This document provides a comprehensive presentation on photo detectors. It begins with an outline of topics to be covered, including the history of photo detectors and classifications of optoelectronic semiconductor devices. It then discusses the principles of photo detection, modes of operation for optical detectors, and laws of conservation and optical absorption. Finally, it describes types of photo detectors with respect to PN junctions, including photo diodes, PIN photo diodes, and avalanche photo diodes. Diagrams and equations are provided to illustrate key concepts.

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2. I dedicate this work to my beloved father Syed WaqarKazmi (Late), siblings
and the ummah...
Engr.Syed Absar Kazmi (Pakistan)
0060-182391572
G1220119 MSEE, IIUM Malaysia
engrabsarkazmi@gmail.com

3. A Comprehensive Presentation on
" Photo detectors"
Semiconductor Devices (ECE 6317)
Lecturer Dr.Fadzlin Hasbullah

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Outline
• Elements Periodic Table
• Classifications of Opto Electronics
• History of Opto Detectors
• Photo detection principle
• Modes of Operation of Optical Detector
• Law of Conservation and Optical Absorption
• Types of Photo Detectors (w.r.t PN junction)
a) Photo Diode
b) PIN photo Diode
c) Avalanche Photo Diode

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Semiconductors
Table. 1. Periodic Table [1].

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Classification of optoelectronics
semiconductor devices
Figure. 1. Classification of semiconductor devices [2].

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• John N. Shive was an American physicist and inventor who made remarkable
contributions in solid state physics and electronic engineering during the early
days of development of transistors at the Bell Telephone Laboratories.
• Graduation in physics and chemistry in the year 1934 and PhD in the year 1939.
• In 1948 the Shive implicated light beam instead of emitter terminal as wire for
transistor, generating holes that flow via collector; latterly this device was
appellated as phototransistor [3].
History
Figure. 2. Phototransistor inventor
Accessed on 27 Mar 15.

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What is Photo-detector ?
• Semiconductor devices which can be implicated
to detects the photons presence. Such devices
are known as photo detectors [5].
• They transform optical signals into electrical
signals.
• When excess electrons and holes are generated
in a semi-conductor.
• Response is proportional to the power in the
beam.
Figure.3. Photo detectors [6].

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Photodetection principle is based on two types of photoelectric effects which are
as follows [4].
 External photoelectric effect : Electrons become free from the metal
surface by energy absorption obtained by streams of incident photons.
Photomultiplier tube and vacuum photodiode are the implications of
external photoelectric effect.
 Internal photoelectric effect : Free charge carriers are generated by
absorption of incident photons in semiconductor junctions detectors.PIN
photodiode, pn junction photodiode and avalanche photodiode.
• Important detector properties are responsivity, spectral response, and rise
time.
Principles of Photodetection

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Operation regimes of an illuminated
junction
Figure.4(a) .(photo detector)
Power is delivered to
the device by the external circuit [7].
Figure 4(b). Photovoltaic
Power is delivered to the load by
the device(solar cell/ energy
harvesting) [7].

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Modes of Operation
Photoconductive vs. Photovoltaic
A photodiode can be operated in one of two modes:
Photoconductive (reverse bias)
• The photo detectors are operated in photo conductive mode.
Photovoltaic (zero-bias).
• Solar cells are operated in Photo voltaic mode.
• The photo diodes implicated in electrical energy generation are
optimized to have high efficiency of energy conversion.
• The photo diodes used as photo detectors are optimized to have fast
response times [8, 9].
Figure. 5. Operating Modes [10].

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Photoconductive mode
• In photoconductive mode the diode is operated in
reverse biased, which interns dramatically reduce
the response time at the expense of increased noise.
Consequently faster response times can be
achieved by increasing the depletion layer and
decreasing the junction's capacitance [11].
• The photocurrent is linearly proportional to the
luminance for the given spectral distribution.

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Photovoltaic Mode
• This mode exploits the photovoltaic effect, which
is the fundamental for solar cells. When used in
zero bias or photovoltaic mode, the flow of
photocurrent out of the device is restricted and a
voltage builds up [11].

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Characteristic curve for operating
modes
Figure. 6. Photo detectors operating modes [11].

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Current Voltage characteristic
Figure.7. Photodiode Characteristic curves [12].

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Light spectrum versus wavelength and
energy.
Figure. 8. Light spectrum vs. wavelength and energy [13].

17. Figure.9. Momentum conservation law
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MOMENTUM CONSERVATION LAW
Momentum conservation ensures that only vertical
transitions are allowed during absorption and
emission.
 Figure.9 Depicts Band-to-band absorption in
semiconductors.
 An electron from the valence-band going to an
acceptor causes a hole to be generated. Reverse
processes can also occur.
The most imperative mechanism for recombination
and generation is light emission and absorption
respectively [14].However my concern is in
absorption.

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Optical Absorption in Semiconductors
• Interaction in semiconductors as far as devices are
concerned is the band-to-band transition.
• In the photon absorption process, a photon scatters
an electron in the valence band, causing the
electron to go into the conduction band.
• These two processes are of obvious importance for
light-detection [14].
Figure.10. absorption and recombination [15].

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•–
• Most modern photo detectors operate on the basis of the
internal photoelectric effect.
•The photo excited electrons and holes remain within the
material, increasing the electrical conductivity of the material
in a semiconductor.
•Electron-hole photogenerationin semiconductor
Absorbed photons generate free electron-hole pairs.
Transport of the free electrons and holes upon an electric
field results in a drift current [7].
Electron-hole photo generation
Figure.11. Photo generation process

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Absorption coefficient of some direct and
indirect gap semiconductors.
For the direct gap material, the absorption
coefficient is very strong once the photon energy
exceeds the band gap.
For indirect materials the absorption
coefficient is small near the band edge, but once
the photon energy is more than the direct gap, the
absorption coefficient increases rapidly [14].
Figure.12. Absorption coefficient for
semiconductors.

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Absorption Coefficient and Photodiode Materials
– –
Photon absorption in a direct and indirect band gap semiconductors [16].
Direct Band gap
E
CB
VB
kk
Eg
EC
EV
Photon
Figure.13(a).direct band gap
k
Indirect Band gap
E
k
VB
CB
Photon
Phonons
EgEC
EV
Figure.13(b).indirect band gap

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Responsivity vs. wavelength
Figure.12. . Typical responsivity curves for silicon, InGaAs, and germanium
[29].

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Photo detectors Significant parameters
• Some of the imperative parameters of photo detector are as follows [18].
Quantum Efficiency It is the ratio of primary electron-hole pairs created photon to the photon
incident on the diode material.
• Detector Responsivity This is the ratio of output current to input optical power. Hence this is
the efficiency of the device. The unit of responsivity is amperes per watt.
• Spectral Response Range This is the range of wavelengths over which the device will operate.
The spectral response refers to the curve of detector responsivity as a function of wavelength
• Noise Characteristics
• The level of noise produced in the device is critical to it operation at low levels of input light.
• Response Time
• This is a measure of how quickly the detector can respond to variation in the input light
intensity
• Rise time
The rise time is the time for the detector output current to change to change from 10 to 90% of
its final value when the optic input power variation is a step.
• Gain
The APDs have more gain than PIN diode due to the effect of impact ionization as it is
operated at high electric field

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Types of photo detector (pn junction)
• Photodiode
• P-I-N Photodiode
• Avalanche Photodiode
Figure.15. Photodiodes [ 19].

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Photo diode
• Photo means light and diode means a device pertaining of two
electrodes [20].
• A photo diode is a light sensitive electronic device capable of
converting light into a voltage or current signal.
• It works on the principle of photo generation.
Figure. 16. Photodiode [21]

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Photodiode Equivalent Circuit
The simplified photodiode equivalent model represents the key elements.
• The irradiance (light intensity) is proportional current source ID which is in
parallel with diode in figure 17.
• Performance is dependent upon parasitic components RD and CD can plays an
imperative role [22].
Figure. 17. Photodiode Equivalent circuit

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Working of photodiode is based on Internal photoelectric effect : Free charge
carriers are generated by absorption of incident photons in semiconductor
junctions detectors [4].
The diode is connected reverse biased under incident photons.
•Free charge carriers are created by photon absorption.
•The pair of electron and hole is generated by every single incident photon.
•These moving charges cause current flow through the external circuit [23].
Photodiode Working
Figure. 18. Photodiode in reverse biased

28. Figure.18. Photodiode energy band dgm03/31/15
The reverse biased photo diode is under luminance (incident photons) is
represents the energy bandgap diagram in figure. 18(b) and 18(c).
• which absorbed in the junction via window.
• The absorbed energy raises a bound electron across the band gap from the
valence to the conduction band.
• A free hole is left in the valence to the conduction band.
• under electric field the generated free charge
carrieirs produce the drift current.
Photodiode Working...

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P-I-N photodiode
• A PIN diode is a diode with a wide, lightly
doped 'near' intrinsic semiconductor region
between a p-type semiconductor and an n-type
semiconductor region.
• The p-type and n-type regions are typically
heavily doped because they are used for ohmic
contacts [11].
Figure. 19. PIN diode

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P-I-N photodiode.....
Figure.20 (d). Photodiode Energy Band Dgm [16].
Figure. 20 (a). PIN diode [24]. Figure. 20 (b). PIN diode [24]. Figure. 20 (c). PIN diode [24].
Figure. 20.a shows the biased PIN photodiode vs.
energy band diagram.
Figure. 20.b depicts reversed biased PIN photodiode.
Figure. 20.c represents construction layers of PIN.
Figure. 20.d shows descriptive energy band diagram.

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Generic Operating Parameters of an pin
Photodiode InGaAs
Table. 2. PIN Photodiode operating parameters [25].

32. Avalanche Photodiode
• Avalanche Photodiodes ( APDs ) are high sensitivity, high speed
semi-conductor "light" sensors.
• Compared to regular PIN construction photodiodes, APDs, have
an internal region where electron multiplication occurs, by
application of an external reverse voltage, and the resultant
"gain" in the output signal means that low light levels can be
measured at high speed.
• Incident photons create electron – hole pairs in the depletion
layer of a silicon photodiode structure and these move towards
the respective PN junctions depending on the electric field
strength.
Figure. 21(a). APD construction [27].

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Avalanche Photodiode
Figure. 21(c). APD characteristic curve
[26].
Describing Avalanche photo-detecter construction and current voltage
characterstic curve with higlighted circle, depicts avalauche region in
figure.21(a) and 21(b) accordingly.
Operated with much higher reverse bias.
•This allows each photo-generated carrier to be multiplied by avalanche
breakdown.
•The effect is known as impact ionization/avalaunche effect.
•Resulting in internal gain within the photodiode, which increases the effective
responsivity of the device.
Figure. 21(b). Typical parameters for semiconductors [29]

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Significance of Photodiodes
• Photodiodes are unique among light detectors in that when illuminated, they
generate an output which is proportional to light level. Their characteristics and
features can be summarized as follows [8]:
Low cost visible and near-IR photo detector
• Excellent linearity in output photocurrent according to light intensity
• Fast response times.
• Available in a wide range of packages
• Low noise
• Mechanically rugged yet compact and light weight
• Available as duals, quads or as linear arrays
• Usable with almost any visible or near infrared light source such as LEDs;
neon, fluorescent, incandescent bulbs; lasers; flame
• sources; sunlight; etc.

35. Figure.22. Applications of photodiodes [28].
• Spectroscopy.
• Photography.
• Analytical instrumentation.
• Optical position sensors.
• Beam alignment.
Silicon photodiodes are utilized in such
diverse implications as
• Surface characterization.
• Laser range finders.
• Optical communications.
• Medical imaging instruments.

36. Qur’an about Light
• It is He who made the sun a shining light and the moon a
derived light and determined for it phases - that you may know
the number of years and account [of time]. Allah has not
created this except in truth. He details the signs for a people
who have knowledge [10:5].
• Indeed, in the alternation of the night and the day and [in] what
Allah has created in the heavens and the earth are signs for a
people who fear Allah [10:6].

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References
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[3] www.circuitstoday.com/invention-history-of-phototransistor-and-photodiode,Acessed 27 mar.
[4]
[5]
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[7] Photo Devices by Jia-Ming ch#14 and Fundamental of Photonics 2nd Edition by Saleh & Teich ch#18.
[8] http://ecetutorials.com/analog-electronics/photo-diode/Accessed on 22 Mar 2015.
[9]
[10] http://sub.allaboutcircuits.com/images/quiz/01031x01.png Accessed on 29 Mar 2015.
[11]
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Opto Electronic Device March, 2003 20th Edition Issued by Matsushita Electric Industrial Co., Ltd.
(C) Matsushita Electric Industrial Co., Ltd
www.powershow.com/view1/1b295a-ZDc1Z/Chapter_7_Light_Detectors_powerpoint_ppt_presentation 28 mar
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http://www.thorlabs.com/tutorials.cfm?tabID=31760 Accessed on 28 Mar 2015.
National seminar on Electric Energy : Safety & Conservations Department of Physics, University of Delhi India
http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/photdet.html#c2
BOOK; Semiconductor Physics and Devices Basic Principles Fourth Edition Donald A. Neamen University of
New Mexico ,Ch # 14 ,Pg # 622.

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[24]
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2015.

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http://www.pacer.co.uk/Assets/Pacer/User/Photodiode%20Typical%20Applications.pdf 27 Mar 2015.
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