[SOLVED] EEEE3089 2021-2022

PowerPoint Presentation

Sensing Systems and Signal Processing
Dr Richard

Copyright By Assignmentchef assignmentchef

Sensing Light

Sensing Light
Nature of light

Silicon photodiodes

Avalanche photodiodes

Photomultiplier tubes

https://www.hamamatsu.com/eu/en/product/optical-sensors/photodiodes/index.html

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Sensing Light Nature of light
Light has a dual nature: it exhibits wave and particle properties

The particle associated with light is called a photon

The energy of a photon depends upon the frequency (or wavelength) of the light

E is the energy of a photon (J)
h is Plancks constant (6.626 x 10-34 Js)
f is the frequency of the radiation (s-1)
short wavelengths (UV) -> higher energies
long wavelengths (IR) -> lower energies

https://commons.wikimedia.org/wiki/File:EM_spectrum.svg, CC BY-SA 3.0

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Sensing Light Nature of light
The frequency of light is related to the wavelength
c is the speed of light (2.998 x 108 ms-1)
is the wavelength (m)

Example: red HeNe laser, = 632.8 nm

What is Energy of this photon?

https://commons.wikimedia.org/wiki/File:EM_spectrum.svg, CC BY-SA 3.0

EEEE3089 2021-2022

Sensing Light Nature of light
The frequency of light is related to the wavelength
c is the speed of light (2.998 x 108 ms-1)
is the wavelength (m)

Example: red HeNe laser, = 632.8 nm

f = 2.998108 / 632.810-9 = 4.74 x 1014 ~ 500 THz

E = hf = 6.62610-34 x 4.74 x 1014 = 3.14 x 10-19 J

https://commons.wikimedia.org/wiki/File:EM_spectrum.svg, CC BY-SA 3.0

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Sensing Light Nature of light
The frequency of light is related to the wavelength
c is the speed of light (2.998 x 108 ms-1)
is the wavelength (m)

Example: laser pointer
= 532 nm

How many photons per second is emitted by the laser pointer?

https://commons.wikimedia.org/wiki/File:EM_spectrum.svg, CC BY-SA 3.0

EEEE3089 2021-2022

Sensing Light Nature of light
The frequency of light is related to the wavelength
c is the speed of light (2.998 x 108 ms-1)
is the wavelength (m)

Example: laser pointer
= 532 nm
E = hf = hc / = 3.73 x 10-19 J

P = 2 x 10-3 Js-1

N = P / E = 5.36 x 1015 photons.s-1

https://commons.wikimedia.org/wiki/File:EM_spectrum.svg, CC BY-SA 3.0

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Sensing Light photoconductor
A photoconductor converts energy (and information) from an optical form into an electrical form
Photoconductors can obviously be used as light detectors
An opto-electrical phenomenon where a materials conductivity increases due to the absorption of em radiation
When light illuminates a semiconductor:
some photons with the right energy are absorbed
electrons from the valence band obtain enough energy to jump to the conduction band
conductivity increases because of the higher number of conduction electrons
An electron requires a minimum energy to jump from the valence to the conduction band
This minimum energy is the energy gap between the valence band and the conduction band
Photons with energies greater than the bandgap can be absorbed

Unfilled Bands
Filled Bands
Valence Band
Conduction Band

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Sensing Light photodiode
Photodiodes are semiconducting devices that convert light into electrical signals
A photodiode is a reverse-biased p-n junction
positive bias applied to n side of the diode
negative bias applied to p side of the diode
In an (ideal) reverse-biased p-n junction there is no current flow

If an photon with sufficient energy is incident on the junction, it can be absorbed E>Eg

The absorption creates an electron-hole pair as an electron jumps from the valence to the conduction band

The electron and the hole are swept through the junction in opposite directions

This creates a (photo-)current in the photodiode

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Sensing Light photodiode
Photodiode Characteristics

Different semiconductors are sensitive to different wavelengths of light due to their particular energy bandgap

We shall concentrate on silicon photodiodes
Si bandgap = 1.12 eV
Si long wavelength detection cut-off ~ 1.1 m

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Sensing Light Silicon photodiode

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Sensing Light Quantum Efficiency
The primary factor defining the sensitivity of a photodiode is its quantum efficiency (QE)

QE is defined as the percentage of incident photons generating electron-hole pairs which subsequently contribute to the output signal

QE = Nelectrons /Nphotons

depends on
wavelength
internal electric fields
sensor architecture

Can be as high as 80% for Si PDs in the visible
Silicon PD limits:
Red (long wavelength) limit:
band gap of Si = 1.12 eV corresponds to = 1.1 m
Blue (short wavelength) limit:
not intrinsic, due to surface structure
transparency of electrodes decreases
reflection losses increase

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Sensing Light Responsivity
The sensitivity of a photodiode can also be expressed in more practical units

The responsivity R(), is the amps of photodiode current (Ip) obtained per watt of incident illumination (P)

R() = Ip / P

Thorlabs: https://www.thorlabs.de/NewGroupPage9.cfm?objectgroup_id=2822

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Sensing Light Responsivity
R() may be derived by multiplying the QE by the electronic charge (e = 1.602 x 10-19 C) and dividing by the photon energy for a particular wavelength (hc / )

R() = QE.e/(hc/ )
= QE . .e/hc
= QE. /1.24
R() is in AW-1
is in microns for final form

Ip = QE.e. .P./(h.c)
= QE.e. .P./(h.f)
= P . R()
P is incident power, Ip is photocurrent

Thorlabs: https://www.thorlabs.de/NewGroupPage9.cfm?objectgroup_id=2822

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