The ratio of the number of

photogenerated carriers to

incident photons and thus a

unitless quantity.




Responsivity (R)


where Ip is the output photocurrent in Amperes and P0 is theincident optical power in Watts. Typical value ranges from 0.5 A/W

to 1.0 A/W

The relationship for Rmay be developed to include quantumefficiency as follows:

1= AWhf

eR

1

0

= AWP

IR

p

The responsivity is a useful parameter as it gives the transfer

characteristic of the detector (i.e. photocurrent per unit incident

optical power) and is defined as:




Responsivity (R)


This equation may be developed a further stage to include the

wavelength of the incident light where is in nm.

The ideal responsivity against

wavelength characteristic for asilicon photodiode with unit

quantum efficiency is as shown.

1248

==

hc

eR





Response Time ( tr)

A measure of how long it takes a detector to respond to achange in light power falling on it

usually measured with reference to a square inputpulse

both rise and fall times are often quoted A good working rule is

choose detector with rise time of ~1/10 of shortest pulseduration to be detected




Noise Equivalent Power (NEP) All detectors produce a small output signal in darkness

Sets a lower limit to the intensity of detected light

In photomultipliers and semiconductor devices, the backgroundsignal is thermally generated

A few electrons are excited into the conduction energy levels toproduce a background current

the dark current

dark currents are typically in the picoamp to nanoamp region. To be seen by the detector the incident light needs to produce an

output greater than that of the noise signal

NEP is defined as the radiant flux which produces

an output signal equal in magnitude to thatproduced by the noise signal

The units of NEP are W/HzDepends on reciprocal of square root of bandwidth, detector area &

temperatureGood detectors have a NEP value of around 10-12 to 10-14 W/Hz



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EXAMPLEEXAMPLE

Calculate the responsivity of a photosensitive

material with a quantum efficiency of 1% at 500 nm.

Solution

Responsivity is

= 0.01 x 1.6x10-19 J x 500x10-9 m /(6.63x10-34 J s x 3x108 m/s)

= 4.0 mA W-1

1248==

hc

eR



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SEMICONDUCTOR PHOTODIODESSEMICONDUCTOR PHOTODIODES

Semiconductor diodes can be classified into two categories:

with internal gain (APD)

without internal gain (PN and PIN photodiode)


Semiconductor photodiodes without internal gain generate a

single electron hole pair per absorbed photonSemiconductor photodiodes with internal gain, at the

depletion region, while most of the photons are absorbed and

the primary carrier pairs generated, there is a high field

region in which holes and electrons can acquire sufficientenergy to excite new electron-hole pairs.


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THE JUNCTION PHOTODIODETHE JUNCTION PHOTODIODE

pn junction is operated under reverse potential bias positive terminal is connected to n-side and negative to p side

Electrons in the n-side are pulled out of the depletion region and

holes are pulled from the p side

This leaves more fixed ions (immobile carriers) of both kindsin the depletion region causing it to widen

Consequently, the energy barrier increases in accordance

with the applied potentialThe width of the depletion region is therefore dependent upon

the doping concentrations for a given applied reverse bias (i.e.

the lower the doping, the wider the depletion region).


Under equilibrium conditions a potential barrier, Vo, existsacross the depleted areas on either side of the pn-junction

no net current flows through the diode.


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pnpn--JUNCTION PHOTODIODEJUNCTION PHOTODIODE


Under illumination, the

photogenerated electron-hole

pairs separate and drift under

the influence of the electric field,

whereas outside this region the

hole diffuses towards the

depletion region in order to be

collected.

The diffusion process is very

slow compared to the drift

process and thus limits the

response of the photodiode.

It is therefore important that the

photons are absorbed in the

depletion region.

The depletion region width in a p-n

photodiode is normally 1-3 m and is

optimized for the efficient detection of

light at a given wavelength.


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pnpn--JUNCTION PHOTODIODEJUNCTION PHOTODIODE


Typical output characteristics for the reverse-biased p-n photodiode.The different operating conditions may be noted moving from no

light input to a high light level.

PHOTODIODEPHOTODIODE


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PIN PHOTODIODEPHOTODIODE


PIN diode is a variation on standard

pn-diode

An intrinsic (pure) layer of

semiconductor is fabricated between the

p and n-types

Depletion layer widens

Internal electric field is maintained over a

wider layer

Because very few electrons and holesare in this region

Its resistivity is low

Only a small reverse bias is needed

to increase the depletion region Stretches almost entire way

between the terminals

Very fast response times

A few nanoseconds or less

PHOTODIODEPHOTODIODE


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The front illuminated photodiode when

operating in the 0.8-0.9 m band (Fig. (a))

requires a depletion region of between 20

and 50 m.

The side illuminated structure (Fig. (b)),where light is injected parallel to the

junction plane, exhibits a large absorption

width and hence is particularly sensitive at

wavelengths close to the bandgap limit

(1.09m).

PIN PHOTODIODEPHOTODIODE


Germanium p-i-n photodiodes which span the

entire wavelength range of interest are also

commercially available, but the dark current isrelatively high.

Other material of interest is In1-xGaxAsyP1-y.

The structure for such a p-i-n photodiode is

shown in Fig. (1.0 to 1.7 m).


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AVALANCHE PHOTODIODESAVALANCHE PHOTODIODES -- with internal gain

Basic pn junction is highly doped Operated at high reverse bias

> 50 V usually

Diode operates in avalanche region of I-V characteristics

Electrons and holes which cross depletion region

gain enough energy to produce more electrons and holes

Avalanche multiplication

A guard-ring is fabricated around the active area Reduces leakage current if biased to same voltage as diode

Restricts avalanche effect to middle of illuminated area

Fast response

Internal amplification of number of electrons

Because of avalanche effect

The main advantage compared to pin photodiode is the

multiplication or gain factor, M.



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September 2004 Prof. John WatsonFACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIAASMS05 FACULTY OF ELECTRICAL ENGINEERING UNIVERSITI TEKNOLOGI MALAYSIA

AVALANCHE PHOTODIODES (APD)AVALANCHE PHOTODIODES (APD)

The new carriers created

by impact of ionizationGuard-ring around the activearea

Reduces leakagecurrent

Restricts avalancheeffect to middle ofilluminated area


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The response time limited by three factors.

(1) The transit time of the carriers across the absorption region (i.e. the

depletion width).

(2) The time taken by the carriers to perform the avalanche multiplication

process.

(3) The RC time constant incurred by the junction capacitance of the

diode and its load.

AVALANCHE PHOTODIODES (APD)AVALANCHE PHOTODIODES (APD)


Hence, although the use of suitable materials and structures may give risetimes between 150 and 200 ps, fall times of 1 ns or more are quite common

which limit the overall response of the device.

DRAWBACKS OF APDDRAWBACKS OF APD


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DRAWBACKS OF APDDRAWBACKS OF APD


(a) Fabrication difficulties due to their more complex structure and hence

increased cost.

(b) The random nature of the gain mechanism which gives an additional

noise contribution.

(c) The high bias voltages required (100-400 V).

(d) The variation of the gain with temperature as shown in Fig. below for asilicon reach-through APD (RAPD).

MULTIPLICATION FACTORMULTIPLICATION FACTOR


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MULTIPLICATION FACTORMULTIPLICATION FACTOR


The multiplication factor M is a measure of the internal gain provided by

the APD. It is defined as:

Where:

I is the total output current at the operating voltage.

Ip is the initial or primary photocurrent.

The gain M, increases with the reverse bias voltage, Vd

pI

IM =

n

BR

d

VV

M

=

1

1

where n = constant and VBR is the breakdown voltage of the detector

which is usually around 20 to 500 V.

II--V CHARACTERISTICS OF IRRADIATEDV CHARACTERISTICS OF IRRADIATED pnpn--DIODEDIODE


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With no illumination, the responseof the diode corresponds to the

situation described by the diode

equation

On increasing the irradiance, thereverse photon current

increases to iph and the whole

curve shifts downwards by this

amount The forward voltage drop across the

open circuit diode for a given

irradiance is given by the point at

which the curve intersects thevoltage axis at i = 0

For a given reverse voltage, say

VR, the near linear increase in iphwith irradiation can be seen.

II V CHARACTERISTICS OF IRRADIATEDV CHARACTERISTICS OF IRRADIATED pnpn DIODEDIODE


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Example 1:


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Example 2:


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Example 3:

Example 4:


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