F.Y.B.Sc. (IT)

Normalization

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Boyce-Code Normal Form (BCNF)

  • A relationship is said to be in BCNF if it is already in 3NF and the left hand side of every dependency is a candidate key.
  • A relation which is in 3NF is almost always in BCNF. These could be same situation when a 3NF relation may not be in BCNF the following conditions are found true.
  1. The candidate keys are composite.
  2. There are more than one candidate keys in the relation.
  3. There are some common attributes in the relation
Professor Code Department Head of Dept. Percent Time
P1 Physics Ghosh 50
P1 Mathematics Krishnan 50
P2 Chemistry Rao 25
P2 Physics Ghosh 75
P3 Mathematics Krishnan 100

Consider, as an example, the above relation. It is assumed that:

  1. A professor can work in more than one department
  2. The percentage of the time he spends in each department is given.
  3. Each department has only one Head of Department.
  4. The relation diagram for the above relation is given as the following:

 

Untitled

The given relation is in 3NF. Observe, however, that the names of Dept. and Head of Dept. are duplicated. Further, if Professor P2 resigns, rows 3 and 4 are deleted. We lose the information that Rao is the Head of Department of Chemistry.

The normalization of the relation is done by creating a new relation for Dept. and Head of Dept. and deleting Head of Dept. form the given relation. The normalized relations are shown in the following.

Professor Code Department Percent Time
P1 Physics 50
P1 Mathematics 50
P2 Chemistry 25
P2 Physics 75
P3 Mathematics 100

 

Department Head of Dept.
Physics Ghosh
Mathematics Krishnan
Chemistry Rao

See the dependency diagrams for these new relations.

y1

 

 

 

 

 

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COMPOSITION AND INHERITANCE

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  • In real-life, complex objects are often built from smaller, simpler objects. For example, a car is built using a metal frame, an engine, some tires, a transmission, a steering wheel, and a large number of other parts.
  • A PC is built from a CPU, a motherboard, some memory, etc. This process of building complex objects from simpler ones is called composition (also known as object composition).
  • Composition is used for objects that have a has-a relationship to each other.
  • A car has-a metal frame, has-an engine, and has-a
  • A personal computer has-a CPU, a motherboard, and other components.
  • Composition is nothing but relationships between the different objects. Sometimes Object made up from another objects like Airplane is Wings, Landing gears, engines etc. this relationship is called composition
  • Inheritance is extends one class to another class like

Public class A

{//Here methods and variable etc.}

public class B:A

{//here methods and properties variable etc}

public static void main()

{B b = new B();

b.method();

b.Variable ..}

 INHERITANCE

  • One of the most important concepts in object-oriented programming is that of inheritance. Inheritance allows us to define a class in terms of another class, which makes it easier to create and maintain an application.
  • This also provides an opportunity to reuse the code functionality and fast implementation time.
  • When creating a class, instead of writing completely new data members and member functions, the programmer can designate that the new class should inherit the members of an existing class. This existing class is called the base class, and the new class is referred to as the derived
  • The idea of inheritance implements the is a For example, mammal IS-A animal, dog IS-A mammal hence dog IS-A animal as well

            Base & Derived Classes:

  • A class can be derived from more than one class, which means it can inherit data and functions from multiple base classes.
  • To define a derived class, we use a class derivation list to specify the base class. A class derivation list names one or more base classes and has the form:

class derived-class: access-specifier base-class

  • Where access-specifier is one of public, protected, or private, and base-class is the name of a previously defined class. If the access-specifier is not used, then it is private by default.
  • Consider a base class Shape and its derived class Rectangle as follows:

#include <iostream.h>
// Base class
class Shape
{ public:
voidsetWidth(int w)
{ width = w; }
voidsetHeight(int h)
{ height = h; }
protected:
int width;
int height;};
// Derived class
class Rectangle: public Shape
{ public:
intgetArea()
{ return (width * height); }};
void main()
{ Rectangle Rect;
Rect.setWidth(5);
Rect.setHeight(7);
// Print the area of the object.
cout<< “Total area: ” <<Rect.getArea() <<endl;}

      When the above code is compiled and executed, it produces the following result:

 Total area: 35

 Total paint cost: $2450

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Application of Zener diode as voltage regulator:

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  • Zener diode works on reverse bias or in zener region the voltage across it is substantially constant for a large change of current through it. This characteristic permits the zener diode to be used as a voltage regulator.
  •  The zener diode maintains a constant voltage across the load spite of any change in load current or input voltage. Above figure is a very simple voltage regulator circuit requiring just one zener diode and one resistor. As long as the input voltage is a few volts more than the desired output voltage, the voltage across the zener diode will b stable.
  • As the input voltage increase the current through the zener diode increases but the voltage drop remains constant-a feature of zener diodes.
  • Review Questions

1)      Define semiconductor.

2)      What is meant by biasing of p-n junction diode?

3)      What is meant by the term “barrier potential”? What is its value for silicon and germanium diodes?

4)      Explain the formation of Depletion region in the unbiased p-n junction.

5)      Explain the working and characteristics of p-n junction diode.

6)      Compare ideal and practical diode.

7)       Compare zener and p-n junction diode.

8)      Zener diode can be used as voltage regulator. Justify.

9)      What is rectifier? With the help of neat circuit diagram and waveform explain the operation of a half rectifier circuit. Why this circuit is called ‘half wave’ circuit?

10)  Compare HW, FW and bridge rectifier.

 

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ZENER DIODE:

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Fig 1.13 Symbolic Representation of Zener Diode.

  • A conventional diode does not permit large current to flow when in reverse biased. When a p-n junction is reversing biased, the current through the junction is very small. However if the magnitude of reverse bias reaches a critical value, avalanche breakdown may take place. Thus a rapid avalanche breakdown occurs and the diode conducts a large current in the reverse biased mode and may get damaged permanently.
  • However, diodes may be specially built to operate in the breakdown region. By varying the degree of doping, diodes the specific breakdown voltages (ranging from about one to several hundred volts) can be fabricated. If the junction is well-designed, the breakdown will be very sharp and the current after the breakdown will be independent of voltages; such diodes designed for a specific breakdown voltage are shown as Zener diodes.
  • They are useful in voltage regular circuits. As the load current or supply voltage changes, the current through a Zener diode will accommodate itself to these changes to maintain a constant load voltage. The upper limit on the diode current is determined by the power dissipation rating of the diode.
  • Working Of  Zener Diode (Biasing of Zener Diode):
  • Forward Biasing of Zener Diode:
  • When the anode of the zener diode is connected to the positive terminal if DC Source and  the cathode is connected to the Negative terminal, the zener diode is said to be forward biased.
  • The forward biased zener diode is behaves identical to the forward biased diodeThe fig shows the forward biased connection of zener diode.

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  • Zener diode generally not used in forward biased condition.
  • Reverse  Biasing of Zener Diode:
  • When the cathode is connected to the positive terminal of the dc source and the anode is connected to the negative terminal of the dc sourse, the zener diode is said to be reverse biased.
  • Zener diode in the reverse biased condition is used as a voltage regulator.

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  • The breakdown voltage depends upon the following.

1)      Width of the depletion region.

2)      Doping level.

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The characteristics curve has three regions viz. forward, leakage and breakdown. In the diode forward region, it starts conducting at 0.7 V as any other silicon diode. The region between zero and breakdown is the leakage region and only small reverse current flows in this region. The breakdown region is very sharp. When the voltage reaches – 15V, the characteristics becomes almost vertical and the voltage becomes constant at- 15V.

The minus sign in the specification of the breakdown voltage does not have any significance. It only indicates that the Zener diode is reversing biased. It is preferable to say that the Zener diode has a breakdown voltage of (say) 15 V.

The following mechanism is responsible for breakdown under increasing reverse voltage,

1)      Zener Breakdown: The Zener breakdown occurs in the junction which being heavily doped and has a very narrow depletion layers, very strong electric field of the order of 10^8 V/M is developed at breakdown voltage. This electric field is strong enough to break the covalent bonds thereby generating electron hole pairs. Further a very small increase in reverse voltage produces a very large number of current carriers.

2)      Avalanche breakdown: The avalanche breakdown occurs in junction which are lightly doped, have wide depletion layer where the electric field is not strong enough to generate zener breakdown. The avalanche breakdown occurs when the accelerated free electron acquire

sufficient energy to ionize atoms by bombardment. The additional free electrons created in this manner are accelerated by the reverse field causing more and more ionization.

                  The zener diode uses a p-n junction is reverse bias to make use of the zener effect, which is a breakdown phenomenon which holds the voltage close to a constant value called the zener voltage. It is useful in zener regulator to provide a more constant voltage, for improvement of regulated power supplies, and for limiter application. Characteristics of zener diode is quite similar to that of simple PN junction diode in forward bias and have a Sharpe breakdown in reverse bias condition.

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VOLT AMPERE CHARACTERISTICS OF A PN JUNCTION:

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                                  Fig 1.12 Characteristics of PN Junction Diode

The generalized voltage-current characteristic for a p-n junction in Figure above shows both the reverse-bias and forward-bias regions.

  1. At zero voltage: The barrier does not permit any current to flow through it.
  1. Forward-bias: Current rises rapidly as the voltage is increased and is quitehigh.
  1. Reverse-bias: The junction offers a very high resistance called reverse resistance.

Some amount (very small) of free holes and electrons still manage to cross the junction and constitute a reverse curren

Other Important terms:

  1. If the reverse bias is made very high, a large number of electron-hole pairs are created and the reverse current increases to a relatively high value. The maximum reverse potential difference, which a diode can tolerate without breakdown is called reverse break down voltage or zener voltage.

In other words, the minimum reverse voltage at which a pn junction breaks down is called the breakdown voltage.

  1. Knee Voltage: The voltage at which the pn junction begins to conduct currentand shows rapid rise in the current.
  1. Maximum forward voltage: The highest forward current that the pn junctioncan conduct without any damage to the junction
  1. Peak Inverse Voltage(PIV): It is the maximum reverse voltage that can beapplied to a pn junction without any damage to the junction

Beyond PIV, the junction diode is destroyed due to excessive heat.

  1. Maximum power rating: It is the maximum power that can be dissipatedthrough the junction without damaging it.
  2. It is equal to the product of junction current and voltage across the junction.

 

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FULL WAVE RECTIFIER

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  • A rectifier which rectifies both halves of each A.C. input cycle is called a full wave rectifier.To make use of both the halves of input cycle, two junction diodes are used.
  • Principle. It also works on the principle that a junction diode offers lowresistance during forward bias and high resistance, when reverse biased. Here, two junction diodes are connected in such a way that if one diode gets forward biased during first half cycle of A.C. input, the other gets reverse biased but when the next opposite half cycle comes, the first diode gets reverse biased and the second forward biased. Thus, output is obtained during both the half cycles of the A.C. input.
  • Arrangement: The a.c. supply is fed across the primary coil P of a step-downtransformer. The two ends of the secondary coil S of the transformer are Connected to the p-sections of the junction diodes D1 and D2. A load resistance RL is connected across the n-sections of the diodes and the central Tapping of the secondary coil. The d.c. output will be obtained across load resistance RL.
  • Theory:
  • Suppose that during first half of the input cycle upper end of coil S isat positive potential and the lower end is at negative potential, the junction diode D1 will get forward biased, while the diode D2 reverse biased. The conventional current due to the diode D1 will flow along the path of full arrows.

1  2Full Wave rectifier (Center tap)               Input output Waveform Of Full wave  rectifier

                         Fig 1.11 Full Wave Rectifiers with its Waveform

  • When the second half of the input cycle comes, the situation will be exactly reverse. Now, the junction diode D2 will conduct and the conventional current will flow along the path of the dotted arrows
  • Since current during both the half cycles’ flows from right to left through the load resistance RL, the output during both the half cycles will be of the same nature. The right end load resistance RL will be at positive potential w.r.t. its left end.
  •    Thus, in a full wave rectifier, the output is continuous but pulsating in nature. However, it can be made smooth by using a filter circuit.

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JUNCTION DIODE AS RECTIFIER:

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An electronic device which converts A.C. power into DC. power is called a rectifier.

The junction diode offers a low resistance path, when forward biased and a high resistance path when reverse biased. This feature of the junction diode enables it to be used as a rectifier.

The two half cycles of alternating input e.m.f. provide opposite kinds of bias to the junction diode.

If the junction diode gets forward biased during first half cycle, it will get reverse biased during the second half cycle and vice-versa.

 

In other words, when an alternating e.m.f. signal is applied across a junction diode, it will conduct only during those alternate half cycles, which bias it in forward direction.

7.1. HALF WAVE RECTIFIER:

  • A rectifier, which rectifies only one half of each A.C. input supply cycle, is called a half wave rectifier.
  • Principle: It is based on the principle that junction diode offers low resistancepath, when forward biased and high resistance when reverse biased. When A.C. input is applied to a junction diode it gets forward biased during one half cycle and reverse biased during the next opposite half cycle. Thus output is obtained during alternate half cycles of the A.C. input.
  • Arrangement:  The A.C. supply is fed across the primary coil P of a step-down

Transformer. The secondary coil S of the transformer is connected to the junction diode and a load resistance RL shown in Fig. 1.11. The output D.C. voltage is obtained across the load resistance RL.

  • Theory:
  • Suppose that during the first half of the input cycle, the junction diode gets forward biased. The conventional current will flow in the direction of the arrow heads.
  • The upper end of RL will be at positive potential w.r.t. the lower end. The magnitude of output across RL during first half cycle at any time will be proportional to the magnitude of current through it.
  • Hence, during the first half of the input cycle, when junction diode conducts, output across RL vary in accordance with A.C. input

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Half wave Rectifier                                       Input Output Waveform of H.rectifier

Fig 1.10 HW Rectifier with its waveform

  • During the second half cycle, junction diode will get reverse biased and hence no output will be obtained across RL·
  • Critically, a small current will flow due to minority carriers and a negligible output will be obtained during this half cycle also.
  • During the next half cycle, output is again obtained as the junction diode gets forward biased.
  • Thus a half wave rectifier gives discontinuous and pulsating output across the load resistance as shown in Fig. Hence half wave rectification involves a lot of wastage of energy and hence it is not preferred.

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ADVANTAGES OF SEMICONDUCTORS:

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Because of their merits over vacuum tubes, semiconductor devices (junction diodes, transistors, integrated circuits) have practically completely replaced them in all the fields of electronics. Some of the advantages of the semiconductor devices are as given below:

Advantages:

 

  1. As semiconductor devices have no filaments, hence no power is needed to heat them to cause the emission of electrons.
  1. Since no heating is required, semiconductor devices are set into operation as soon as the circuit is switched on.
  1. During operation, semiconductor devices do not produce any humming noise.
  1. Semiconductor devices require low voltages for their operation as compared to vacuum tubes.
  1. Owing to their small sizes, the circuits involving semiconductor devices are very compact.
  1. Semiconductor devices are shock proof.
  1. Semiconductor devices are cheaper as compared to vacuum tubes.
  1. Semiconductor devices have almost unlimited life.
  1. As no vacuum has to be created in semiconductor devices, they have no vacuum deterioration trouble.

 

Disadvantages:

  1. Noise level is higher in semiconductor devices as compared to that in the vacuum tubes.
  1. Ordinary semiconductor devices cannot handle as much power as ordinary vacuum tubes can do.
  1. In high frequency range, they have poor response.
  1. The semiconductor devices are temperature-sensitive. The maximum temperature, the semiconductor devices can withstand, is very low (about 50° C). Even a small over-heating damages the semiconductor device. This is because, at a higher temperature, the covalent bonds break up and the semiconductor material forming the semiconductor device becomes conducting.

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FORWARD AND REVERSE BIASING ON A JUNCTION DIODE

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A junction diode can be biased in the following two ways:

5.1 Forward Bias.

  • When an external D.C. source is connected to the junction diode with p-section connected to positive pole and n-section to the negative pole, the junction diode is said to be forward biased.

 

  • Action of junction diode:

 

  • The p-n junction is forward biased as shown in Fig. 1.9 When forward biased; the positive holes in the p-section are repelled by the positive pole of the battery towards the p-n junction. Simultaneously, the negative electrons in the n-section are repelled by negative pole of the battery towards the junction.
  • However, the movement of electrons and holes across the junction is opposed by the barrier voltage or depletion voltage (0·3 V to 0·7 V) developed across the junction. Just near the p-n junction, electrons and holes combine and cease to exist as mobile charge carriers after the potential barrier is overcome by the applied potential.

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                                   Fig. 1.9 forward biased p-n junction diode

  • For each electron-hole combination that takes place near the junction, a covalent bond breaks in the p-section near the positive pole of the battery. Of the electron and the hole produced, the electron is captured by the positive terminal, while the hole moves towards the junction.

 

  • On the other hand, as soon as the hole is created in the p-section due to the breaking of a covalent bond, an electron is released from the negative terminal of the battery into the n-section to replace the electron lost by the combination with a hole at the junction. These electrons move towards the junction, where they again get neutralized on meeting the new holes coming from left. As a consequence, a relatively large current, called forward current

 

flows through the junction.

 

  • The current in the external circuit is due to the electrons and is from negative terminal of battery to positive terminal through the junction diode.

 

  • During the forward bias, the applied D.C. voltage opposes the barrier voltage developed across the p-n junction. Due to this, the potential drop across the junction decreases and as a result, the diffusion of holes and electrons across the junction increases. It makes the depletion layer thin and the junction diode offers low resistance during forward bias.

 

5.2 Reverse Bias:

  • When a battery is connected to junction diode with p-section section connected to negative pole and n-section connected to the positive pole, the junction diode is said to be reverse biased.
  • Action of junction diode.

 

  • When the p-n junction is reverse biased as shown in Fig. 1.9 the holes (majority carriers) in the p-section get attracted towards the negative terminal of battery and therefore, the holes move away from the junction. At the same time, the electrons (majority carriers) in the n-section get attracted towards the positive terminal and move away from the junction.
  • As a very small number of holes and electrons (minority carriers) are left in the vicinity of the junction, practically no flow of current cakes place. However, due to thermally generated electron-hole pairs within p-region as well as n-region, a small current (a few microamperes) still flows. Some covalent bonds always break because of the normal heat energy of the crystal molecules. Electrons liberated by this process in the p-region move to the left across the junction, while holes generated in the n-region move to the right under the electric field produced by the battery.
  • Thus, a small electron-hole combination current, called reverse current is maintained by the minority carriers. If the reverse bias is made very high, all the covalent bonds near the junction break and a large number of electron-hole pairs are liberated and the reverse current increases abruptly to a relatively high value.
  • The maximum reverse potential difference, which a diode can tolerate without breakdown is called reverse break down voltage or zener voltage.

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  • During the reverse bias, the applied D.C. voltage adds to the barrier voltage developed across the junction. Due to this, the potential drop across the junction increases and as a result, the diffusion of holes and electrons across the junction decreases. It makes the depletion layer thick and the junction diode offers high resistance during reverse bias.
  • It may be noted that the potential barrier opposes the forward current, while it adds the reverse current.

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Electronics and Communication Technology

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UNIT I   SEMICONDUCTOR DEVICES                          

1. INTRODUCTION:

  • A solid is a large collection of atoms. The energy levels of an atom get modified due to the presence of other surrounding atoms and the energy levels in the outermost shells of all the atoms form valence band and the conduction band separated by a forbidden energy gap.

 

  • The energy band formed by a series of energy levels containing valence electrons is called valence band. At 0 K, the electrons start filling the energy levels in valence band starting from the lowest one. The highest energy level, which an electron can occupy in the valence band at 0 K, is called Fermi level. The lowest unfilled energy band formed just above the valence band is called conduction band.
  • At 0 K, the Fermi level as well as all the lower energy levels is completely occupied by the electrons. As the temperature rises, the electrons absorb energy and get excited. The excited electrons jump to the higher energy levels. These electrons in the higher energy levels are comparatively at larger distances from the nucleus and are more free as compared to the electrons in the lower energy levels.
  • Depending upon the energy gap between valence band and the conduction band, the solids behave as conductors, insulators and semiconductors.

 

2. CLASSIFICATIONS IN SOLIDS

We know that some solids are good conductors of electricity while others are insulators.

  • There is an intermediate class of semiconductors. The difference in the behavior of solids as regards electrical conductivity can be beautifully explained in terms of energy bands.
  • The electrons in lower energy band are tightly bound to the nucleus and play no part in the conduction process. However the valence and conduction bands are of particular importance in ascertaining the behavior of various solids.

2.1 Metals ( Conductors ):

  • Conductors (e.g. copper, aluminum) are those substances which easily allow the passage of electric current through them. It is because there are a large number of free electrons available in a conductor.
  • In terms of energy band, the valance and conduction bands overlap each other as shown in Fig. 1.1. Due to this overlapping, a slight potential difference across a conductor causes the free electrons to constitute electric current. Thus the electrical behavior of conductors can be satisfactorily explained by the band energy theory of solids.
  • For example, (i) in sodium, the conduction band is partially filled, while the valence band is completely filled. (ii) The valence band is completely filled and the conduction band is empty but the two overlap each other. Zinc is an example of band overlap metals.

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Fig. 1.1 Energy Bands in Metals (Conductors)

  • In both the situations, it can be assumed that there is a single energy band, which is partially filled. Therefore, on applying even a small electric field, the metals conduct electricity.

 

2.2 Insulators:

  • Insulators are those substances which do not allow the passage of electric current through them.
  • In terms of energy band, the valence band is full while conduction band is empty. Further, the energy gap between valence and conduction band is very high (say 6 eV or above). Therefore, a very high electric field is required to push the electrons to the conduction band. For this reason, the electrical conductivity of such materials is extremely small and may be regarded as nil.

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FIG. 1.2 Energy band in insulators

  • In insulators, the forbidden energy gap is quite large For example, the forbidden energy gap for diamond is 6 eV, which means that a minimum of 6 eV energy is required to make the electron jump from the completely filled valence band to the conduction band. When electric field is applied across such a solid, the electrons find it difficult to acquire such a large amount of energy and so the conduction band continues to be almost empty. No electron flow occurs i.e. no current flows through such solids. So they behave as insulators.
  • 2.3 Semiconductors.
    • Semiconductors (e.g. germanium,. silicon etc.) are those substances whose electrical conductivity lies in between conductors and insulators.

    In terms of energy band, the valence band is almost filled and conduction band is almost empty.

    • The energy gap between valence and conduction bands is very small as shown in Fig. therefore comparatively smaller electric field (smaller than insulators but much greater than conductors) is required to push the electrons fro valence band to the conduction band. In short, a semiconductor has :

     

    I)  almost full valence band

     

    II) Almost empty conduction band

     

    III) Small energy gap (1 ev) between valence and conduction bands.

    • At low temperature, the valence band is completely full and conduction band is completely empty. Therefore, a semiconductor virtually behaves as an insulator at low temperatures.
    • The energy band structure of the semiconductors is similar to the insulators but in their case the size of the forbidden gap is much smaller than insulators. For examples forbidden gap os silicon is 1.1 eV and that of germanium is 0.69eV.

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Fig. 1.3 Energy bands in semiconductors

  • In semiconductors due to smaller width of forbidden energy gap the electrons in find easier to shift to the conduction band. So the conductivity in semiconductors lies between that of metals and insulators.

3. TYPES OF SEMICONDUCTORS:

The semiconductors are classified as intrinsic and extrinsic semiconductors on the basis of their purity.

3.1. Intrinsic Semiconductors:

In a pure semiconductor, each atom behaves as if there are 8 electrons in its valence shell (due to formation of covalent bonds) and therefore the entire material behaves as an insulator low temperature.

A semiconductor atom needs energy of the order of 1·1 e V to shake off the valence electron. This energy becomes available to the semiconductor even at room temperature. Due to thermal agitation of the crystal structure, electrons from a few covalent bonds come out. The bond from which electron is freed, a vacancy is created there. The vacancy in the covalent bond (where there should have been an electron) is called a hole. This hole can be filled by some other electron in a covalent bond.

As an electron from a covalent bond moves to fill the hole, the hole is created in the covalent bond from which the electron has moved. In other words, the hole shifts from one covalent bond to another in a similar way as an electron does in an attempt to fill the hole. Since the direction of movement of the hole is opposite to that of the negative electron, a hole behaves as a positive charge carrier.

Thus, at room temperature, a pure semiconductor will have electrons and holes wandering in random directions. These electrons and holes are called intrinsic carriers and such a semiconductor is called intrinsic semiconductor.

As the crystal is electrically neutral, the number of free electrons will be equal to the number of holes. If we apply potential difference across the semiconductor, the electrons will move towards positive terminal and the holes towards the negative terminal of the battery.

The electrons and holes are not current in themselves but act only as the negative and positive charge carriers of the current respectively.

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Fig. 1.4 Intrinsic Silicon structure

Also, when an electron is raised from the valence band to the conduction band, a vacancy created in the valence band. This vacancy created in the valence band (where electron was present before moving to conduction band) acts as the hole.

3.2 Extrinsic Semiconductors :

A pure semiconductor at room temperature possesses free electrons and holes but their number is so small that conductivity offered by the pure semiconductor cannot be made of practical use.

By the addition of impurities to the pure semiconductor in a very small ratio, its conductivity can be remarkably improved. The process of adding impurity to a pure semi conductor crystal (Si or Ge-crystal) so as to improve its conductivity, is called doping.

The impurity atoms are of two types:

 

  1. Pentavalentimpurity atoms i.e. atoms having 5 valence electrons such asantimony (Sb) or arsenic (As). Such atoms, when added to a pure semiconductor, produce excess of free electrons i.e. donate electrons to the semiconductor. For this reason, pentavalent impurity atoms are called donor impurity atoms. The semiconductor so produced is called n-type extrinsic semiconductor.

 

  1. Trivalentimpurity atoms i.e. atoms having 3 valence electrons such as indium(In) or gallium such atoms on being added to a pure semiconductor, instead of producing free electrons, accept electrons from the semiconductor. For this reason, trivalent impurity atoms are called acceptor impurity atoms. The semiconductor so produced is called p-type extrinsic semiconductor.

 

3.3 n-Type Semiconductor :

Fig 1.5 shows the effect of adding pentavalent impurity arsenic to silicon crystal.

When the arsenic impurity atoms are added to the silicon crystal in a small, its atoms replace the silicon atoms here and there. The four electrons out of five valance electrons of As-atom take part in covalent bonding with four silicon atoms surrounding it. The fifth electron is set free.

Obviously the extra free electrons created in the crystal will be as many as the number of the pentavalent impurity atoms added. As the pentavalant impurity increases the number of free electrons also increases, hence, it is called donor impurity.

The silicon crystal so obtained is termed as n-type Si-crystal. The electrons so set free in the silicon crystal are called extrinsic carriers and the n-type Si-crystal is n-type extrinsic semiconductor.

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Fig. 1.5 Si-Crystal as n-type Semiconductor

Due to thermal agitation, even the pure silicon crystal possesses a few electrons and holes. Therefore, n- type Si-crystal will have a large number of free electrons (majority carriers) and small number of holes (minority carriers).

3.4 p-Type Semiconductor :

Fig 1.6 shows the effect of adding trivalent impurity indium (In) to silicon crystal.

The four silicon atoms surrounding the In-atom can share one electron each with the In-atom, which has got three valance electrons. All three of the In-atom‘s valence electrons are used in the covalent bonds; and, since four electrons are required, a hole results when each trivalent atom is added.

Thus, for every trivalent impurity atoms added, an extra hole will be created. As the impurity atoms accept electrons from the silicon it is called acceptor impurity. The Si-crystal so obtained is called p-type as it contains free holes.Each hole is equivalent to positive charge. The holes so created are extrinsic Carriers and p-type Si-crystal so obtained is called p-type extrinsic Semiconductor.

1Fig. 1.6 p-Type Si semiconductor

4. FORMATION OF p-n JUNCTION:

  • A p-n junction is a basic semiconductor device.
  • When a p-type crystal is placed in contact with n-type crystal so as to form one piece, the assembly so obtained is called p-n junction or junction diode or crystal diode. The surface of contact of p and n- type crystals is called junction. In the p-section, holes are the majority carriers; while in n-section the majority carriers are electrons.
  • Due to the high concentration of different types of charge carriers in the two sections, holes from p-region diffuse into n-region and electrons from n-region diffuse into p-region. In both cases, when an electron meets a hole, the two cancel the effect of each other and as a result, a thin layer at the junction becomes devoid of charge carriers. This is called depletion layer.
  • The diffusion continues back and forth until the number of electrons which have crossed the junction have a large enough electrical charge to repel or prevent any more charge carriers from crossing over the junction. Soon enough, a state is reached where the pn junction is electrically neutral due to the creation of a potential barrier. The potential difference developed across the junction due to migration of majority charge carriers is called potential barrier.
  • It opposes the further diffusion of charge carriers. The magnitude of the potential barrier is different for ex. For germanium junction diode for silicon junction diode. However, the value of potential barrier depends on the magnitude of doping of the semiconductor crystal.

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Fig. 1.7 Formation of p-n junction Diode.

 

  • The junction diode is represented by the symbol as shown in Fig. 1.8

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Fig 1.8 Junction Diode

  • The arrow-head represents the p-section of the junction diode and points in the direction in which the hole current or conventional current will flow, when junction diode is forward biased. The electron current or the electronic current will flow in opposite direction.

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