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11. ELECTRONICS SALIENT FEATURES

Band theory of solids Conductors, Insulators, and Semi conductors Types of semiconductors Junction Diode, properties and uses Junction transistor, properties and uses Important stages in Radio broadcasting Important stages in Television broadcasting Computer - principles and working

BAND THEORY OF SOLIDS Most natural solids are known to have crystalline structure. A crystal consists of three

dimensional array of atoms separated by equal distance.

A crystal is formed by bringing a large number of atoms together. Since the separation between atoms in a crystal is extremely small, of the order of 1A 0, the energy levels of outer shell electrons are significantly affected. However the energy levels of inner shell electrons are not much affected by the presence of neighbouring atoms and need not be considered for determination of properties of solids.

During formation of a crystal, when atoms come very close to each other, superposition of energy levels of outer shell electrons of the atoms take place. In the process, the discreteness of energy levels of individual atoms is lost altogether. Therefore, the superposition or grouping of levels leads to a range of energy values for the electrons in a crystal. This group of energy levels is broadly called an “energy band” of a solid.

The energy band structure in a solid consists of conduction band, valence band and a separation between them (EG). The band structure determines the electrical behaviour of a solid.

CONDUCTORS, INSULATORS, AND SEMICONDUCTORS

INSULATORS

A very poor conductor of electricity is known as and insulator. In an insulator, the forbidden gap (Energy Gap) is large and the concentration of free electrons is small. Electrical conduction is impossible in an insulator because electrons in them require very high energy.

Ex: Plastic, Rubber, Wood etc.

CONDUCTORS

Substances that conduct electricity efficiently are known as electric conductors. Conductors contain plenty of electrons that are free to move throughout the material. In a conductor Valence band and conduction band overlap and there is no physical distinction between them.

Ex: All metals, human body etc. www.sakshieducation.com


Semi conductors:

In a solid, if the forbidden gap is relatively small, that is about 1ev, the solid is called semiconductor. The electrical properties of a semiconductor are intermediate between those of insulators and good conductors.

Ex: Silicon, Germanium, etc.

Effect of temperature on a semiconductor: The conductivity of a semiconductor depends on its temperature. It increases with the

increase in temperature, because the energy gap of a semiconductor decreases with the rise in temperature.

At 0 K, a semiconductor is an insulator. As the temperature is gradually raised, some of the electrons from valance band can occupy the conduction band, leaving a gap in the valence band. This gap is known as “HOLE’’

The absence of an electron in the valence band of a semiconductor is called “HOLE”

TYPES OF SEMICONDUCTORS

Semi conductors are of two types. They are 1) Intrinsic semiconductor 2) Extrinsic semiconductor

1) Intrinsic Semiconductor: In an intrinsic semiconductor, the number of electrons in the conduction band and the

number of holes in the valence band are equal. Ex: Pure Silicon, Germanium etc.

2) Extrinsic semiconductor:

An extrinsic semiconductor is one which contains small quantities of selected impurities introduced into an intrinsic semiconductor.


www.sakshieducation.com The process of introducing impurities in small quantities into a material is called “doping”

Depending on the material doped, Extrinsic semiconductors are two types. They are 1) p-type Semiconductors 2) n-type semiconductors.

p-type semiconductors: A p-type semiconductor is formed when a small quantity of acceptor impurity is added to the pure

semiconductors.

Formation: When a trivalent impurity (Gallium) is doped in a pure semiconductor (Germanium) crystal, a p-type semiconductor is formed. Since the valency of Gallium atom is 3, it forms 3 covalent bonds with 3 Germanium atoms. In the fourth covalent bond, only Germanium atom contributes one valency electron, while Gallium has no electron to contribute. This missing electron is called a hole. Thus every Gallium atom creates an extra hole. Hence, the number of holes created becomes more than that of the conducting electrons in the conduction band produced by thermal agitation. Therefore holes are majority charge carriers. As holes are positive charge carriers, this substance is called p-type semiconductor.

n-type Semiconductor

When a donor impurity is doped into a pure semiconductor, an n-type semiconductor is formed.

Formation:



• When a pentavalent impurity (Phosphorous) is doped in a pure semiconductor crystal (Germanium) an n-type semiconductor is formed.

• The valency of Phosphorous atom is five hence, it forms four covalent bonds with four Germanium atoms.

• The fifth electron does not participate in the bond and is free. • Thus, every Phosphorous atom has a free electron which contributes for conduction of

electric current. • Hence, the number of free electrons becomes more than the number of holes produced by

thermal agitation. • Therefore, electrons are majority charge carriers. As electrons are negative charge carriers,

this substance is called n-type semiconductors.

JUNCTION DIODE JUNCTION:

When a p-type semiconductor is suitably joined to a n=-type semiconductor, a contact is established between them and is called a p-n junction.

Junction Diode: The p-n junction consisting of p-type and n-type semiconductors provided with two terminals is

called a p-n Junction Diode.

Properties of Junction Diode:

Forward bias: 1) In a p-n junction diode, when p-side is connected to the positive terminal and n-side is

connected to thenegative terminal of a battery, diode is said to be in ‘forward bias’ condition.

2) In this Forward bias condition a junction diode offers low resistance and conducts electricity.

Reverse bias:



1) In a p-n junction diode, when p-side is connected to the negative terminal and n-side is connected to the positive terminal of a battery, diode is said to be in “reverse bias” condition.

2) In this reverse bias condition, a junction diode offers high resistance and no current flows in the circuit.

Uses: 1) A p-n junction diode can be used as an electronic switch. 2) In a rectifier circuit, a p-n junction diode is used.

Conversion of Alternating Current (AC) into Direct Current (DC) is known as rectification.

3) Certain p-n junction diodes glow on supply of electricity. Such diodes are called Light Emitting Diodes (LEDs). These are used in digital clocks and digital calculators.

JUNCTION TRANSISTOR

The transistor was invented in 1948 by J.Barden, W.H. Btattain, and William Shockley.

A transistor consists of two p-n junctions formed by sandwiching either a p-type semiconductor or a n-type semiconductor between a pair of opposite type semiconductors.

Transistors are of two types, they are 1) p-n-p transistor 2) n-p-n transistor

p-n-p transistor:


www.sakshieducation.com Consider a p-n junction diode such that p-type semiconductor is on the left of n-type semiconductor.

Addition of one p-type semiconductor on to the right of n-type semiconductor gives a device consisting of three extrinsic semiconductors in the order on p,n,p. This device is called p-n-p transistor.

n-p-n transistor:

Similaryly, additionof one n-type semiconductor on to the left of p-type semiconductor, gives a different device of three extrinsic semiconductors in the order of n,p,n. This device is called a n-p-n transistor.

The three terminals of a transistor are 1) Emitter 2) Base 3) Collector.

Properties of a Junction transistor:

1) In a circuit, one junction of the transistor is forward biased and the other is reverse biased. The forward biased junction offers less resistance to the current flow. A reverse biased junction offers a high resistance to the current flow.

Uses: 1) Transistor acts as an amplifier.

An electronic device which can raise the magnitude of current or of voltage input signals is called as amplifier.

2) Transistors are used in electronic circuits called ‘oscillators’. 3) Transistors are also used in stabilized power supplies. 4) Transistors form important components of micro electronic systems called ICs (Integrated

Circuits) or ‘Chips’.

RADIO BROADCASTING AND RECEPTION



1) Production of sound and modulated wave: In a radio, the song rendered by a person is converted into electrical signals using microphone. These signals are in the audio frequency range. An oscillator produces suitable radio frequency (r.f.) carrier waves.

2) Radio Transmitter: The audio frequency (a.f) signal and radio frequency (r.f.) waves are combined in the modulator. The modulated waves are fed to an antenna fixed on a tower. The transmitting antenna broadcasts the modulated r.f. waves into space.

Modulation The process of fixing message (a.f.waves) to a radio frequency (r.f.) carrier waves is called “Modulation”. Modulation is of two types. 1) Amplitude Modulation (A.M) 2) Frequency Modulation (F.M.)

3) Radio receiver: The aerial of a radio receiver receives the modulated waves from different transmitting stations. The tuning circuit selects only the desired signal. The detector circuit extract (demodulate) the a.f. signal which is fed to an amplifier. The amplified electrical signals are fed to a speaker and converted into music or speech.

Demodulation The process of extracting information from the modulated r.f. carrier waves is called “demodulation”. In a radio, the detector circuit which consists a local oscillator demodulates the a.f. signals from r.f. carrier waves.



TELEVISION BROADCASTING

1) Production of audio and video signals - modulation The audio and video signals are converted into electrical signals by microphone and iconoscope respectively. When a T.V.Camera (Iconoscope) focused on to an object and the image thus formed is divided into a very large number of small squares, called scanning.

1. Scanning is a process of dividing an image on the screen into a very large number of very small squares formed by sets of horizontal and vertical lines.

2. Iconoscope is a camera consisting of Photo electric cells and cathode ray tubes.

2) T.V.Transmitter:

The audio signals are frequency modulated in F.M Sound transmitter and the video signals are amplitude modulated in A.M. Picture transmitter. Thus the audio and video modulated carrier waves are amplified and are together fed to the transmitting antenna. The transmitting antenna broadcasts the combined signal radiation into space.

2) T.V. Receiver: The receiving antenna in the television set receives the modulated wave and feeds to the tuning circuit. The tuning circuit selects the desired signals. The selected signals are demodulated. Audio, video signals are separated and amplified. The amplified waves are reconverted into image by picture tube and into sound by speaker.

An ordinary Television set is also known as “Kinescope”

The frequency ranges of r.f. carrier waves for radio transmission will be in the range of 300 KHZ to 30 MHZ and for Television transmission will be in the range of 30 MHZ to 300MHZ.

COMPUTER - BASIC PRINCIPLES OF WORKING

Microprocessor: • A combination of ICs is called a microprocessor. • This microprocessor can perform mathematical operations and can understand instructions

given to it. • A microprocessor can also take logical decisions. In order to do this it requires a device

called ‘input device’. • The data are processed according to the instructions given to it. • To process the data, the microprocessor consists three components, they are 1) Control Unit,

2) Memory Unit, 3) Arithmetic Logical Unit. The combination of these three is known as Central Processing Unit (CPU).

Computer can understand the binary system only. The binary digit 0 or 1 is known as a “bit”. A group of 8 bits is known as a “Byte”. One or more bytes make a “Word”

Memory

The data given to a computer can be stored in the form of BCD (Binary Coded Decimal) before processing. This BCD consists 8 bits. The first four bits are called Zone bits ( indicates letters or symbols) and the next four bits are called numeric bits (indicates numbers).


www.sakshieducation.com BCD (Binary Coded Decimal)

Computer can understand the instructions given in this form only. This is called as machine language. One well known machine language is “Assembler” Machine language depends on hardware of the computer.

All the physical components like CPU, input and output devices put together are called Hardware.

The set of instructions given to a computer are known as programme. A set of programmes is called as Software.

Block diagram of computer:

High level language

Writing programmes in ‘machine language’ is tedious. So a programme can be written in ‘high level language’.

A high level language is a collection ofa limited number of English words and symbols which are used according to a set of rules called “Syntax”.

Some of the high level languages are BASIC, COBOL, FORTRAN, C, C++, JAVA, ORACLE etc.

The computer requires the help of another programme called compiler which translates the high level language instructions into machine language.

High level language does not depend on hardware of a computer.

Thus to solve a problem one has to write a suitable program in one of the high level languages, when fed to the computer, will give you a result in the form of printout or display on the monitor within a fraction of second.

Generally the output devices are Monitor, Printer or external storage devices like CDs, DVDs, Pendrives etc.

Importance and uses of a computer

1) Computer became a part of life, because of its wide variety of uses in banking, industry, Commerce, science, education, medicine, law, weather prediction, satellite communication, war fare etc.


www.sakshieducation.com 2) Because of the computer, a technology called Information Technology (IT) is now widely in use. 3) The computer is modern man’s super servant but man has to be intelligent enough to give correct

instructions to get proper service from it.

IMPORTANT QUESTIONS

4 MARKS

1) Explain the formation of p-type semiconductor? (March-06)2) Explain the formation of n-type semiconductor? (March-06)3) State the properties and uses of Junction diode? (March-10,05)

4) State the properties and used of Junction Transistor?(Mar-08,07,03)5) Explain different stages in radio communication with a block diagram? (Mar-2000)6) Explain different stages in T.V. communication with a block diagram? (Mar-04,Jun-04)7) What do you know about microprocessor? (March-2000)8) Draw the block diagram of computer? Describe the functions of each of the components?

(March-08,06,01,June- 02)9) What is the classification of solids according to their conductivity? Explain?

(Jun-07,00)10) What is the importance of computer in modern life? (June-2001)

2 MARKS

1) Draw the diagrams of Forward bias and reverse bias of Diode? (June-04)2) Draw the diagrams of p-n-p and n-p-n transistors? (Mar-05, Jun-08,01)3) What are the ranges of frequencies used in radio and T.V. communication?4) What are the hardware and software of a computer? (Jun-07,Mar-07,05)5) Draw the block diagram of computer? (Mar-09, Apr-08)6) Mention the differences between p-type and n-type semiconductors?

1 MARK

1) What is a hole? (June-08)2) What is doping? (Apr-08, June- -6, March-06,04)3) What are the three terminals of a transistor?4) What is an IC?5) What is a BIT?6) Define Byte? (Mar-2003)7) What do you mean by a transistor? (March-2005)8) What is an energy band?9) What is the effect of temperature on the energy gap of a semiconductor? (Mar-06)10) What is the role of a compiler in the computer?11) What is modulation?12) Mention any three high level languages? (March-2010)13) What is scanning?14) What is demodulation?

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1. At 00K conduction band is completely EMPTY.2. At 0 K valence band is completely SATURATED.3. Rubber is an INSULATOR.4. The energy gap is highest in the case of INSULATOR.5. When temperature of a semi conductor is raised its energy gap DECREASES.6. The gap between conduction band and valence band is known as ENERGY GAP or FORBIDDEN GAP. 7. At 00K semi conductor behaves as an INSULATOR.8. The absence of an electron in the valence band of a semi conductor is called HOLE.9. Pure semi conductors are called INTRINSIC semi conductors.10. In an intrinsic semi conductor the number of ELECTRONS is equal to the number of HOLES.11. Doped or impure semi conductors are called EXTRINSIC semi conductors.12. The process of introducing impurities in small quantities into a material is called DOPING.13. In a p type semiconductor the majority carriers are HOLES and the minority carriers are ELECTRONS. 14. Example of trivalent impurity Ga, I, Al, B.15. Pentavalent impurities are called as DONAR IMPURITIES.16. In an n type semiconductor the majority carriers are ELECTRONS, and the minority carriers are HOLES. 17. In a semiconductor the charge carriers are ELECTRONS and HOLES.18. The combination of p type and n type semi conductors results in the formation of JUNCTION.19. The p-n junction consisting of p type and n type semiconductors provided with two terminals is

called a P-N JUNCTION DIODE.20. If the p side of a diode is connected to the positive terminal of a battery then the diode is said to

be in FORWARD BIAS21. If the n side of a diode is connected to the positive terminal of a battery then the diode is said to

be in REVERSE BIAS.22. A diode offers LESS resistance in forward bias condition.23. A diode offers HIGH resistance in reverse bias condition.24. Under FORWARD bias condition a diode conducts.25. Diode is used as RECTIFIER.26. A device which consist two p-n junctions is called JUNCTION TRANSISTOR.27. The three terminals of a transistor are EMITTER, BASE, and COLLECTOR .28. The arrow mark in the symbol of transistor indicates EMITTER.29. Transistor acts as AMPLIFIER.30. In radio communication the range of carrier frequency is 300K.Hz to 30 M.Hz.31. In T.V. transmission, the range of carrier frequency is 30 M.Hz to 300 m. Hz.32. The process of fixing message of radio frequency (r.f) carrier waves is called MODULATION.33. The process of extracting information from the modulated r.f. carrier waves is called DEMODULATION . 34. SCANNING is a process of dividing an image on the screen into a large number of very small

squares formed by sets of horizontal and vertical straight lines.35. The camera consisting of a cathode ray beam and a photo cell is called ICONOSCOPE.36. T.V. set is also known as KINESCOPE.37. The combination of diodes and transistors is called INTEGRATED CIRCUITS (IC).38. A group of IC s is called MICROPROCESSOR.39. Input devices are KEYBOARD, or MOUSE.40. Out put devices are PRINTER or MONITOR.41. CPU stands for CENTRAL PROCESSING UNIT.42. The main components of CPU are MEMORY , CU, ALU.43. The binary digit of 1 or 0 is called a BIT.44. A group of 8 bits is called a BYTE.45. Set of instructions is called a PROGRAMME.46. The language used by a computer is called MACHINE LANGUAGE.47. Machine language is dependent of HARDWARE.48. Computer programmes will be written in HIGH LEVEL LANGUAGES.49. Set of programmes is called SOFTWARE.50. High level languages are independent of HARDWARE.


www.sakshieducation.com 51. Number of bits in a BCD code is 8.

MATCHINGGROUP: A GROUP: B

1. Positively charged particles ( D ) A) Intrinsic semiconductor2. Pure silicon ( A ) B) Transistor3. Oscillator ( B ) C) Extrinsic semi conductor4. Rectifier ( E ) D) Hole5. n-type semiconductor ( C ) E) Diode

F) Positive ChargeG) Negative Charge.

GROUP: A GROUP: B 1. Boron ( B ) A) Low resistance.2. Phosphorous ( E ) B) Transistor.3. Amplifier ( F ) C) Trivalent impurity.4. Forward bias ( A ) D) High resistance.5. Reverse bias ( D ) E) Pentavalent impurity.

F) Diode.

GROUP: A GROUP: B 1. Mouse ( E ) A) High level language.2. Basic ( A ) B) Out put device.3. Assembler ( D ) C) Outer parts of a computer.4. Printer ( B ) D) Machine language.5. Hardware ( C ) E) Input device.

GROUP: A GROUP: B 1. Programme ( E ) A) Combination of diodes and transistors2. IC ( A ) B) Group of Programmes3. Bit ( D ) C) Group of 8 bits.4. Byte ( C ) D) 0 or 1.5. Software ( B ) E) Group of commands.


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picsciences.weebly.compicsciences.weebly.com/uploads/1/8/8/4/18842038/11-electronics.docx · Web viewJunction transistor, properties and uses . Important stages in Radio broadcasting