The document provides information on various optoelectronic devices including light emitting diodes, photodiodes, phototransistors, and photomultiplier tubes. It discusses their construction, working principles, characteristics and applications. The key optoelectronic devices covered are LEDs, which convert electricity to light, and photodiodes and phototransistors, which convert light to electricity. Photomultiplier tubes are also discussed as devices that amplify light-generated electrons to detect very weak light signals.

1. B.Sc III Semester
ELE 3: Opto and Digital Electronics PPT
by
Mahiboob Ali K Mulla
M.Sc. , M.Phil.
Assistant Prof.in Electronics,ssgfgcollege,Naragund

2. UNIT 1 :OPTO ELECTRONICS

3. Unit 1: Opto Electronics:
Opto electronics means the electronics materials and devices whose electrical properties
depends on Light energy.Ex.LDR,LED,Photo Diode,Photo transistor, Photo Multiplier Tube
,Solar cells, Photo Voltaic cells,Laser diodes etc.
spectral response of human eye

4. Electronics:
optoelectronic devices are predominantly made using III–V
semiconductor compounds such as GaAs, InP, GaN, and GaSb, and
their alloys due to their direct-band gap

5. OPTO ELECTRONICS DEVICES: TREE
The devices that converts electricity in to light (called emitters) and converts light in to electricity (called
detectors).That is Optics Plus light energy.
OPTO ELECTRONICS DEVICES
COUPLERS/ISOLATORS
EMITTERS SENSORS/DETECTORS
LED
VISIBLE
LED IR LED LASER
PHOTO DETECTOR PHOTO EMITTER
PHOTO
CONDUCTOR
PHOTO
VOLTAIC
PHOTO TUBE
PHOTO
MULTIPLIER
BULK
TYPE
JUNCTION
TYPE
SI SE
Cds Cdse Pbs
PHOTO
DIODE
PHOTO
TRANSISTOR
PIN
Pn Jn
APD
BJT
FET
LASER

6. Photo Electric Effect:

7. •The resistance of the material changes.
•The output current of the semiconductor
changes.
•The output voltage of the semiconductor
changes.

8. Photoemissive Cell

9. Photoconductive Cell

10. Photo-voltaic cell

11. LIGHT EMITTING DIODE:
The light emitting diode is P-N junction diode, which consists of two leads and
semiconductor light source. When it is activated by applying the suitable voltages across its
leads then it emits the light energy in the form of photons and the color of this light was
determined by the band gap of semiconductor material. This light energy is produced by
the recombination of electrons and electron holes with in this device and this process is
called electroluminescence process. This process was stated in twentieth century from solid
state material when it is heated at room temperature then it emits the light energy. At the
beginning the infrared light emitting diode was developed and its light intensity was low
but it is still frequently used in variety of consumer electronics circuits such as remote
controls. Different sizes of light emitting diodes are available in market form 1mm2 to
onward. One of the light emitting diode with electrical symbol and practical structure
diagram is shown in figure 1. It is also used to make LED matrix.

12. • Construction of Light Emitting Diode :
• The construction of light emitting diode is so much simple, it is made by depositing the three layers of
semiconductor material on a substrate. These three semiconductor material layers are made three regions
which are called a P-type region which is top one, active region which is middle one and N-type region
which is bottom one. The figure 2 shows all of three semiconductor material regions.
• The Construction Diagram of Light Emitting Diode
• As Per the figure 2 the P-type region have the holes, N-type region have the elections
and active region have both electron and holes. In normal condition when there are no
any voltages are applied at anode and cathode then all the holes and electrons are
present at their places but when the voltages are applied at this light LED then it is
forward biased. Then the holes from p-type region and electrons from n-type region are
pushed up towards the active region, which is also called the depletion region. Because
the holes have positive charge and electron have negative charge then the light is
produced by the recombination of these opposite polarity charges.

13. Working Principle Light Emitting Diode :
The light LED works on the same principle of simple PN junction diode means when the anode is connected
to positive terminal of dc supply and cathode is connected to the negative terminal of dc supply then the PN
junction is forward biased. When the PN junction is forward biased then the holes’ form P-type region and
electrons from N-type region are recombined then the conduction band is formed for emitting the light
energy in the form of light energy photons. This whole phenomenon is called electroluminescence
phenomena and this light energy depends upon the amount of current absorbed by the LED. In other words,
this light energy is directly proportional to the absorbing current means when it absorbs more current then
the light would be high similarly when it absorbs low current then the light would be low. All the absorbing
current not converted into light energy some of the portion of this current is converted into heat which is
dissipated by the light emitting diode during light emitting into external environment. This heat is known as
electron dissipate energy and this dissipate energy also depends upon the semiconductor material of light
emitting diode. The inner working diagram of this light emitting diode is also shown in figure .

14. I-V Characteristics of Light Emitting Diode:
Applications of Light Emitting Diode
1.These are used in security alarm system such as burglar alarm system.
2.There are used in electronic calculators for showing the digital data.
3.These are used in mobile phones for taking the pictures.
4.These are used in traffic signals for controlling the traffic crowds in cites.
5.These are used for lighting purpose such as in homes lights, factory lights and street lights etc. Instead
of incandescent lamps for saving the energy.
6.These are used in digital computers for displaying the computer data.
7.These are used in digital multimeter for showing the current, voltage and resistance in digital form.
8.These are used in aviation lights for air craft warning signal.
9.There are used in remote control systems such TV or LCD remote.
10.There are also used in digital watches and automotive heat lamps.

15. PHOTO DIODE:
Definition: Photodiode is a two terminal electronic device which, when exposed to
light the current starts flowing in the diode. It is operated in reverse biased mode
only. It converts light energy into electrical energy. When the ordinary diode is
reverse biased the reverse current starts increasing with reverse voltage the same
can be applied to the photodiode.
But in the case of photodiode the current can flow without application of reverse
voltage, the P-N junction of the photodiode is illuminated by light and light energy
dislodge valence electrons and the diode starts conducting.
• Construction of Photodiode
• The photodiode is made up of two layers of P-type and N-type semiconductor. In
this, the P-type material is formed from diffusion of the lightly doped P-type
substrate. Thus, the layer of P+ ions is formed due to the diffusion process. And N-
type epitaxial layer is grown on N-type substrate. The P+ diffusion layer is
developed on N-type heavily doped epitaxial layer. The contacts are made up of
metals to form two terminal cathode and anode.

16. The front area of the diode is divided into two types that are active surface and
non-active surface. The non-active surface is made up of SiO2 (Silicon di
Oxide) and the active surface is coated with anti-reflection material. The active
surface is called so because the light rays are incident on it.
While on the non-active surface the light rays do not strike. The active layer is coated with
anti-reflection material so that the light energy is not lost and the maximum of it can be
converted into current. The entire unit has dimensions of the order of 2.5 mm.

17. Working Principle of Photodiode:
When the conventional diode is reverse biased, the depletion region starts expanding and the
current starts flowing due to minority charge carriers. With the increase of reverse voltage, the
reverse current also starts increasing. The same condition can be obtained in Photodiode without
applying reverse voltage.
The junction of Photodiode is illuminated by the light source, the photons strike the junction surface. The
photons impart their enrgy in the form of light to the junction. Due to which electrons from valence band get
the energy to jump into the conduction band and contribute to current. In this way, the photodiode converts
light energy into electrical energy.

18. The current which flows in photodiode before light rays are incident on it is
called dark current. As leakage current flows in the conventional diode, similarly
the dark current flows in the photodiode.
Modes of Operation of Photodiode
It operates in two modes that are Photo-conductive and Photo-voltaic.
1.Photo-Conductive: When the Photo diode operates in reverse biased mode it is
called Photoconductive mode. In this, the current flowing in diode varies linearly
with the intensity of light incident on it. In order to turn-off the diode, it should be
provided with forward voltage.

19. 1.Photo-Voltaic: When the diode is operated without reverse biased it is said to be operated in
photovoltaic mode. When the reverse biased is removed, the charge carriers are swept across the
junction. The barrier potential is negative on N-side and positive on P-side.
When an external circuit is connected to photodiode after removal of reverse biasing, the minority
carriers in both P, as well as N-region, return to their original region. It means the electrons which
crossed the junction from N-type to P-type again move to N-side with the help of external circuit.
And the holes which crossed the junction and moved from P-type to N-type during junction fabrication
will now again move to P-side with the help of external circuit.
Thus, the electrons can now flow out from N-type and holes can flow out from P-type thus in this
condition they behave as voltage cell having N-type as the negative terminal and P-type as a positive
terminal. Thus, the photodiode can be used as a photoconductive device or a photovoltaic device.
V-I Characteristics of Photodiode
The characteristics curve of the photodiode can be understood with the help of the below diagram.
The characteristics are shown in the negative region because the photodiode can be operated in
reverse biased mode only.

20. The reverse saturation current in the photodiode is denoted by I0. It varies linearly with the intensity of
photons striking the diode surface. The current under large reverse bias is the summation of reverse
saturation current and short circuit current.
I = Isc + I0 (1 – eV/ɳVt)
Where Isc is the short circuit current, V is positive for forward voltage and negative for reverse bias, Vt is volt
equivalent for temperature, ɳ is unity for germanium and 2 for silicon.
Advantages of Photodiodes
1.The reverse current is low in the tens of microamperes.
2.The rise and fall times in case of photodiodes is very small making it suitable for high-speed counting and
switching applications.

21. • Disadvantages of Photodiodes
• Photodiodes have lower light sensitivity than cadmium sulphide LDRs (Light
dependent resistors), thus they CdS LDRs are considered more suitable for
some applications.
• Applications of Photodiodes :
• It is used for detection of both visible as well as invisible light rays.
• Photodiodes are used for the communication system for encoding &
demodulation purpose.
• It is also used for digital and logic circuits which require fast switching and
high-speed operation.
• These diodes also find application in character recognition techniques and IR
remote control circuits.
• Photodiodes are considered as one of the significant optoelectronics devices
which is extensively used in the optical fibre communication system.

22. PHOTO TRANSISTOR :
• Definition: The phototransistor is a three-layer semiconductor device
which has a light-sensitive base region. The base senses the light and
converts it into the current which flows between the collector and the
emitter region.
• The construction of phototransistor is similar to the ordinary transistor,
except the base terminal. In phototransistor, the base terminal is not
provided, and instead of the base current, the light energy is taken as the
input.
• The symbol of the phototransistor is similar to that of the ordinary
transistor. The only difference is that of the two arrows which show the
light incident on the base of the phototransistor.

23. • Principle of Phototransistor
• Consider the conventional transistor is having open terminal base circuited.
The collector base leakage current acts as a base current ICBO.
• IC = βIB + (1+B) ICBO
• As the base current IB = 0, It acts as an open circuited. And the collector
current becomes.
• IC = (1+B) ICBO
• The above equations shown that the collector current is directly
proportional to the current base leakage current, i.e., the IC increases with
the increases of the collector base region.
• Phototransistor Operation
• The phototransistor is made up of semiconductor material. When the light
was striking on the material, the free electrons/holes of the semiconductor
material causes the current which flows in the base region. The base of the

24. The light enters into the base region of phototransistor generates
the electron-hole pairs. The generation of electron-hole pairs
mainly occurs into the reverse biasing. The movement of electrons
under the influence of electric field causes the current in the base
region. The base current injected the electrons in the emitter
region. The major drawback of the phototransistor is that they
have low-frequency response.
• Phototransistor Construction
• The construction of the phototransistor is quite similar to the
ordinary transistor. Earlier, the germanium and silicon are used
for fabricating the phototransistor. The small hole is made on the
surface of the collector-base junction for placing the lens. The
lens focuses the light on the surface.

26. • Nowadays the transistor is made of a highly light effective material (like
gallium and arsenides). The emitter-base junction is kept at forward biased,
and the collector-base junction is at the reverse biased.
• When no light falls on the surface of the transistor, the small reverse
saturation current induces on the transistor. The reverse saturation current
induces because of the few minority charge carriers. The light energy falls
on the collector-base junction and generates the more majority charge
carrier which adds the current to the reverse saturation current. The graph
below shows the magnitude of current increases along with the intensity of
light.

27. The phototransistor is widely used in electronics devices likes smoke
detectors, infrared receiver, CD players, lasers etc. for sensing light.
• Photodiode Vs Phototransistor :
• The photodiode and phototransistor both convert the light energy into the
electrical energy. But the phototransistor is mostly preferred over the
photodiode because of their following advantages.
• The current gain in the phototransistor is more than the phototransistor
even if the same amount of light strike on it.
• The sensitivity of the phototransistor is higher than the photodiode.
• The photodiode can be converted into the phototransistor by removing
their emitter terminals.
• The response time of the photodiode is much higher than the
phototransistor. The output current of the photodiode is in microamperes,
and it can switch on or off in nanoseconds. While the response time of the
phototransistor is in microseconds and it provides current in milliamperes.

28. •The Areas of application for the Phototransistor include:
•Punch-card readers.
•Security systems.
•Encoders – measure speed and direction.
•IR detectors photo.
•electric controls.
•Computer logic circuitry.
•Relays.
•Lighting control (highways etc)

29. • Photomultiplier tube working principle:
• A photomultiplier tube, useful for light detection of very weak signals, is a
photoemissive device in which the absorption of a photon results in the
emission of an electron. These detectors work by amplifying the electrons
generated by a photocathode exposed to a photon flux.

30. Construction:
A photomultiplier tube is a vacuum tube consisting of an input window, a
photocathode, focusing electrodes, an electron multiplier and an anode
usu- ally sealed into an evacuated glass tube. Figure above shows the
schematic construction of a photomultiplier tube.
The operating principle is that – caused by the photoelectric effect –
photons striking a photocathode at the entrance window of a PMT produce
electrons, which are then accelerated by a high-voltage field and multiplied
in number within a chain of dynodes by the process of secondary emission.
Among the photosensitive devices in use today, the photo- multiplier
tube (or PMT) is a versatile device that provides ex- tremely high sensitivity
and ultra-fast response. ... The photomultiplier tube also features fast time
re- sponse, low noise and a choice of large photosensitive areas.

31. V-I charecteristics:
Electrons emitted by the photocathode are accelerated toward the dynode
chain, which may contain up to 14 elements. Focusing electrodes are usually
present to ensure that photoelectrons emitted near the edges of the
photocathode will be likely to land on the first dynode. Upon impacting the
first dynode, a photoelectron will invoke the release of additional electron
that are accelerated toward the next dynode, and so on. The surface
composition and geometry of the dynodes determines their ability to serve
as electron multipliers. Because gain varies with the voltage across the
dynodes and the total number of dynodes, electron gains of 10 million
(Figure 1) are possible if 12-14 dynode stages are employed.
Photomultipliers produce a signal even in the absence of light due to dark
current arising from thermal emissions of electrons from the photocathode,
leakage current between dynodes, as well as stray high-energy radiation.
Electronic noise also contributes to the dark current and is often included in
the dark-current value.

33. Applications:
Photomultipliers are used in research laboratories to measure the intensity
and spectrum of light-emitting materials such as compound
semiconductors and quantum dots. Photomultipliers are used as the
detector in many spectrophotometers.

34. Photovoltaic Cell or Solar cell:
• Definition: The Photovoltaic cell is the semiconductor device that converts
the light into electrical energy. The voltage induces by the PV cell
depends on the intensity of light incident on it. The name Photovoltaic is
because of their voltage producing capability.
• The electrons of the semiconductor material are joined together by the
covalent bond. The electromagnetic radiations are made of small energy
particles called photons. When the photons are incident on the
semiconductor material, then the electrons become energised and starts
emitting.
• The energises electron is known as the Photoelectrons. And the
phenomenon of emission of electrons is known as the photoelectric
effect. The working of the Photovoltaic cell depends on the photoelectric
effect.

35. • The semiconductor materials like arsenide, indium, cadmium, silicon,
selenium and gallium are used for making the PV cells. Mostly silicon and
selenium are used for making the cell.
• Consider the figure below shows the constructions of the silicon
photovoltaic cell. The upper surface of the cell is made of the thin layer of
the p-type material so that the light can easily enter into the material. The
metal rings are placed around p-type and n-type material which acts as
their positive and negative output terminals respectively.
• The multi-crystalline or monocrystalline semiconductor material make the
single unit of the PV cell. The mono-crystal cell is cut from the volume of
the semiconductor material. The multicell are obtained from the material
which has many sides.
Construction of Photo Voltaic Cell:

37. The output voltage and current obtained from the single unit
of the cell is very less. The magnitude of the output voltage is
0.6v, and that of the current is 0.8v. The different
combinations of cells are used for increasing the output
efficiency. There are three possible ways of combining the PV
cells.
• Series Combination of PV Cells
• If more than two cells are connected in series with each
other, then the output current of the cell remains same, and
their input voltage becomes doubles. The graph below
shows the output characteristic of the PV cells when
connected in series.

38. • Series Combination of PV Cells
• If more than two cells are connected in series with each other, then the
output current of the cell remains same, and their input voltage becomes
doubles. The graph below shows the output characteristic of the PV cells
when connected in series.

39. • Parallel Combination of PV cells
• In the parallel combination of the cells, the voltage remains same,
and the magnitude of current becomes double. The characteristic
curve of the parallel combination of cells is represented below.

40. • Series-Parallel Combination of PV cells
• In the series-parallel combination of cells the magnitude of both the voltage
and current increases. Thereby, the solar panels are made by using the
series-parallel combination of the cells.

41. The solar module is constructed by connecting the single solar cells. And
the combination of the solar modules together is known as the solar panel.

42. • Working of PV cell
• The light incident on the semiconductor material may be pass or reflected
through it. The PV cell is made of the semiconductor material which is
neither a complete conductor nor an insulator. This property of
semiconductor material makes it more efficient for converting the light
energy into electric energy.
• When the semiconductor material absorbs light, the electrons of the
material starts emitting. This happens because the light consists small
energise particles called photons. When the electrons absorb the photons,
they become energised and starts moving into the material. Because of the
effect of an electric field, the particles move only in the one direction and
develops current. The semiconductor materials have the metallic
electrodes through which the current goes out of it.

43. Solar Cell:
• A solar cell (also known as a photovoltaic cell or PV cell) is defined as an
electrical device that converts light energy into electrical energy through the
photovoltaic effect. A solar cell is basically a p-n junction diode. Solar cells
are a form of photoelectric cell, defined as a device whose electrical
characteristics – such as current, voltage, or resistance – vary when exposed
to light.
• Individual solar cells can be combined to form modules commonly known as
solar panels. The common single junction silicon solar cell can produce a
maximum open-circuit voltage of approximately 0.5 to 0.6 volts. By itself
this isn’t much – but remember these solar cells are tiny. When combined
into a large solar panel, considerable amounts of renewable energy can be
generated.

44. Construction of Solar Cell:
• A solar cell is basically a junction diode, although its construction it is little
bit different from conventional p-n junction diodes. A very thin layer of p-
type semiconductor is grown on a relatively thicker n-type semiconductor.
We then apply a few finer electrodes on the top of the p-type
semiconductor layer.
• These electrodes do not obstruct light to reach the thin p-type layer. Just
below the p-type layer there is a p-n junction. We also provide a current
collecting electrode at the bottom of the n-type layer. We encapsulate the
entire assembly by thin glass to protect the solar cell from any mechanical
shock.

46. • Working Principle of Solar Cell
• When light reaches the p-n junction, the light photons can easily enter in
the junction, through very thin p-type layer. The light energy, in the form of
photons, supplies sufficient energy to the junction to create a number of
electron-hole pairs. The incident light breaks the thermal equilibrium
condition of the junction. The free electrons in the depletion region can
quickly come to the n-type side of the junction.
• Similarly, the holes in the depletion can quickly come to the p-type side of
the junction. Once, the newly created free electrons come to the n-type
side, cannot further cross the junction because of barrier potential of the
junction.
• Similarly, the newly created holes once come to the p-type side cannot
further cross the junction became of same barrier potential of the junction.
As the concentration of electrons becomes higher in one side, i.e. n-type
side of the junction and concentration of holes becomes more in another
side, i.e. the p-type side of the junction, the p-n junction will behave like a
small battery cell. A voltage is set up which is known as photo voltage. If we
connect a small load across the junction, there will be a tiny current flowing
through it.

47. V-I Characteristics of a Photovoltaic Cell

48. • Materials Used in Solar Cell
• The materials which are used for this purpose must have band gap close to
1.5ev. Commonly used materials are-
• Silicon.
• GaAs.
• CdTe.
• CuInSe2
• Criteria for Materials to be Used in Solar Cell
• Must have band gap from 1ev to 1.8ev.
• It must have high optical absorption.
• It must have high electrical conductivity.
• The raw material must be available in abundance and the cost of the
material must be low.

49. • Advantages of Solar Cell
• No pollution associated with it.
• It must last for a long time.
• No maintenance cost.
• Disadvantages of Solar Cell
• It has high cost of installation.
• It has low efficiency.
• During cloudy day, the energy cannot be produced and also at night we will
not get solar energy.

50. • Uses of Solar Generation Systems
• It may be used to charge batteries.
• Used in light meters.
• It is used to power calculators and wrist watches.
• It can be used in spacecraft to provide electrical energy.
• Conclusion: Though solar cell has some disadvantage associated it, but the
disadvantages are expected to overcome as the technology advances, since
the technology is advancing, the cost of solar plates, as well as the
installation cost, will decrease down so that everybody can effort to install
the system. Furthermore, the government is laying much emphasis on the
solar energy so after some years we may expect that every household and
also every electrical system is powered by solar or the renewable energy
source.

51. LIGHT DEPENDENT RESISTOR:
An LDR or a photo resistor is a device that is made up of high resistance
semiconductor material.
Construction of an LDR
The construction of an LDR includes a light-sensitive material that is placed on
an insulating substrate like as ceramic. The material is placed in a zigzag shape
in order to get the required power rating and resistance. The area of zigzag
separates the metal placed areas into two regions.
Where the Ohmic contacts are made either on the sides of the area. The
resistances of the contacts must be as less as possible to make sure that the
resistance, mainly varies due to the light effect only. The use of lead &
cadmium materials is avoided as they are injurious to the environment.

52. Working Principle of Light Dependent Resistor
The working principle of an LDR is photoconductivity, that is nothing but an optical
phenomenon. When the light is absorbed by the material then the conductivity of
the material reduces. When the light falls on the LDR, then the electrons in the
valence band of the material are eager to the conduction band. But, the photons in
the incident light must have energy superior than the bandgap of the material to
make the electrons jump from one band to another band (valance to conduction).

53. Fig.LDR Working Principle

54. •Applications of LDR :
Light-dependent resistors are simple and low-cost devices.
These devices are used where there is a need to sense the
presence and absence of light is necessary. These resistors
are used as light sensors and the applications of LDR mainly
include :
• alarm clocks,
• street lights,
• light intensity meters,
•burglar alarm circuits.