SCR, DIAC, TRIAC, Power BJT, Power MOSFET, IGBT Heat sinks and junction temperature, LED, LCD, Photo transistor, Opto Coupler, Solar cell, CCD.

1. VEL TECH MULTI TECH DR.RANGARAJAN AND
DR.SAKUNTHALA ENGINEERING COLLEGE,AVADI
CHENNAI
BY
LIGI S
ASSISTANT PROFESSOR

2. 191EC211 / ELECTRONIC DEVICES AND CIRCUITS
UNIT – 4
POWER DEVICES AND DISPLAY DEVICES
SCR, DIAC, TRIAC, Power BJT, Power MOSFET, IGBT Heat sinks and
junction temperature, LED, LCD, Photo transistor, Opto Coupler, Solar cell,
CCD.

3. THYRISTOR (SCR)
 Thyristor is also known as SCR. It is a three terminal, four layer
semiconductor device. Many layers of silicon is used, gate terminal controls
and once the device is ON it acts like a rectifier and so it is called as SCR.
It has common properties of diodes, resistors and transistors.
 Thyristor is a unidirectional device and conducts current in only one
direction. It can be used only for switching and cannot be used for
amplification.
Symbol of SCR:
 The symbol of SCR looks like the diode symbol and Gate terminal entering
at the junction. It has three terminals Anode, Cathode and Gate.
Construction of SCR:
 Thyristor or SCR consists of four P-N-P-N layer and have three PN
junctions J1,J2, J3 in series. It has three terminals Anode, Cathode and
Gate. Gate terminal is attached to the P type layer near the Cathode
terminal.
 The two transistor model shows that SCR is a combination of one PNP
transistor and one NPN transistor. The emitter of PNP transistor is taken as
the Anode terminal, emitter of NPN transistor is taken as Cathode and base
of PNP is taken as the Gate terminal. The base of PNP is connected to the
collector of NPN and collector of PNP is connected to the NPN transistor.

4. Working of SCR:
Reverse blocking mode of SCR:
 The thyristor is switched off if no voltage is applied at the Gate terminal. When SCR is
reverse biased the anode is connected to the negative end and Cathode is connected to
the positive end, the junctions J1 and J3 are reverse biased and J2 is forward biased. It
acts like the conventional diode in reverse biased condition. Only reverse saturation
current flows. Reverse breakdown happens like diode when it exceeds the safe voltage.
Forward blocking mode of SCR:
 When SCR is forward biased the anode is connected to the positive end and Cathode is
connected to the negative end, the junctions J1 and J3 are forward biased and J2 is
reverse biased. It acts like the conventional diode in forward biased condition. Here also
no current flows, only small saturation current flows.
Forward conduction mode of SCR:
 When the thyristor is forward biased and small voltage is applied at the gate terminal,
the device is ON.
 In the two transistors model when the gate voltage is applied the lower transistor
switches ON which in turn switches ON the upper transistor. This continues till the
power supply applied to gate and anode terminals is stopped. The current flow continues
even when the applied voltage at the gate terminal is stopped.

5. Characteristics of SCR:
 When the gate voltage is not applied and when the anode and cathode terminals are forward or reverse
biased, it blocks the current flow and only saturation current flows.
 When the voltage is applied at the gate terminal the current flows in the lower transistor which turns ON the
upper transistor and this continues like a latch. Once the device is ON gate terminal has no effect in
controlling the current. To stop the device all the power supplies should be switched OFF.
Advantages:
 It is easy to turn ON
 It is possible to control AC power
 It can be protected with the help of fuse
 It is of low cost
 SCR or thyristor can handle large voltage,
current and power

6. Disadvantages:
 It conducts in only one direction, so it controls during only half cycle.
 It cannot be used at high frequencies
 Gate current should be positive
 It is very difficult to turn off SCR
Applications of SCR:
 Controlled Rectifiers
 Inverters
 Pulse circuits
 Timing circuits
 Latching relays
 Temperature control systems
 Remote switching units

7. DIAC (DIode for Alternating Current)
 Diac is a two terminal bidirectional semiconductor switch. ‘Di’ stands for
two and ‘ac’ stands for alternating current. Thus Diac is a diode for
alternating current. It belongs to Thyristor family. It can be turned ON in
both forward and negative direction. It starts conducting current when the
applied voltage is above breakover voltage.
Symbol of Diac:
 The symbol of Diac consists of two diodes connected in parallel but
opposite to one another. It has two terminals, anode1 and anode2. MT
stands for main terminal. Since it is bidirectional the terminals are
reversible like resistor and capacitor. It does not have a gate terminal.
Construction of Diac:
 Diac is made up of five layers and two terminals. It has two P type layer
and three N type layer. It has no base or gate terminal. The layers which
are close to the terminals are made up of both N type and P type. The
doping concentration in all the layers is identical, whereas in transistor
the doping concentration is different in each layer. Since all the layers are
identical they are bidirectional and terminals can be used the either way.

8. Working of Diac:
 When the terminal MT1 is positive the direction of the flow of current will be in the order P1-N2-
P2-N3. The junction between P1 and N2 is forward biased, the junction between N2 and P2 is
reverse biased and the junction between P2 and N3 is forward biased.
 When the terminal MT2 is positive the direction of the flow of current will be in the order N1-P1-
N2-P2. The junction between N1 and P1 is forward biased, the junction between P1 and N2 is
reverse biased and the junction between N2 and P2 is forward biased.

9. V-I characteristics of Diac:
 When the external voltage is applied at the terminals, Diac does not conduct immediately. Only a small
leakage current flows. When the applied voltage is further increased and when it crosses the breakover
voltage the junction which is reverse biased breaks and they start conducting.
 Till the breakover voltage is reached they remain in forward and reverse blocking state. After the
applied voltage is increased above the breakover voltage avalanche breakdown takes place and the
current increases. It happens for both the polarity of voltages. To turn OFF the device the applied
voltage is decreased below the breakover voltage.
Application of Diac:
 Used in TRAIC triggering circuit
 Used in Lamp dimmer circuit
 Used in heat control circuit
 Used in speed control of a universal motor.

10. TRIAC (Triode for Alternating Current)
 Triac is a three terminal bidirectional semiconductor device. Triac stands for Triode
for Alternating Current. It conducts current in both directions and gate terminal
controls it. Triac belong to the Thyristor family. The difference between thyristor and
Triac is SCR conducts current in only one direction, but Triac conducts in both
directions. It is used as AC Switch.
Symbol of Triac:
 The symbol of Triac has three terminals Anode1, Anode2 and Gate. The Anode1
and Anode2 terminals are commonly called as Main terminal1 and Main
terminal2. Gate terminal acts like a trigger to turn the device ON. The symbol
looks like two thyristors connected in inverse parallel direction merged together
with Gate terminal in common.
Construction of Triac:
 Triac is a four layer, six doped region and a three terminal device. Gate terminal
is connected to both N3 and P2 so that gate triggers the device when both
positive and negative voltage is applied. In the Same way MT1 or Anode1 is also
connected to N2 and P2 regions, and MT2 or Anode2 is connected to the P1 and
N4 regions. So the polarity between the terminals decide the direction of the
current through the layers.

11. Working of Triac:
 There are four possible combinations of the potentials applied to the
terminals.
Mode1: MT2 is positive and Gate terminal is positive.
 When the MT2 terminal is made positive with respect to the
terminal MT1 and when positive voltage is applied at the gate
terminal the path of the current flow from MT2 to MT1 will be P1-
N1-P2-N2. The junction between P1N1 and P2N2 are forward
biased and junction between N1P2 is reverse biased and breakdown
occurs at this junction.
Mode2 : MT2 is positive and Gate terminal is Negative
 When the MT2 terminal is made positive with respect to the
terminal MT1 and when negative voltage is applied at the gate
terminal, initially the path of the current flow from MT2 to MT1
will be P1-N1-P2-N3. When the voltage applied at the MT2
terminal is further increased the junction P2N2 is forward biased
and the path of the current flow will be P1-N1-P2-N2. More Gate
current is needed to turn the Triac.

12. Mode3: MT2 is negative and Gate terminal is positive.
 When the MT2 terminal is made positive with respect to the
terminal MT1 and when negative voltage is applied at the
gate terminal the path of the current flow from MT2 to MT1
will be P2N1P1.
 The Junctions P2N1 and P1N4 are forward biased and the
junction N1P1 is reverse biased. So in this mode Triac work
in a negative biased region.
Mode4: MT2 is negative and Gate terminal is negative.
 When the MT2 terminal is made negative with respect to the
terminal MT1 and when negative voltage is applied at the
gate terminal the path of the current flow from MT2 to MT1
will be P2N1P1N4.
 Mode2 and mode3 are less sensitive and need more gate
current to turn ON the device. Mode1 and mode4 have
greater sensitivity when gate polarity and MT2 are of same
polarity.

13. V-I characteristics of Triac:
 Triac is made up of two SCRs in inverse parallel. It operates in four modes. Initially the Triac operates in
forward and reverse blocking mode and only small leakage current flows through it. When the applied voltage
at the MT2 terminal is further increased and when it crosses the breakover voltage Triac starts conduction.
The current start to flow and the voltage applied at the Gate terminal controls this current flow.
Application of Triac:
 Used in high power lamp switching
 Used to Control AC power
 Used in light dimmers
 Used to control the speed of electric fan and small motors

14. POWER BJT
Construction of Power BJT:
 The power BJT has three terminals Collector (C), Emitter (E) and Base (B). It has a vertically oriented four-
layers structure. The vertical structure uses to increase the cross-sectional area.
 There are two types of BJT; n-p-n transistor and p-n-p transistor. Out of these two types, the n-p-n transistors
widely use compare to the p-n-p transistor.
 It has four layers. The first layer is a heavily doped emitter layer (n+). The second layer is moderately doped
the base layer (p). The third region is lightly doped collector drift region (n-). The last layer is a highly
doped collector region (n+).
 The drift layer (n-) increase the voltage blocking capacity of the transistor due to the low doping level. The width
of this layer decides the breakdown voltage. The disadvantage of this layer is that the increase on state voltage
drops and increase on state device resistance, which increases power loss.
 The power handling capacity of the power transistor is very large. So, they have to dissipate power in the form of
heat. Sometimes, heatsink uses to increase effective area and therefore increase power dissipation capacity. the
heatsink made from metal.
 The power bipolar junction transistor (BJT) blocks a high voltage in the off
state and high current carrying capacity in the on-state. The power handling
capacity is very high.
Symbol:

15. I-V characteristic:
The I-V characteristic of Power BJT divides into four regions.
1. Cut-off region 2.Active region
3. Quasi-saturation region 4.Hard saturation region
In the structure of BJT, there are two junctions; Emitter junction (BE) and Collector junction (CB).
1. Cut-off region:
The BE and CB both junctions are reverse bias. The base current IB=0 and collector current IC is equal to the reverse
leakage current ICEO. The region below the characteristic for IB=0 is cut-off region. In this region, BJT offers large
resistance to the flow of current. Hence it is equivalent to an open circuit.
2. Active region:
The BE junction is forward bias and CB junction is reverse bias. The collector current IC increase slightly with an
increase in the voltage VCE if IB is increased. The relation of IB and IC is, IC=βdcIB is true in the active region.
If BJT uses as an amplifier or as a series pass transistor in the voltage regulator, it operates in this region. The dynamic
resistance in this region is large. The power dissipation is maximum.
3. Quasi-saturation region:
Quasi-saturation region is between the hard saturation and active region. This region exists due to the lightly doped drift
layer. When the BJT operates at high frequency, it is operated in this region. Both junctions are forward bias. The device
offers low resistance compared to the active region. So, power loss is less. In this region, the device does not go into deep
saturation. So, it can turn off quickly. Therefore, we can use for higher frequency applications.

16. 4. Hard-saturation region:
The Power BJT push into the hard-saturation region from the quasi-saturation region by increasing the base current.
This region is also known as deep saturation region. The resistance offers in this region is minimum. It is even less than
the quasi-saturation region. So, when the BJT operates in this region, power dissipation is minimum. The device acts as
a closed switch when it operates in this region. But it needs more time to turn off. So, this region is suitable only for
low-frequency switching application. In this region, both junctions are forward bias. The collector current is not
proportional to the base current, IC remains almost constant at IC(sat) and independent from the value of base current.
Application of Power BJT:
1. Switched Mode Power Supply (SMPS)
2. Power Amplifier
3. Relay and Drivers
4. AC motor speed controller
5. DC/AC inverter
6. As series pass transistor in the regulated power supply
7. The audio amplifier in the stereo system
8. Power control circuit

17. POWER MOSFET
 Metal Oxide Semiconductor Field Effect Transistor MOSFET is a type of transistor used to switch
electronic signals. It has four terminals namely; source S, Drain D, Gate G and Body B.
 The MOSFET’s body is normally connected to the terminal of the sourceSS, which results in three-
terminal device similar to other field effect transistors FET.
 Since these two main terminals are usually interconnected via short circuit, only three terminals are visible
in electrical diagrams.
 It is the most common device in circuits that are both digital and analog. Compared to the regular
transistor, a MOSFET needs low current less than one mill−ampere to switch ON. At the same time, it
delivers a high current load of more than 50 Amperes.
Operation of a MOSFET
 MOSFET has a thin layer of silicon dioxide, which acts as the plate of a capacitor. The isolation of the
controlling gate raises the resistance of the MOSFET to extremely high levels almost infinite.
 The gate terminal is barred from the primary current pathway; thus, no current leaks into the gate.

18. MOSFETs exist in two main forms −
 Depletion state − This requires the gate-source voltage (VGB) to switch the component OFF. When the gate is
at zero (VGB) the device is usually ON, therefore, it functions as a load resistor for given logic circuits. For
loading devices with N-type depletion, 3V is the threshold voltage where the device is switched OFF by
switching the gate at negative 3V.
 Enhancement state − The gate-source voltage (VGB) is required in this state to switch the component ON.
When the gate is at zero (VGB) the device is usually OFF and can be switched ON by ensuring the gate
voltage is higher than the source voltage.
Symbol and Basic Construction
Where, D − Drain; G − Gate; S − Source; and Sub − Substrate

19. IGBT
 The insulated gate bipolar transistor IGBT is a semiconductor device with three terminals and is used mainly as an
electronic switch. It is characterized by fast switching and high efficiency, which makes it a necessary component
in modern appliances such as lamp ballasts, electric cars and variable frequency drives VFDs.
 Its ability to turn on and off, rapidly, makes it applicable in amplifiers to process complex wave-patterns with
pulse width modulation. IGBT combines the characteristics of MOSFETs and BJTs to attain high current and low
saturation voltage capacity respectively. It integrates an isolated gate using FET Field effect transistor to obtain a
control input.
IGBT Symbol
 The amplification of an IGBT is computed by the ratio of its output signal to its
input signal. In conventional BJTs, the degree of gain β is equal to the ratio of its
output current to the input current.
 IGBT has a very low value of ON state resistance ON than a MOSFET. This implies
that the voltage drop (I2R) across the bipolar for a particular switching operation is
very low. The forward blocking action of the IGBT is similar to that of a MOSFET.
 When an IGBT is used as controlled switch in a static state, its current and voltage
ratings equal to that of BJT. On the contrary, the isolated gate in IGBT makes it
easier to drive BJT charges and hence less power is required.

20.  IGBT is switched ON or OFF based on whether its gate terminal has been activated or deactivated. A constant
positive potential difference across the gate and the emitter maintains the IGBT in the ON state. When the
input signal is removed, the IGBT is turned OFF.
IGBT Principle of Operation
 IGBT requires only a small voltage to maintain conduction in the device unlike in BJT. The IGBT is a
unidirectional device, that is, it can only switch ON in the forward direction. This means current flows from
the collector to the emitter unlike in MOSFETs, which are bi-directional.
Applications of IGBT
 The IGBT is used in medium to ultra-high power applications, for example traction motor. In large IGBT, it is
possible to handle high current in the range of hundred amperes and blocking voltages of up to 6kv.
 IGBTs are also used in power electronic devices such as converters, inverters and other appliances where the
need for solid state switching is necessary. Bipolars are available with high current and voltage. However, their
switching speeds are low. On the contrary, MOSFETs have high switching speeds although they are expensive.

21. LIGHT EMITTING DIODE
 Light emitting diode or LED is similar to the semiconductor PN Junction
diode. When it is forward biased the holes from P type and electrons from
N type combine with each other and it emits energy in the form of light
and when reverse biased the current does not flow. The colour of the LED
depends on the nature of the semiconductor material used in the
manufacturing of the diode. Available LED colours are red, green , blue ,
yellow , amber and white.
Symbol of LED:
 Looks like tiny bulb. LED’s are directional devices and it is connected in
the specified direction only. It has 2 leads Anode and Cathode. It is shown
in the figure above .
Construction of LED:
 The structure and the construction of LED differs from the normal
semiconductor diodes. PN junction is formed by material which have low
energy band gap like gallium antimonide, gallium arsenide, indium
antimonide and indium arsenide. The PN junction is then covered with
hemispherical shaped shell body made up of transparent solid plastic
epoxy resin.

22.  Some are made with rectangular or cylindrical shaped dome also. This protects the LED from atmospheric
disturbances , vibration and thermal shock. The LED is constructed in such a way that the light emiited by the
photons in the junction is focused at the top of the dome.
 The P type material is the surface of LED. The anode is deposited at the edge of the P type material. Below the P
type material N type material is placed and the cathode is made of gold film which is placed below the N type
material. Gold film is used for better reflection.
Working of LED:
 When it is forward biased,that is when P type is connected with
positive terminal and when N type is connected to negative terminal
the current starts to flow. So the majority carriers and minority carriers
combine each other and it neutralizes the charge carriers in the
depletion region which is the junction of P and N type semiconductors.
 When the energy of electrons decreases from higher level to lower
level it emits energy in the form of photons. So the movement of
majority and minority charge carriers releases some amount of photons
in the form of monochromatic light. Its wavelength is in nm which
resembles the colour of the LED.

23.  The LED colours depends on the materials used, so depending on the application where it is used the
wavelength specifications are customized. This is because of the different energy gaps of different
semiconductor materials and the amount of photons emitted with varying frequencies.
 LED needs very low voltage of about 0.3v to turn on the device.When reverse biased current does not
flow. When the applied external voltage is increased, it permanently damages the device.
V-I characteristics of LED:
 Initially only red colour LED was used. After some research the different colour LED was formed by
using different types of semiconductor material. Each colour have particular wavelength.
Application of LED:
 Used in general lighting as bulbs
 Used in traffic light signal
 Used in vehicles used to dim the light
 Used in remote controls
 Used in camera flashes
 Used in lighted wallpaper
 Used in horticulture grow lamps

24. Liquid Crystal Display:
 Liquid Crystal Display (LCD) is an flat display screen used in
electronic devices such as laptop, computer, TV, cellphones and
portable video games. As the name says liquid crystal is a material
which flows like a liquid and shows some properties of solid.
These LCD are vey thin displays and it consumes less power than
LEDs.
Molecular arrangement of Liquid Crystal:
 molecular structure of liquid crystal is in between solid crystal and
liquid isotropic. In Liquid crystal display (LCD) nematic type of
liquid cyrstal molecular arrangement is used in which molecules
are oriented in some degree of alignment. For example when we
increase the temperature the ice cube melts and liquid crystal is
like the state in between ice cube and water
Construction of Liquid Crystal Display:
 Construction of LCD consists of two polarized glass pieces. Two
electrodes are used, one is positive and the other one is negative.
External potential is applied to LCD through this electrodes and it
is made up of indium-tin-oxide. Liquid crystal layer of about
10µm- 20µm is placed between two glass sheets. The light is
passed or blocked by changing the polarization.

25. Working of Liquid Crystal Display
 The basic working principle of LCD is blocking of
light. It does not produce light on its own. So
external light source is used. When the external light
passes from one polarizer to the next polarizer,
external supply is given to the liquid crystal ,the
polarized light aligns itself so that the image is
produced in the screen.
 The indium oxide conducting surface is a
transparent layer which is placed on both the sides
of the sealed thick layer of liquid crystal . When no
external bias is applied the molecular arrangement is
not disturbed.
 When the external bias is applied the molecular
arrangement is disturbed and it and that area looks
dark and the other area looks clear.
 In the segment arrangement, the conducting segment
looks dark and the other segment looks clear. To
display number 2 , the segments A,B,G,E,D are
energized.

26. Positive and Negative LCDs:
 In positive LCD display the segments are dark and the background is white and the polarizers are placed
perpendicular to each other. In the negative LCD display the segments are white in the dark background and
the polarizers are aligned to each other.
Advantages:
 It is thin and compact
 Low power consumption
 Less heat is emitted during operation
 Low cost
Disadvantages:
 Speed of operation is low
 Lifespan is less
 Restricted viewing angles
Applications:
 Used in digital wrist watch and numerical counters
 Display images in digital cameras and mobile screens
 Display screen in calculators and television
 Used in image sensing circuits and video players

27. PHOTO TRANSISTOR
 Photo Transistor is a three terminal semiconductor device which
converts the incident light into photocurrent. Light is incident on the
base terminal and it is converted into current which flows through
emitter and collector. It is the combination of photo diode and
transistor an amplifier. The current produced by the photo diode is
low, so it is sent through the transistor and amplified.
Symbol of Photo Transistor:
 The symbol of Photo Transistor is similar to the transistor. The arrows
shows the light incident on the base terminal.
Construction of Photo Transistor:
 When compared to normal transistor, in photo transistor the base and
collector area is large. The base area is increased to increase the
amount of current generated. Because more the light falls more the
current is generated. Earlier it was made up of single semiconductor
material like silicon or germanium.
 Recently photo transistors are made up of Gallium and Arsenic to
obtain higher efficiency. Finally photo transistor is placed inside a
metallic case and a lens is kept at the top of the case to absorb the
incident radiation.

28. Working of Photo Transistor:
 From the above circuit we can know that base is not
connected to any external bias and only light is incident on
the base terminal. Collector terminal is connected to the
positive side of external supply and output is taken from the
emitter terminal.
 When no light is incident on the base terminal only some
leakage current flows and it is called as dark current. When
light is incident on the lens at the base collector junction,
base current is generated which is proportional to the
intensity of the incident light.
Characteristics of Photo Transistor:
 From the above figure we can observe how the collector
current varies with the intensity of the incident light. The
collector current increases with the intensity of the incident
light. Collector current differs with the wavelength and the
intensity of the light.

29. Advantages:
 Efficiency is high
 Faster response
 Less noise interference
 Low cost
 Small in size
Disadvantages:
 Poor performance at high frequency
 Slower than photodiode
Applications:
 Used in Counting systems
 Used in Optical tape reader
 Used to detect Object
 Used in printers

30. OPTOCOUPLER
 Optocoupler is a electronic device which connects two isolated
circuits by light. Basically Optocoupler consists of LED and a
photo sensitive device. Both the circuits are enclosed in a
package. The circuits cannot be changed externally.
Optocouplers are used to prevent the system from high voltage.
Structure of Optocoupler:
 It consists of two circuits which are electrically isolated. The
first circuit infra red emitting diode and the second circuit is
infra red sensitive device, it can be photo diode, photo
transistor, photo TRAIC, photo SCR.
 The space between the two circuit can be made of glass, air or
transparent plastic. The LED emits the light and the photo
transistor receives the light and amplifies it. The 1st and 2nd
pins are the anode and cathode of LED, 3rd and 4th pins are the
emitter and collector of the photo transistor.

31. Working of Optocoupler:
 The basic working principle of Optocoupler is the output of the electrically isolated circuit is controlled by
varying the input of the circuit. Input is given to the Infra red LED by a voltage source, the intensity of the
voltage source is adjusted by varying the input voltage. The emitted light is of particular wavelength. The
photo detector detects this light and converts light energy into photo current. The output current produced is
then amplified. The output current is proportional to the intensity of the light incident on it.
Advantages of Optocoupler:
 Compact and less weight
 Low cost
 Works very fast
 Less noise
Disadvantages of Optocoupler:
 Optocouplers are not capable to handle high current
Applications of Optocoupler:
 Used for ground isolation
 Used in high voltage monitoring circuits
 Used in lighting control circuits
 Used in dimmer circuits

32. SOLAR CELL
 Solar cell is also called as photovoltaic cell and this is a device
which converts light energy into electrical energy by using
photovoltaic effect. Solar cell is basically a normal PN Junction
diode.
Symbol of Solar cell:
Construction of Solar cell:
 It consists of N type and P type semiconductor material. N type is
highly doped and P type is lightly doped. Top and bottom is of
conducting electrode to collect the current.
 The bottom is fully covered with the conductive layer and top layer is
not fully covered because the sun rays should not be fully blocked.
 Since semiconductors are reflective in nature, antireflective coating is
used. The whole arrangement is kept inside a thin glass to avoid
mechanical shock.

33. Working of Solar cell:
 The working of solar cell is based on photovoltaic effect. It is a
effect in which current or voltage is generated when exposed to
light. Through this effect solar cells convert sunlight into
electrical energy.
 A depletion layer is formed at the junction of the N type and P
type semiconductor material. When light energy of the sun rays
falls on the solar panel, the photons which is the small bundle
of energy whose energy is higher than the energy gap gives
energy to the electrons and holes in the depletion region.
 The electrons and holes move to the higher level which is the
conduction band. The electrons move towards N type and holes
move towards P typeand they act as a battery.
 So this movement of electrons and holes forms the electric
current.

34. Solar cell to Solar farm:
 Solar cell is the basic building module and it is in
octagonal shape and in bluish black colour. Each cell
produces 0.5 voltage. 36 to 60 solar cells in 9 to 10
rows of solar cells are joined together to form a solar
panel.
 For commercial use upto 72 cells are connected. By
increasing the number of cells the wattage and voltage
can be increased. The thickness of solar panel is in the
range 2.5 to 4cm. Many modules together form the
solar array.
V-I characteristics of Solar cell:
 Isc is the short circuit current and it is measured by
short circuiting the terminals. Voc is the open circuit
voltage and it is measured when no load is connected.
Pm is maximum power, Im is maximum current, Vm is
maximum voltage and it occurs at the bend of the
characteristic curve.

35. Advantages of Solar cell:
 It uses renewable energy
 No pollution so it is environment friendly
 It lasts for many years
 No maintenance cost
Disadvantages of Solar cell:
 Energy is not produced during rainy, cloudy days and during night times.
 Cost of installation is high.
Applications of Solar cell:
 It is used in calculators and in wrist watches
 Used in storage batteries
 Street lights
 Portable power supplies
 Satellites

36. CHARGE COUPLED DEVICE
 CCD is the acronym for Charge coupled device. It is an
integrated circuit which consists of light sensitive
elements and it captures and stores the image in the form
of electrical charge. These electrical charges are then
shifted inside the device and it is manipulated and
digitized. It is based on Metal Oxide Semiconductor.
Rain water analogy of charge coupled device:
 The working of Charge coupled device can be understood
clearly by rain water analogy. The cups are spread over
rectangular conveyor belt and after the cups are filled by
the rain water, it is transferred from the cups in the vertical
conveyor belt to the cups in the horizontal belt. From the
horizontal conveyor belt the rain water is shifted to the
final storage container.
 In the same way the pixels are collected in the
photosensitive area and it is transferred vertically and
horizontally by shift registers. Finally it is transferred to
the last capacitor and it is amplified, digitized and saved in
the memory.

37. Architecture of charge coupled device:
1) Full frame: Full area is active and used to collect the
light. To read out the data a mechanical or camera
shutter is used. This results in image streaking.
2) Frame transfer: Only half of the area is exposed to
incident light which is image store area and half of
the area is covered with opaque mask which is the
frame store area. Image is transferred very quickly
from image store area to frame area store. No shutter
is required and no image streaking.
3) Interline : In this type alternate columns are masked
and used for frame store. So the shutter time is very
less and image streaking is totally eliminated. But the
cost is high to manufacture complex architecture.

38. Working of charge coupled device:
There are two main regions
1) Photo active region
2) Transmission region
Photo active region: This photoactive region is an
epitaxial layer of silicon. It consists of an array of
capacitors. The image is projected onto this photoactive
region through the lens. So the electric charge proportional
to the light intensity of the image at that location is
accumulated in the array of capacitors
Transmission region: The accumulated charge in the
photoactive region is then transferred to the shift registers
by transfer gates. After that by horizontal and vertical shift
registers the charge is transferred to the last capacitor
which is the storage capacitor. From the last capacitor it is
transferred to the amplifier where the current is converted
into voltage. This voltage is then sampled, digitized and
stored in the memory.

39. Advantages of charge coupled device:
 Smaller in size
 Consumes less power and works at low voltage
 Low noise and high sensitivity
 Very faint and bright targets can also be measured
 No chemical processing is needed
Disadvantages of charge coupled device:
 Slower readout
 No direct access to the pixel, since it is read out serially
Applications of charge coupled device:
 Used in astronomy for imaging, photometry and spectroscopy
 Used in digital photography. It converts captured light into digital data.
 Used in image sensors
 Used in signal processing
 Used in Medical fluoroscopy