CH-3 OPTOELECTRONIC DEVICES Marks: 10 (15)
Optoelectronics is the study and application of electronic devices and systems that detect and control light. Optoelectronic devices are electrical to optical or optical to electrical transducers.
Photodiode:
A photodiode is a semiconductor device that converts light into an electrical current.
The current is generated when photons are absorbed in the photodiode.
Photo means light and diode means a device consisting of two electrodes.
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.
Principle of photodiode:
It works on principle of photo junction effect.
It is two terminal junction device operated by reverse biasing junction.
A reverse biased pn-junction has small amount of reverse saturation current. It is due to thermally generated electron-hole pairs.
Same effect occurs if the junction is illuminated. Electron-hole pairs are also generated by incident light.
The number of carriers depends on the intensity of light incident on junction.
As intensity of incident light increases, number of carriers i.e. electron-hole pairs increases and hence increases level of reverse saturation current.
Photodiode Construction:
Photodiode is formed by diffusion of lightly doped P-type impurity into heavily doped N-type impurity.
Area between P-type impurity and N-type impurity is depletion region.
The contact terminal at N-type area is cathode and that of P-type area is anode.
Active area on front side is coated with anti reflection material to reduce the reflection of light and other area is coated with thick layer of silicon dioxide(SiO2).
Photodiode operation:
The circuit has reverse biased photodiode, resistor R and DC power supply.
When no light is incident on photodiode, the reverse current is extremely small. This is called dark current.
When light is incident on pn-junction of photodiode, there is transfer of energy from incident light to atom in junction. This will cause more free electrons.
This will increase the reverse current. As intensity of light increases, reverse current goes on increasing till it becomes maximum.
Characteristics of photodiode:
For a given reverse voltage reverse saturation current increases with increase in the intensity of incident light.
Increasing the reverse voltage does not increase the reverse current because all available charge carriers are already being swept across junction.
To reduce the current to zero it is necessary to forward bias junction by an amount equal to the barrier potential.
Dark current means the current that flows when no light is incident.
A photodiode can turn ON and OFF in nanoseconds. Hence it is fastest photo detector. Hence it is used where it is required to switch ON and OFF at maximum rate.
Applications/uses of photodiode:
Consumer electronics devices like smoke detectors, compact disc players, and televisions and remote controls in VCRs.
Automotive devices
Detection of visible and invisible light.
In optical communication instrument.
Character recognition.
Cameras
Medical devices
Bar code scanners
Phototransistor:
Phototransistors are either tri-terminal (emitter, base and collector) or bi-terminal (emitter and collector) semiconductor devices which have a light-sensitive base region.
Symbol
Principle of phototransistor:
Phototransistor works on principle of photo junction effect.
Phototransistor is similar to an ordinary bipolar transistor, except that no base terminal is provided.
Instead of a base current the input to the transistor is provided in the form of light.
In phototransistor collector current increases, when collector base junction is illuminated. Phototransistor is more sensitive than photodiode.
Construction and Working:
It consists of two silicon or germanium p-n junctions as ordinary transistor.
Window lens is provided on the base.
The light falls on the base region through window lens the and direct voltage is applied across emitter and collector.
The collector current and emitter current is a function of light intensity which falls on the base region.
Due to amplifier action very small changes in light intensity can be detected by photo transistor.
It has very high sensitivity for the small change in the intensity of the light, compared to LDR or photo diode.
Application of phototransistor: Phototransistors are often used in smoke detectors, infrared receivers, and CD players. Phototransistors can also be used in astronomy, night vision, optocouplers and laser range-finding.
Optocoupler
LDR (Light Dependent Resistor):
Principle:
It works on principle of photo conductivity. It is also called photoresistor.
Light can provide sufficient energy to break away electrons from their atoms within material and liberating electrons from surface of material.
Free electrons and holes are created within material and hence its resistance decreases.
Such materials are called photosensitive or photoconductive materials. Cadmium sulfide (CdS) and (CdSe) are the common photosensitive materials.
Construction of Photoresistor:
Photosensitive material is arranged in the form of long strip, zigzagged across disc shaped base with protective sides.
For protection, a glass or plastic cover may be included.
Two ends of the strip are brought out.
Two metal grids are cemented to the these ends which serve as terminals for external connection
Symbol Construction
Characteristics of Photoresistor:
When it is not illuminated its resistance is greater than 100KΩ. This is known as the dark resistance of LDR.
When illuminated, the LDR resistance falls to few ohms.
Photosensitive materials respond slowly to changes in light intensity.
Types of photoresistors: It depends on the type of photoconductive material used for construction of photoresistor. Types of photosensitive materials are CdS, CdSe, PbS(lead sulphide) and TlS(thallium sulphide).
Applications/uses:
CdS type photoresistors are used in counting applications.
These are used for detecting ships and aircrafts by radiation given out by their exhausts.
CdS photoresistors are used in light meters.
As ON-OFF switch.
For relay control.
In street light control.
As volume control that is controlled by light levels.
Fiber Optic Cables (FOC):
Instead of conventional cables, fiber optic cables (FOC) are used in communication.
Signal to be transmitted is converted into light pulses by means LED or LASER.
Then light pulses are allowed to pass through FOC.
Optical fibers are widely used in fiber-optic communications.
An optical fiber is a flexible, transparent fiber made of glass or plastic, slightly thicker than a human hair. It functions as a waveguide or light pipe to transmit light between the two ends of the fiber.
Construction of FOC:
Optical fibers typically include a transparent core surrounded by a transparent cladding material with a lower index of refraction.
Light is kept in the core by total internal reflection. This causes the fiber to act as a waveguide.
Types of FOC:
Multi mode fibers: Fibers that support many propagation paths or transverse modes are called multi-mode fibers (MMF). Multi-mode fibers generally have a wider core diameter (50 to 1000 µm). These are used for short-distance communication links and for applications
Single mode fibers: Fibers that only support a single mode are called single-mode fibers (SMF). These fibers have core diameter about 2 to 15 µm. Single-mode fibers are used for most communication links longer than 1,050 meters.
Advantages of FOC over conventional electric cables:
Fiber optics cables can carry much information at a time.
It provides widest bandwidth.
Fiber optics cables are very small in size.
It provides fastest communication..
These are lightweight cables.
Free from electrical interference.
Greater safety. Fiber optic cable cannot be tapped like cable TV signals.
They are strong and flexible.
No corrosion due to water or chemicals.
Applications of fiber optic communication:
National telephones systems.
Inter connection from TV studio to main transmitter.
Closed circuit TV system.
Secured communication systems at military bases.
Aircraft communications and aircraft controls.
In Computer network like WAN, LAN.
Ship-board communications
In railway communications and controls.
In private organization for internal communications systems.
Nuclear plant interconnections
Questions
Marks wise Questions: 1M(3), 3M(3), 4M(1), 6M(1)
1. Fill in the blanks-
LDR is based on ————— effect. [M-14]
(a) Photoconductive (b) Photojunction (c) Photovoltaic (d) Photoemissive.
Phototransistor works on principle of————— effect. [M-15]
(a) Photoconductive (b) Photojunction (c) Photovoltaic (d) Photoemissive.
In dark, resistance of LDR is ————.
(a) Low (b)High (c)Zero (d) none
Photodiode works on principle of --—--————-— effect.
(a) Photoconductive (b) Photojunction (c) Photovoltaic (d) Photoemissive.
--------- deals with converting light energy to electrical energy and vice versa.
(a) Electronic devices (b) Mechanical devices (c) Optoelctronic devices
--------- is formed by lightly doped P-type and heavily doped N-type semiconductor.
(a) LDR (b) Photodiode (c) Phototransistor
In phototransistor -----------area is illuminated by light.
(a) Photodiode (b) Phototransistor (c) LDR
-------- deals with converting light energy to electrical energy and vice versa.
(a) Electronic devices (b) Mechanical devices (c) Opto-electronic devices
Photodiode is ----------operted.
(i) Reverse bias (ii) Forward bias (III) Unbiased
2. Match the following-
(1) A B
(1) LDR [M-11, M-13,M-15] (1) Photojunction effect
(2) Photodiode/phototransistor [M-11] (2) Photoconductive effect
(2) A B
(1) LDR [M-12] (1) CD player/bar code scanner/reverse biased
(2) Photodiode (2) Optocoupler [M-09]
(3) Phototransistor (3) Photorelay [O-09]
3. True or false-
Forward current of photodiode increases as intensity of incident light increases.
LDR is used in Optocoupler.
LDR is photoconductive cell. [O-09]
Photodiode is display device.
LDR works on principle of photo junction effect. [M-10]
Photo diode is always reverse biased.
Phototransistor is unipolar transistor. [M-14]
4. Answer the following-
What is optoelectronics?
What is photodiode?
Draw and explain light dependent resistor. [M-11]
What is LDR? Write the names of materials used for LDR.. [O-11]
Explain working principle and applications of photodiode. [M-14]
Write a note on LDR. [M-13,M-16]
Explain the construction and working of photo diode. [O-11]
Write any four applications of photodiode.
Explain construction and working of photoresistor/LDR
Explain characteristics of LDR.
Write applications of LDR
Explain in short— (i) Phototransistor (ii) Photodiode.
Explain working principle and construction of photodiode. Draw its symbol. [M-10]
What is phototransistor? Explain its principle and construction. Draw its symbol.[M-17]
Write a note on photo transistor. [O-11]
Write the applications of phototransistor.
Define fiber cable (FOC) and explain its construction.
Explain types of FOC.
State advantages of FOC over conventional cables.
Explain types of FOC.
